preventing perseveration in language production

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Preventing perseveration in languageproductionJoseph Paul Stemberger aa University of British Columbia , Vancouver, BC, CanadaPublished online: 13 May 2009.

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Preventing perseveration in language production

Joseph Paul StembergerUniversity of British Columbia, Vancouver, BC, Canada

This paper investigates the effect of the repetition of phonological elements onaccuracy in spontaneous language production. Using a corpus of naturalisticspeech errors, it is shown that repetition of a whole segment doubles the errorrate on the second token (a perseveratory effect), for onset consonants, vowels,and coda consonants; the effect is present (at a reduced magnitude) in the speechof young children. Repetition also leads to an increased error rate on the firsttoken (an anticipatory effect), but only for word-initial consonants and only foradults. Repetition of subsegmental features has an effect only for word-initialconsonants and only perseveratorily. There are no effects of repetition of largerunits (e.g., syllable onsets) or for general segmental similarity. It is argued thatthe effect is largely due to mechanisms designed to prevent perseveration (bye.g., shifting average activation values downward), and affects early-accessedinformation (whole segments; onset consonants) more than later-accessedinformation (subsegmental features; vowels and coda consonants).

Keywords: Language production; Speech errors; Perseveration; Segmental

representation; Subsegmental features.

In language production, as in other tasks, a speaker must plan and execute

speech in such a way that the target elements (such as phonemes and

subsegmental features) occur at the intended points in time. Outputting the

intended elements is in principle separate from placing them into the

correct points in time. Skilled behaviour demands mechanisms that prevent

correct target elements from showing up multiple times at both correct and

incorrect points in time. Little is known about these mechanisms in human

language, or their functioning.

Correspondence should be addressed to J. P. Stemberger, Department of Linguistics, Totem

Fields Studios, University of British Columbia, Vancouver, BC, V6T 1Z4 Canada. E-mail:

[email protected]

LANGUAGE AND COGNITIVE PROCESSES

2009, 24 (10), 1431�1470

# 2009 Psychology Press, an imprint of the Taylor & Francis Group, an Informa business

http://www.psypress.com/lcp DOI: 10.1080/01690960902836624

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Perseveration (the tendency for an element to appear at the intended time

and at a later point in time) is clearly a problem for cognitive and neural

systems. Systems tend to maintain whatever state they are in. Dell, Burger,

and Svec (1997) argue that perseveration is an especially notable character-istic of unskilled behaviour. There are mechanisms at the cellular level to

prevent the immediate systematic re-occurrence of a state: after firing,

neurons have a refractory period, during which they are less responsive to

input and so are less likely to re-fire. Dell (1986) incorporated a refractory

period for elements at the cognitive level as well: a downward shift in

activation level (level of activity) for some period of time after an element is

produced, making it more difficult for the system to get into that state again.

While there are non-activation-based approaches for mechanisms to preventperseveration (Shattuck-Hufnagel, 1979), the activation-based account

presently makes clearer empirical predictions, and I will use these predictions

in the design and scope of this study. I address an alternative mechanism in

the discussion section.

Anticipation (the tendency for an element to appear earlier than intended)

also occurs. Dell et al. (1997) argue, however, that it is especially a property

of skilled behaviour. Stemberger (1989) reported that very young children are

initially more prone to perseveration errors, but that anticipation errors cometo predominate over time. He notes that anticipation errors can occur only

insofar as there is pre-planning in which the speaker readies elements in

upcoming states. If later states are not being planned until just before they

are actually uttered, there is no way to produce an element early. The extent

of pre-planning may vary; Bock (1982) argues that speakers may often begin

to utter a sentence as soon as the beginning is planned, hoping that later

parts of the sentence will be ready to produce when the speaker gets to them,

paying the price of mid-sentence disfluency if they are not ready in time. Iflater states are being planned in advance, at a time when earlier elements are

still being planned and produced, then there must be mechanisms to prevent

those later states from showing up early. However, this should be a lesser

problem than perseveration. Elements in preceding words are planned to

completion and produced before a later target word is produced, by

definition; these elements are thus present, at full activation (at the time of

production), to interfere with later elements. Elements in upcoming words

may not be planned at all, or may be only partially planned, at the time whenan earlier target word is produced; some later elements thus have no effect at

all on preceding elements, and others have a reduced effect, leading to a

smaller anticipatory effect as compared with the perseveratory effect. The

following prediction is also made: the earlier an upcoming element is

planned, the more impact it will have on the processing of earlier elements.

The appropriate mechanisms for preventing anticipation are much less clear,

and will be addressed in the discussion section. Of course, even in the most

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skilled language processing systems (such as those of adult native speakers of

a language), the mechanisms that are designed to prevent perseverations and

anticipations fail on a small percentage of tokens, leading to occasional

errors known as speech errors or slips of the tongue.Errors might also arise if the mechanisms designed to prevent anticipation

and perseveration errors are too efficient. These mechanisms are designed to

prevent the systematic incorrect recurrence (or early occurrence) of an output

state. But output states can unsystematically and correctly arise twice during a

short period of time. In a phrase such as big bill, the phonemes

/b/ and /=/ appear twice in two adjacent words, simply because they are

independent characteristics of those words. An overly efficient mechanism for

preventing perseveration or anticipation would erroneously prevent suchnonsystematic recurrence of the same output state in such a short period of

time. Shattuck-Hufnagel (1979) refers to such over-application as the trigger

effect; she does not provide quantitative evidence for such an effect, and

presents a mechanism (a pre-articulatory output monitor/editor; see below)

that makes few inherent predictions about the relative strength (or even

existence) of this effect for different types of elements. Dell (1986) provides the

refractory period as a mechanism for a perseveratory repetition effect (with an

increase in the rate of error on the second of two identical elements), butprovides no mechanism for an anticipatory repetition effect (with an increase in

the rate of error on the first of two identical elements). Even with explicit

mechanisms for both perseveratory and anticipatory repetition effects, the

uncertain nature of preplanning leads us to the prediction that the persevera-

tory effect will be greater in magnitude than the anticipatory effect.

There are only a few studies that have addressed the repetition effect

quantitatively. MacKay (1969), examining speech errors in German in which a

consonant is lost from a word, demonstrated that repetition increases errorrates, but did not address directionality. Berg (1988), examining the same type

of error in a different German error corpus, reports that the effect is primarily

perseveratory. Stemberger (1991) used the repetition effect as a tool to examine

asymmetries in substitution errors, but did not address directionality. We do

not know whether the repetition effect is in general only perseveratory for all

types of phonological errors; I address this issue in Studies 1�4.

The prediction that the repetition effect will be stronger in a perseveratory

direction is not the only prediction that derives from the fact that thepreplanning of later elements lags behind the production of earlier elements.

In some models of language production, certain types of elements are

planned more quickly than others; we would predict that faster elements

would have larger anticipatory repetition effects than slower elements. Meyer

(1991) and Levelt, Roelofs, and Meyer (1999) argue that word-initial

phonemes are planned earlier than following phonemes, in an incremental

fashion. Dell (1985, 1986) and Stemberger (1982) suggest that word-initial

PREVENTING PERSEVERATION 1433

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phonemes are accessed at higher levels of activation than phonemes later in

the word, and so would be expected to reach the threshold of activation

needed to create interference more quickly, to interfere for a longer period of

time during processing, and to show a larger anticipatory repetition effect.Dell (1985, 1986; Oppenheim & Dell, 2008), Stemberger (1982, 1990), and

Levelt et al. (1999; Roelofs, 1999) also argue that there are two levels for the

content of phonological representations: a segmental level, in which

segments (phonemes such as /b/) are identified as to type without internal

structure, and a feature level, in which the features or gestures of the

phoneme’s pronunciation are represented as separate elements (e.g., [Labial],

[�nasal], [�consonantal], [�sonorant], [�continuant], etc.). They argue

that segments are accessed earlier in time than their subsegmental features(and that phenomena arising at this early level show effects of whole-segment

identity and not of feature-based similarity). This also entails that whole

segments interfere with the processing of sounds in other words for a longer

period of time than do subsegmental features, and that whole segments

should show a larger anticipatory repetition effect than subsegmental

features. I address these issues in Study 2.

This last point leads us to a more general issue: the nature of the elements

in phonological processing. While Dell (1986), Stemberger (1990), and Leveltet al. (1999) all predict repetition effects of whole segments (e.g., that the two

tokens of /b/ in big bill will interfere with each other) and of subsegmental

features (e.g., the two tokens of [Labial] in those two tokens of /b/), other

theories do not. Browman and Goldstein (1986), Studdert-Kennedy and

Goodell (1995), and Pouplier (2007) posit only the featural level and assume

that words are not organised into segments. In theories without segments, all

effects must be at the level of individual subsegmental features; effects on

repeated segments such as /b/ must arise via the independent effects onrepeated features such as [Labial], [�continuant], [�voiced], etc. In

principle, quantitative effects of repetition involving whole segments might

not be derivable from effects on individual features, thereby providing

evidence for a segmental level of representation during processing. Shattuck-

Hufnagel and Klatt (1979) argued for a model in which segments are

elements of processing, but features are not; features define similarity across

segments, and similarity leads to greater error rates, but features themselves

are not independent elements that can be involved in errors. This leads to thepossibility that the repetition of features per se will have no affect on error

rates; in addition to an increased error rate on the second of two tokens of

e.g., /b/, however, there might also be a greater error rate when /b/ follows a

very similar phoneme (such as /p/, sharing all features except laryngeal

features) than when it follows a dissimilar phoneme such as [l]. If we find no

general effect of phoneme similarity, but do find an effect for repeated

subsegmental features such as [Labial], that will suggest that elements at

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both levels are independently present during processing. In addition to

segments and subsegmental features, there are theories that posit the

existence of larger elements, at a level above the segment. Some researchers

(e.g., Kupin, 1982; Menyuk, 1972; Moskowitz, 1970) have suggested(especially for young children) that consonant clusters (e.g., /pl/, /tr/, /kw/)

may be analysed and stored as single unanalysed chunks. In that case, errors

on a target such as /pl/ would be increased by a preceding /pl/ cluster to the

same extent that there is an effect of repeating whole phonemes, and would

be increased by a preceding /bl/ only to the extent that there is an effect of

repeating subsegmental features. Similarly, we would expect that /p/ not only

increases the rate of substitution errors (such as [k]) on a following /p/, but

should also increase the rate of addition errors (where /p/ is pronounced ase.g., /pr/). I address these issues in Study 3.

If the mechanism underlying repetition effects is based on changes in

activation levels (e.g., a refractory period), then more subtle effects may also

be predicted. There should in principle be effects of the repetition of

nontarget (source) segments. In a sequence such as /p...b...p/, does the

repetition of the /p/ affect the likelihood of an error on which the /p/ replaces

the /b/ (yielding the error /p...p...p/), since the two tokens of /p/ in the

surrounding context should lead to changes in their activation levels? In asequence of three identical phonemes such as /b...b...b/, should the presence

of the first /b/ affect the activation level of the second /b/ in a way that

impacts on the error rate on the third /b/? And should the third /b/ affect the

activation level of the second /b/ in a way that impacts on the error rate on

the first /b/? I address these issues in Study 5.

Lastly, there is a developmental issue: are perseveratory and anticipatory

effects of repetition a characteristic of just skilled adult processing, or are

they already present during childhood? If they are present in the speech ofchildren, do we find the same patterns of presence vs. absence, and of relative

effect size, as we do in adult speech? This will be addressed in Study 4.

This paper examines these issues looking at errors that arise sponta-

neously in natural speech. I begin with a study of substitution errors in adult

speech, examining the effects of directionality (perseveratory vs. anticipa-

tory) and position in the word or syllable (word-initial onset vs. stressed

vowel vs. word-final coda).

STUDY 1. REPETITION OF WHOLE PHONEMES IN ADULTLANGUAGE PRODUCTION: SUBSTITUTION ERRORS

Method

The speech error corpus. The data in this section come from a corpus of

7500 errors that I collected from natural speech over a 7-year period. Speech


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was carefully monitored for errors; when detected, they were recorded in a

notebook, along with the date and speaker’s identity. Errors were collected

only when enough attention could be spared for the task and after a

conscious decision had been made to monitor for and collect errors; this

served as a partial control for biases due to differences in salience between

different types of errors. Many of the patterns observable in this naturalistic

corpus have been replicated under experimental conditions (e.g., Stemberger,

1992). All of the data were collected before I had any interest in the repetition

effect. I examined all errors in which a singleton phoneme (i.e., not one that

appeared in a cluster) was mispronounced as a phoneme that appeared in a

nearby word. Exchanges and other complex errors in which there were errors

on more than one word were entered twice, one for each phoneme that was

mispronounced. The preceding and following context was examined to see

whether another token of the target phoneme appeared (with correct

pronunciation) before the target phoneme, after the target phoneme, or

both. I then calculated the observed proportion of errors on each phoneme

(and, summing across phonemes, on each particular subset of phonemes),

distinguishing between syllable-initial onset consonants, syllable-final coda

consonants, and stressed vowels (plus syllabic consonants). Because most

speech errors involve a target and source phoneme from the same position in

the syllable (e.g., Dell, 1986; Fromkin, 1971; Stemberger, 1989; Vousden,

Brown, & Harley, 2000), a phoneme was counted as ‘repeated’ only if it was

in the same position in the syllable as the target, i.e., onset-onset, coda-coda,

and vowel-vowel. Here are some examples with repeated vs. nonrepeated

whole-segment onset consonants (target and repeated consonants in bold

font, source and error consonants both in bold and in italics):

Repeated phoneme:

. . .in Bulgaria, so doath � both countries claim this dance.

No repeated phoneme, no consonant similar to target (/b/):

Looks like the same parn � barn we passed up there.

Operationalising repetition. In all probability, there is some time windowfor repetition, beyond which it has little effect on phonological processing;

while a phoneme that occurred 30 seconds earlier is unlikely to be relevant,

the exact window is unclear. Because in the majority of phonological errors

the source appears within four words of the target word on which the error

occurs (Nooteboom, 1969), I have operationally adopted a window of four

words before and after the error, including all closed-class lexical items. This

operational decision will allow us to see what the basic effects are. A more

detailed determination of the appropriate window, whether the effects

include closed-class lexical items, and whether the effects are on a gradient

as a function of distance from the target word, must be left for the future. All

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other phonemes within the target word were excluded from the analysis; this

paper is limited to the effects of repetition of phonemes across different

words, and ignores repetition within the same word.

Estimating chance. Some level of repetition of phonemes occurs by

chance. Evidence for a repetition effect comes from above-chance repetition

of phonemes in association with errors. Dell and Reich (1981) proposed

using the error corpus itself to estimate chance, and this has been followed by

e.g., Stemberger (1990) and many others. For each type of error (e.g.,

substitution errors on onset consonants), the sentences containing the

phonological error were examined. All phonemes in the same position in

the syllable (e.g., onsets) within the operationally defined window (fourwords before and after the target word in which the error occurred, excluding

the source word) were identified and summed into the categories before, after,

and both. If a phoneme occurred twice before or after the target word, it was

counted only once. This yields an estimate of chance: e.g., for an error not

involving /b/ as a source, the target word was preceded by /b/ in a particular

proportion of errors; if there is no effect of repetition, that same proportion

of errors on target /b/ should also be preceded by another token of /b/. By

computing chance before vs. after as separate measures, we can assesswhether there are differences between perseveratory vs. anticipatory effects.

By computing chance separately for surrounding repetition (e.g., /b. . .b. . .b/,

with the error occurring on the second token of /b/), we can assess whether

this greater level of repetition shows different effects. Given observed values

and an estimate of chance, chi-squared tests can be used to assess the

statistical significance of any observed numerical differences.

Comparing across positions. It would be useful to compare the strengthof the repetition effect across syllable positions, e.g., in onsets vs. in codas vs.

in vowels. However, such comparisons are difficult because chance levels of

repetition differ greatly across positions. If e.g., chance were 10% in onsets

and 20% in codas, and observed values were 15% in onsets and 25% in codas,

would this be a comparable effect (5% of tokens above chance for both

onsets and codas) or not (errors at 1.5 times chance in onsets vs. at 1.25 times

chance in codas)? Stemberger (2007) proposes a measure, Dell’s a/b ratio

(originally suggested by Gary Dell, personal communication), that allows forthe estimate of effect size independent of the level of chance, with a

procedure for testing whether a given effect size is significantly different,

at different levels of chance for a specific number of observed errors.

(1) Suppose that chance is the same for error types N and M. If we observed

400 errors of type N and 100 errors of type M, we can say that error type

N occurs at four times the rate of error type M (ratio 400/100�4.00).

(2) Suppose that chance is twice as great for error type N than for error type


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M (because, for example, the targets on which error type N occurs are twice

as frequent as the targets on which error type M occurs). If the error rates are

the same on N and M, then, in a given sample of speech, we expect to

observe errors in the ratio 2/1 (2.00), reflecting the fact that there are twice asmany opportunities for N to occur. If we observed 400 errors of type N and

100 errors of type M (a ratio of 4/1), and we expect by chance a ratio of 2/1,

then error type N occurs at a rate twice that of error type M (observed ratio

4/1, divided by predicted ratio 2/1). This adjustment of the ratio of the

observed number of errors (N/M) to take into account the predicted ratio

that would be observed by chance (p/q, with p being chance for error type N

and q being chance for error type M) is Dell’s a/b ratio:

a=b�(N=M)=(p=q)

This ratio provides a measure of how frequent one type of error is with

respect to another: the error rate for error type N may be equal to (a/b�1.00), less than (a/bB1.00), or greater than (a/b�1.00) the error rate for

error type M.

For a more practical example, consider the first line of Table 1 below. For

the 507 observed errors on onset consonants (column labelled ‘# targets’),

there were 79 errors in which the target phoneme was preceded by anothertoken of the same phoneme (15.6% of tokens, column labelled ‘observed

repetition’), and 428 errors in which there was no phoneme repetition

(number of targets minus observed repetition, not explicitly given in Table 1);

the ratio of N to M is 79/428 (�.185). Calculations of chance find that

repetition is expected on 9.1% of tokens (column labelled ‘chance repeti-

tion’), with no repetition on 90.9% of tokens (not explicitly given in Table 1);

the ratio of p to q is .091/.909 (�.100). Dividing N/M (.185) by p/q (.100)

yields a/b�1.85 (column labelled ‘a/b ratio’). The error rate on onsetconsonants when the target phoneme is preceded by another token of the

same phoneme is close to twice the error rate when the target phoneme has

not been repeated. In all the tables below, each line contains information

about the number of observed errors with vs. without repetition (‘observed

repetition’ column) plus the corresponding level of chance (‘chance repeti-

tion’ column); consequently, an a/b ratio for the effect of repetition can be

calculated for every line in every table, giving an effect size for repetition that

can (at least in principle) be compared to the effect size for every other line inall the tables.

While the a/b ratio provides a measure of effect size and allows to us to

compare the magnitude of the effect of repetition in onsets vs. vowels vs.

codas, a further step is needed before we can evaluate such comparisons

statistically. We need some way to compare the effect size in, for example,

onsets vs. codas despite the substantial difference in levels of chance

repetition (which derive from differences in phoneme frequencies in onsets

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vs. codas). Stemberger (2007) notes that the a/b ratio formula above has an

interesting consequence: given a particular (fixed) value of a/b, any change in

chance (p/q) automatically derives the observed values (N/M) that would be

necessary in order to have the same effect size. For example, in Table 1 line 1,

the effect size (a/b�1.85) derives from the N/M ratio (.185) divided by the p/

q ratio (.100). If chance had been the same as for codas in line 4 (15.5% of

tokens, p/q�.155/.845�.183), in order to have a/b�1.85, the N/M ratio

would have to be .338; and since N�M�507, that entails N�128 and

M�379 (rounding to the nearest whole numbers). We now have the adjusted

values of the ‘observed’ number of onset errors with vs. without repetition

which would give the same effect size if chance in onsets had been the same

as chance in codas. Since chance has now been equalised, we can use chi-

square to directly compare these two numbers in onsets (128 vs. 379) with the

observed numbers in codas (44 vs. 116): x2(1)�0.27, ns. Similarly, given the

observed number of coda errors (n�160) and a/b�2.07 in Table 1 line 4, we

can calculate the values of N and M that would yield this effect size if chance

had been the same as in onsets in line 1 (N�27, M�133) and compare them

to the observed numbers for onsets (79 vs. 428): x2(1)�0.15, ns. Stemberger

(2007) proposes that, if both chi-square tests come out significant, it is

reasonable to conclude that the observed difference in a/b ratio is significant.

If neither test is significant, it is reasonable to conclude that the observed

difference is either unreliable or too low in magnitude to be detected at this

sample size. If one test is significant and the other is not, we can take it as a

marginal result that requires further investigation.

One caveat is in order relative to the a/b ratio. Because it is based on

ratios, it is overly sensitive when sample size is small. With chance at .100,

and with the observed number of errors being one more than expected by

chance (an obviously meaningless difference), the a/b ratio ranges from 2.25

TABLE 1Substitution errors on singleton consonants and vowels

Location # Targets Context

Chance

repetition

Observed

repetition a/b ratio x2(1) pB

Onset 507 Before .091 .156 1.85 22.77 .001

After .071 .107 1.56 9.56 .002

Both .012 .032 2.68 17.37 .001

Coda 160 Before .155 .275 2.07 17.62 .001

After .102 .094 0.91 0.11 ns

Both .022 .044 2.03 3.64 .10

Vowel 243 Before .153 .280 2.15 30.11 .001

After .101 .095 0.93 0.10 ns

Both .027 .049 1.87 4.57 .05


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(N�M�10) to 1.23 (N�M�50) to 1.11 (N�M�100) to 1.02 (N�M�500). Similarly, very low chance values can lead to large a/b ratios even with

large sample sizes; with 500 observed errors and chance repetition at 0.2% of

tokens, 2 observed errors with repetition would yield an a/b ratio of 2.00. The

a/b ratio is skewed upwards when numbers get small, which will, of course,

be reflected in lack of statistical significance.

Results

Results for substitution errors in onsets, codas, and nuclei are presented in

Table 1.

Substitution errors on onset consonants show a significant effect of

repetition in both preceding and following contexts. The a/b ratio is slightly

lower when the repeated consonant follows than when it precedes, and the

greatest effect is when there is a repeated consonant in both the preceding

and following contexts. Substitution errors on coda consonants show a

significant effect of repetition in the preceding context and (marginally) in

the surrounding context, with no effect of repetition in the following context;

effects are perseveratory only. The a/b ratios are comparable when the

repetition is only before the target or both precedes and follows the target.

Substitution errors on vowels show a significant effect of repetition in the

preceding context and in the surrounding context, with no effect of repetition

in the following context; effects are perseveratory only. The a/b ratios are

slightly greater when the repetition is only before the target or both precedes

and follows the target.

Because chance is about the same for codas and vowels, we can directly

compare the size of the repetition effect in the three contexts, using chi-

square on the raw numbers of errors with vs. without repetition. There are no

differences in any of the three contexts: repetition before, x2(1)�0.01, ns;

after, x2(1)�0.01, ns; or both, x2(1)�0.07, ns. The effects are of exactly the

same magnitude for errors on coda consonants and vowels.

Because chance is lower for errors on onset consonants, we cannot

compare the raw numbers, but can adjust the observed numbers to find the

number of repeated vs. non-repeated errors that would lead to the same effect

size at a different level of chance. The size of the repetition effect does not

differ for onsets vs. codas for the preceding context: adjusting chance for

onsets, x2(1)�0.27, ns; adjusting chance for codas, x2(1)�0.15, ns; or when

the repeated phoneme both precedes and follows: adjusting chance for

onsets, x2(1)�0.43, ns; adjusting chance for codas, x2(1)�0.18, ns; but the

repetition effect is marginally greater on onsets than on codas when the

repeated phoneme follows the target: adjusting chance for onsets, x2(1)�3.26, p�.071; adjusting chance for codas, x2(1)�2.72, p�.099. The size of

the repetition effect does not differ for onsets vs. vowels for the preceding

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context: adjusting chance for onsets, x2(1)�0.74, ns; adjusting chance for

vowels, x2(1)�0.54, ns; or when the repeated phoneme both precedes and

follows: adjusting chance for onsets, x2(1)�1.08, ns; adjusting chance for

vowels, x2(1)�0.73, ns; but the repetition effect is marginally greater on

onsets than on vowels when the repeated phoneme follows the target:

adjusting chance for onsets, x2(1)�4.38, pB.05; adjusting chance for

vowels, x2(1)�1.99, ns.

The effect sizes can be compared for preceding vs. following vs. both

contexts for onsets, codas, and vowels, using the method of adjusting

observed values for chance. For onsets, the difference between before vs. after

contexts is not significant: adjusting chance for before, x2(1)�0.78, ns;

adjusting chance for after, x2(1)�0.79, ns; but there is some indication that

the large repetition effect when the repeated phoneme both precedes and

follows the target is greater than for only before: adjusting chance for before,

x2(1)�0.95, ns; adjusting chance for both, x2(1)�5.16, pB.025, or only

after: adjusting chance for after, x2(1)�2.01, ns; adjusting chance for after,

x2(1)�8.49, pB.005. For codas, the difference between before vs. after

contexts is significant: adjusting chance for before, x2(1)�5.82, pB.025;

adjusting chance for after, x2(1)�8.33, pB.005, as is the difference between

the after vs. both contexts: adjusting chance for after, x2(1)�27.16, pB.001;

adjusting chance for both, x2(1)�28.10, pB.001; but the difference between

the before vs. both contexts is not significant: adjusting chance for before,

x2(1)�0.01, ns; adjusting chance for both, x2(1)�0.02, ns. For vowels, the

difference between before vs. after contexts is significant: adjusting chance

for before, x2(1)�9.61, pB.002; adjusting chance for after, x2(1)�13.42,

pB.001; the difference between the after vs. both contexts is marginally

significant: when adjusting chance for after, x2(1)�2.08, ns; adjusting

chance for both, x2(1)�6.41, pB.025, but the difference between the before

vs. both contexts is not significant: adjusting chance for before, x2(1)�0.16,

ns; adjusting chance for both, x2(1)�0.52, ns.

Discussion

There are strong effects of the exact repetition of a phoneme. For errors on

onsets, codas, and vowels, a preceding identical consonant leads to a

doubling of the error rate; the effect is the same size for all three parts of

the syllable. A following identical phoneme also raises the error rate on onset

consonants, but there is no effect on coda consonants or on vowels; this

anticipatory effect in onsets is of a smaller magnitude than the perseveratory

effect, though that difference does not reach significance. When there is both

a preceding and a following identical phoneme, the effect size is the same as

the effect of a preceding identical phoneme for coda consonants and vowels;

because the following consonant has no effect, all observed effects originate


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from the preceding consonant, and we would expect the effect to be of the

same size. When there is both a preceding and a following consonant, the

effect is larger on onset consonants than when there is only a preceding or

following consonant; this is expected, because this environment combines theperseveratory effects of the before-only context and the anticipatory effects

of the after-only context, and the double effect leads to an elevated error rate

(especially when compared with the smaller anticipatory effect in the after-

only context).

The finding that the perseveratory effect is of about the same magnitude for

onset consonants, coda consonants, and vowels is quite interesting. The

observed error rates on phonemes in these different positions in the syllable

are quite different (as is evident in the numbers of targets given in column 2 ofTable 1; see also for naturalistic errors, Cutler, 1981; for errors under

experimental conditions, Shattuck-Hufnagel, 1983, 1987; but cf. Laubstein,

1998, Laubstein & Smyth, 2006). The processing penalty from a preceding

token of the target phoneme, however, is the same. This suggests that an anti-

perseveration mechanism is engaged after the production of a phoneme, and

that it is a general mechanism for all phonemes, regardless of position in the

word or syllable. This also suggests that the strength of the anti-perseveratory

mechanism is not directly tied to how frequent different types of errors are.The presence of an anti-anticipation mechanism only for onset con-

sonants is quite interesting. Various researchers have proposed that there is

at least partially an incremental access to phonemes in the word, with earlier

access to word-initial phonemes than to vowels and word-final phonemes

(e.g., Meyer, 1991). Anti-anticipatory mechanisms can only be engaged when

a phoneme has been processed to a level where its content is recognised by

the system. If word-initial phonemes are planned earlier, that means that the

information is available earlier, and anti-anticipatory mechanisms can beengaged earlier. The existence of an anticipatory repetition effect thus

reinforces the conclusion that processing is earlier/faster on word-initial

phonemes than on phonemes later in the word.

The finding that anti-repetition effects are largely perseveratory appears at

first to conflict with another fact: most models of language production that

address speech errors assume that the most basic type of phonological error

is an anticipation. Nooteboom (1969) observed that the majority of

phonological errors are anticipatory. Shattuck-Hufnagel (1979) noted thatmany of these errors are incomplete, where the speaker stops and repairs the

sentence between the error and the source, so that we cannot be sure whether

the source would have been produced correctly (an anticipation) or

incorrectly (an exchange). To some extent (but see below), whether the

result is an anticipation or an exchange is a moot point; in Shattuck-

Hufnagel’s model, the basic error in the sequence A. . .B is anticipation of the

second phoneme B to displace the first phoneme A, with the subsequent

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perseveration of A into the vacated position for B a consequence of the

original anticipation. Stemberger (1989) argues on the basis of develop-

mental evidence that anticipations are more common than exchanges in

adult speech. But everyone seems to agree that anticipation is a morecommon error than (pure) perseveration. And yet, anti-repetition mechan-

isms are largely perseveratory in nature. If anti-repetition mechanisms were

tailored to look for the sorts of errors that are likely in adult speech, and

most errors in adult speech are anticipatory, then the mechanisms should

weaken the first token of an A. . .A sequence more (treating it as a more-

common anticipation error), not the second token (treating it as a less-

common perseveration error). However, there is evidence that between-word

phonological errors in early child language tend to be perseveratory(Stemberger, 1989; but see Jaeger, 2004). In early child language, it would

make sense for the anti-repetition mechanism to be largely perseveratory, to

correct the dominant type of error pattern. I address whether this mechanism

is in place in early child language in Study 4. We are left with two

possibilities: (1) the largely perseveratory nature of the anti-repetition

mechanism is a legacy from childhood, or (2) the anti-repetition mechanism

is not designed with the details of different errors in mind, and statistical

properties derive from independent effects (such as a low degree ofpreplanning of upcoming elements). As we will see below, this second

possibility fits better with activation-based accounts of the repetition effect.

Up to this point, I have been assuming that the unit relevant to the

repetition effect is the phoneme or segment. Phonemes are made up of

subsegmental elements that govern their articulation (at least for language

production), and it is possible that repetition at the level of subsegmental

elements might have an effect. For example, the element [Labial] (constric-

tion of the lips) in /p/ could affect the processing of the later token of [Labial]in a following /b/. Indeed, it is possible that the effects observed in Study 1

might arise entirely from subsegmental effects. When /p/ precedes /p/, all

subsegmental elements are subject to repetition effects; when /p/ precedes /b/,

laryngeal features are not repeated, but place and manner features are; when

/p/ precedes /n/, few features are repeated. It is possible that each repetition

of a feature increases the error rate on the target phoneme, with the highest

predicted error rate on exact repetition of the phoneme. Alternatively, it is

possible that only repetition of whole phonemes has an effect, and thatrepetition of subsegmental elements has no effect. Shattuck-Hufnagel and

Klatt (1979) argue that speech errors occur in a processing stage in which

features are not independent units of processing, and Stemberger (1990),

Roelofs (1999), and Oppenheim and Dell (2008) have argued empirically that

information about which phoneme is involved is available earlier in

processing than information about the subsegmental elements. I suggested

above that there is no anti-anticipatory repetition effect for vowels and coda


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consonants because they are processed more slowly and information about

them becomes available later than information about onset consonants. If so,

we might expect that there would be no anti-anticipatory effects for

subsegmental elements, even for onset consonants.The mechanism underlying the repetition effect may also be clarified by

investigating the possible role of similarity (distinct from repeated subseg-

mental elements). In an interactive activation model (e.g., Dell, 1986;

Stemberger, 1985), there is a level where phonemes compete with each other

as unanalysed units. Shattuck-Hufnagel and Klatt (1979) assume that

subsegmental features are not independent elements, but do affect the

degree of similarity of segments, and the degree of similarity between the

target and source phonemes impacts on error rates. Dell (1985, 1986) arguesthat most phonological speech errors arise at the segment level, but that

feedback from features leads to the result that similar phonemes are more

likely to outcompete the target phoneme. By extension, perhaps similar

phonemes are more likely to inhibit the target phoneme, even if the error that

actually occurs does not involve that similar phoneme. Perhaps /p/, because

of a high degree of similarity to /b/, weakens a following /b/ in general and

leads to a high error rate (including where /b/ is replaced by e.g., /v/); but /s/

(differing by place, manner, and voicing) is far less similar to /b/ and so haslittle impact on the error rate of a following /b/. A repetition effect of the

subsegmental element [Labial] should affect only errors where /b/ loses its

labiality (e.g., where it becomes dorsal /k, g/ or coronal /t, d, s, z, l/); but with

a general similarity effect, /p/ would also lead to an increased rate of errors

where /b/ becomes labial /f, v, m/. Thus, the presence of an anti-repetition

similarity effect would imply a different sort of mechanism than the presence

of an anti-repetition effect involving independent subsegmental elements.

Study 2 tests for both of these effects.

STUDY 2. SIMILARITY AND REPETITION OF SUBSEGMENTALELEMENTS IN ADULT ERRORS

Method

The speech error corpus. The corpus, and the methods used to determine

whether a target phoneme was repeated in the preceding and/or followingcontext, was the same as in Study 1. The substitution errors in Study 1 were

examined for repetition effects of subsegmental elements and of similar

phonemes. I examine a subset of subsegmental elements. In many instances,

it is difficult to say exactly what the psychologically relevant features are for

certain contrasts. For example, the phonemes /w/ and /j/ are clearly glides, /l/

is clearly not a glide at least in onset position (where it has full closure at the

alveolar place of articulation), and /r/ is probably a glide but is usually

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classed with /l/ as a ‘true consonant’; in some systems, all four phonemes are

classed together as approximants. Because it is unclear whether the four

phonemes should be treated as all being of the same manner of articulation,

I find it best to simply not analyse them. For place of articulation, I restrict

the analysis to shared [Labial] in true consonants (/p, b, f, v, m/), shared

[Coronal,�anterior] (alveolar) (/t, d, s, z, n/), without the additional [Dorsal]

articulation of /l/ or dental component of /u, ð/), and shared [Dorsal, �back]

in velars (/k, g, E/). For manner of articulation, I restrict the analysis to

nasals (/m, n, E/), fricatives (/f, v, u, s, z, s/, eliminating /ð/ because it occurs

mostly in close-class morphology and is involved in speech errors only at low

rates, and /z/ because it did not appear in any words in this study), and oral

stops (/p, b, t, d, k, g/, eliminating the affricates /c, /j:/). For voicing, I restrict

the analysis to voiceless phonemes (/p, t, k, f, u, s, s, c, h/) and voiced

obstruents (/b, d, g, v, ð, z, /j:/, eliminating voicing in sonorant consonants).

For vowels, I restrict the analysis to vowel height in monophthongs ([�high]

in /i:, =, u:, I/, [�low] in /æ, "/, and [�high,�low] in /o, L/), vowel backness

in monophthongs ([�back] in /i:, =, o, æ/ and [�back] in /u:, I, L, "/), and

number of moras (one mora (short) in /=, o, I, L/ and two moras (long or

diphthongal) in /i:, e=, u:, oI, a=, aI, o=/, eliminating the low vowels /æ, "/ and

syllabic /r/, which are the longest vowels in English but often treated by

phonologists as having only one mora). Subsegmental element analyses are

restricted to errors in which the target subsegmental element is not present in

the error. For similarity, all phonemes were classified as matching along 1 vs.

2 vs. 3 phonetic dimensions (place, manner, and articulation for consonants;

height, backness, and number of moras for vowels); while a more fine-

grained analysis would be interesting, it would also lead to far too many cells

with low predicted values. Similarity analyses for consonants include all

errors. Similarity analyses for vowels are restricted to those errors in which

the target and sources are non-low monopthongs (/i:, =, o, u:, I, L/);

diphthongs are excluded because by definition they mix height and/or

backness (with e.g., /a=/ mixing low-back with high-front); /æ, ", r/ are

excluded because the number of moras is unclear. Here is an example with

repeated features in onset consonants (target and repeated consonants in

bold font, source and error consonants both in bold and in italics):

Repeated feature, [Labial] in /m/, /b/: I was undoing my like � bike lock.

Results

Results for errors in onsets, codas, and nuclei are presented in Tables 2�7.

For onset consonants, repetition of place features yields a moderate

perseveratory effect that reaches significance, but no anticipatory effect.

Repetition of manner and voicing features leads to a nonsignificant


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perseveratory effect, with no anticipatory effect. For coda consonants, there

are no effects with place features and no perseveratory effect with manner

features, but there are modest elevations for voicing and for manner

TABLE 2Subsegmental feature repetition in onset errors (n�507)

Context Dimension # Targets

Chance

repetition

Observed


Before Place 164 .211 .293 1.55 5.54 .02

Manner 157 .261 .318 1.32 2.14 ns

Voicing 44 .320 .386 1.34 0.61 ns

After Place 164 .179 .177 0.99 0.00 ns

Manner 157 .192 .172 0.87 0.36 ns

Voicing 44 .226 .250 1.11 0.12 ns

TABLE 3Subsegmental feature repetition in coda errors (n�160)


Chance

repetition

Observed


Before Place 69 .328 .348 1.07 0.10 ns

Manner 66 .345 .348 1.03 0.00 ns

Voicing 18 .334 .445 1.60 0.99 ns

After Place 69 .273 .290 1.07 0.09 ns

Manner 66 .204 .288 1.58 2.85 .10

Voicing 18 .232 .389 2.10 2.65 ns

TABLE 4Subsegmental feature repetition in vowel errors (n�49)


Chance

repetition

Observed


Before Height 72 .346 .389 1.21 0.60 ns

Backness 27 .576 .593 1.07 0.03 ns

# moras 45 .518 .533 1.06 0.04 ns

After Height 72 .238 .264 1.15 0.26 ns

Backness 27 .441 .444 1.01 0.00 ns

# moras 45 .406 .422 1.07 0.05 ns

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(anticipatory only), none of which reach significance. For vowels, there are

no indications of any effect of the repetition of subsegmental elements.

There are no significant effects of the general similarity of preceding or

following consonants or vowels.

TABLE 5Similarity of preceding or following consonants, onset errors (n�507)

Context # Dimensions

Chance

repetition

Observed


Before 1 .346 .323 0.90 1.10 ns

2 .598 .592 0.97 0.09 ns

3 .444 .450 1.02 0.07 ns

After 1 .277 .276 1.00 0.00 ns

2 .459 .450 0.96 0.17 ns

3 .325 .290 0.85 2.83 .10

TABLE 6Similarity of preceding or following consonants, coda errors (n�160)


Chance

repetition

Observed


Before 1 .377 .400 1.10 0.37 ns

2 .377 .394 1.07 0.19 ns

3 .334 .338 1.01 0.01 ns

After 1 .302 .319 1.08 0.99 ns

2 .370 .381 1.05 0.09 ns

3 .276 .288 1.06 0.12 .10

TABLE 7Similarity of preceding or following vowels (n�49)


Chance

repetition

Observed


Before 1 .484 .469 0.94 0.04 ns

2 .388 .327 0.76 0.78 ns

3 .115 .143 1.28 0.38 ns

After 1 .446 .388 0.79 0.67 ns

2 .277 .224 0.75 2.06 ns

3 .076 .082 1.08 0.02 ns


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Discussion

There is evidence for a repetition effect on consonant place features, but only

for onset consonants, and only a perseveratory effect. There appears to be

some perseveratory effect of manner and voicing features for onset

consonants as well, but smaller in magnitude and not reaching significance

with this sample size. There is no hint of an effect for vowel features. Nothing

reaches significance for coda consonants, though three of the six cells reach a

reasonable a/b ratio (but with low predicted values, a circumstance where

small differences between predicted and observed values can lead to large a/b

ratios). Segmental similarity per se has no effect.

The clear lack of an effect of repetition of subsegmental elements for

vowels and coda consonants, and for repetition following the target onset

consonant, is theoretically very important. If the repetition effect is based

entirely on features, then there must be an effect of feature repetition, though

the effect should be largest for whole-segment repetition (because it sums the

effects of repetition of all individual features). The results show that there is

an effect of the repetition of whole segments in the absence of an effect of the

repetition of subsegmental features, for vowels and coda consonants, and for

repetition in the following context for onset consonants. This is impossible in

a model in which only subsegmental elements are represented, without whole

segments being represented in any way (e.g., Browman & Goldstein, 1986;

Pouplier, 2007; Studdert-Kennedy & Goodell, 1995). This reinforces the

similar demonstrations of Stemberger (1990), Roelofs (1999), and Oppen-

heim and Dell (2008) that some phenomena are sensitive to segmental

identity but not to feature similarity. The fact that there is some effect of

repeated features suggests difficulties for whole-segment-only theories, such

as Shattuck-Hufnagel and Klatt (1979). It appears that both segments and

features are represented in phonological representations, but that informa-

tion about whole segments becomes available earlier in processing than

information about subsegmental features (as in the models of Dell, 1986;

MacKay, 1987; Stemberger, 1985).

There is no effect of whole-segment similarity; an identical token of /p/

has an effect on the processing of a later token of /p/, but a highly similar

token of /b/ has no more effect on /p/ than a dissimilar token of /z/ or /j/.

That is to say, an identical token of the target phoneme leads to an increased

rate of error on the target phoneme, where any feature may change. A similar

phoneme does not raise the rate of error where just any feature may change.

Any effect of similar-but-nonidentical segments is feature-based: a feature

such as [Labial] raises the rate of errors where [Labial] is absent in the output

(e.g., replaced by [Coronal] or [Dorsal]). The mechanism behind the

repetition effect is not based on competition between holistic segments

which interfere with each other as a function of similarity, but rather on the

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exact repetition of elements in representations (especially whole segments,

but features to some extent): after an element is produced, there is a period

during which it is more difficult to produce that exact element again.

The limited nature of the repetition effect for subsegmental elements iscompatible with the time-course explanation given above, but some details

are unclear. Given that access to feature information lags behind access to

whole-segment information, the processing of earlier segments is too far

advanced by the time that later feature information becomes available, so

that no anti-anticipatory mechanism is engaged; hence, feature repetition has

only perseveratory effects. But why is there no anti-perseveratory effect for

subsegmental features in coda consonants and vowels? Given that features

were produced, the information should be as active for coda consonants andvowels as for onset consonants, yet increases error rates only on onsets. One

difference between onsets and vowels/codas is that the rate of chance

repetition is substantially lower for onsets. I return to this issue below, when

I discuss the effectiveness of an anti-perseveratory mechanism in relation to

the probability of chance repetition of elements.

Having established that there are both whole-segment and (at least

perseveratory) subsegmental effects on word-initial consonants, we can turn

to the issue of repetition effects at higher levels. Is it possible that such effectsarise not because whole segments are repeated, but because whole onsets are

repeated? It has been argued that consonant clusters may in fact be treated as

single phonemes: for children (Menyuk, 1972; Moskowitz, 1970) and for

adults (Kupin, 1982). If so, then /p/ should raise the rate of addition errors

on a following /p/, so that it becomes e.g., /pl/. Since Berg (1988) has shown

that certain types of consonant loss errors in German show a perseveratory

repetition effect only, we would expect that this would be true of our English

errors as well.

STUDY 3. REPETITION OF WHOLE PHONEMES AND/ORONSETS IN ADULT LANGUAGE PRODUCTION:

LOSS AND ADDITION ERRORS

Method

The speech error corpus. The corpus, and the methods used to determinewhether a target phoneme was repeated in the preceding and/or following

context, was the same as in Study 1.

For loss errors, I limit the focus to consonant clusters ending in /r/, /l/, or

/w/; it has often been argued that in clusters involving /s/�stops (/sp, st, sk/)

and consonant�/j/ (e.g., /fj/ in few), the /s/ and /j/ are not part of the onsets

(e.g., Hammond, 1999). Although Stemberger and Treiman (1986) and

Stemberger (1990) argue that speech errors show mostly parallel behaviour


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for all sorts of word-initial consonant clusters, it is more conservative to

eliminate them from this study. I distinguish three levels of repetition: where

the exact cluster is repeated (e.g., /pl. . .pl/), where there is a consonant

cluster with a different first consonant but the same final consonant which

deletes in the target (e.g., /bl. . .pl/), and where the final consonant that

deletes is repeated as a singleton onset in the context (e.g., /l. . .pl/); these

three levels allow us to see possible effects of exact repetition of an onset vs.

repetition of just the phoneme that deletes in the same position in the word

(the final member of a consonant cluster) vs. in a different position in the

word (word-initial). For addition errors, I limit the focus to singleton target

consonants, and on whether another singleton token of the same phoneme

(an identical onset) also raises the rate of addition errors (resulting in a non-

identical onset).

Estimating chance. The number of consonant loss errors in word-initial

position is too small to estimate chance in the same fashion as in Study 1

(making use of occurrence of phonemes before vs. after the target phoneme

in the same errors). Instead, the 507 errors involving substitution errors on

word-initial onset consonants were used to estimate how often e.g., /br/ in

general precedes or follows target phonemes that undergo an error. The

number of consonant addition errors (n�250) is large enough to use as a

base for estimating the chance repetition of target singleton phonemes (but it

should be noted that the results are almost identical using chance estimates

from onset consonants in Study 1). Here are some examples of loss and

addition errors with repeated onset consonants (target and repeated

consonants in bold font, source and error consonants both in bold and in

italics, and target either in a self-correction or following in quotation marks):

Loss of /l/ from /pl/, with preceding identical cluster /pl/:

We can plant a pant in it. ‘plant’

Loss of /r/ from /dr/, with preceding different cluster containing /r/ (/br/:

part of the brain dain in other countries is.... ‘drain’

Loss of /r/ from /pr/, with preceding singleton /r/:

This is a really pity place. ‘pretty’

Loss of /w/ from /kw/, no other /w/ in context:

You guys kitt � quit creeping up my arm

Addition of /r/ to /g/ to create /gr/, with preceding identical onset /g/,

from following /br/ cluster

You can’t build the Golden Grate Bridge out of straw. ‘Gate’

Results

Results for loss errors involving /r, l, w/ in consonant clusters in word-initial

onsets are presented in Table 8.

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When there is a nearby consonant cluster containing another token of the

target phoneme (and so the target phoneme is in the same location in the

onset as the nearby cluster), there is a very strong perseveratory repetition

effect from a preceding repeated phoneme, but no trace of an anticipatory

effect from a following repeated phoneme. The a/b ratio for an identical

preceding cluster is slightly higher than for a preceding similar cluster, but

the difference is not significant and is probably an artifact of the extremely

low predicted value for identical clusters: less than 1; adjusting chance for

same clusters, x2(1)�0.51, ns; adjusting chance for different clusters, x2(1)�0.12, ns. The difference between before vs. after contexts for different clusters

is significant: adjusting chance for before, x2(1)�11.97, pB.001; adjusting

chance for after, x2(1)�12.20, pB.001. When there is a nearby singleton

/r, l, w/ in the onset (and so the target phoneme is after a consonant but the

repeated token is word-initial), there is no repetition effect; the difference

between before vs. after contexts is not significant: adjusting chance for

before, x2(1)�1.32, ns; adjusting chance for after, x2(1)�1.21, ns.

Results for addition errors are presented in Table 9. There is clearly no

effect of the repetition of whole phonemes (or onsets). If anything, an

identical preceding singleton-consonant onset leads to a decreased error rate;

TABLE 8Loss errors of /r, l, w/ from word-initial CC and CCC onset clusters and whether

the error is preceded or followed by CC or CCC clusters or singleton onsetcontaining the lost phoneme

Context # Targets Source

Chance

repetition

Observed


Before 110 Same CC(C) .008 .045 6.07 15.64 .001

Diff CC(C) .049 .200 4.90 54.68 .001

C�r/l/w .102 .128 1.28 0.74 ns

After 110 Same CC(C) .005 .018 3.61 1.57 ns

Diff CC(C) .043 .036 0.84 0.12 ns

C�r/l/w .090 .073 0.80 0.39 ns

TABLE 9Addition errors to word-initial singleton target consonants and whether the target

singleton is repeated

Context # Targets

Chance

repetition

Observed


Before 250 .094 .060 0.62 3.31 .10

After 250 .056 .052 0.93 0.07 ns


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though this does not quite reach significance, it is clear that there is no

increase in error rate. Note, however, that the decreased error rate from

preceding repetition does not differ significantly from the complete lack of

an effect from a following repetition: adjusting chance before: x2(1)�0.76,

ns; adjusting chance after: x2(1)�0.16, ns. The rate of addition errors is

significantly lower than the rate of substitution errors under the same

conditions (Study 1), for both perseveratory effects, x2(1)�22.04, pB.001,

and anticipatory effects, x2(1)�12.82, pB.001.

Discussion

The data from loss errors replicate previous findings that loss of phonemes

under conditions of identity are only perseveratory. This is interesting,

because consonants in onsets are involved, and there is also an anti-

anticipatory repetition effect for substitution errors on onset consonants.

The second consonant in a cluster shares properties with coda consonants

(non-initial and more marked), and they apparently are processed in such a

way that only perseveratory anti-repetition mechanisms are engaged (like

non-initial vowels and coda consonants). The rate of loss errors is elevated

for both identical clusters (identical onsets) and different clusters (non-

identical onsets), which do not differ from each other. This implies that the

effect is not based on identical onsets, but rather on the repetition of the /r, l,

w/ of the cluster. The data rule out an analysis where clusters such as /pl/ are

treated as single phonemes, because, unlike the clear substitution errors on

Study 1 and 2, where whole-segment errors are increased to a much greater

degree than subsegmental feature errors, there is no difference between

identical clusters and non-identical clusters here. However, the repetition

effect is limited to clusters in the environment; preceding identical singleton

consonants do not lead to more loss errors; the target and triggering

phonemes must be in parallel portions of the syllable (as predicted by the

models of e.g., Dell, 1986; Vousden et al., 2000).

The data from addition errors also show that the repetition effect is not

based on the repetition of identical onsets. In contrast to the doubled rate of

substitution errors when /p/ precedes /p/, the rate of addition errors actually

decreases (though this fails to reach significance). This makes no sense if /pl/

is a phoneme parallel to /p/ and /b/: just as /p/ increases the rate of errors

where a later /p/ is mispronounced as /b/, it should increase the rate of errors

where a later /p/ is mispronounced as /pl/. The data show that /pl/ is not

parallel to singleton consonants, compatible with an analysis as a sequence

of two consonants (see also Stemberger, 1989; Stemberger & Treiman, 1986).

The slight decrease in error rate is interesting. It can be argued that the first

/p/ reinforces an output structure with a single consonant in it, thereby

decreasing the rate of errors with a complex onset containing two

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consonants. Stemberger (2004) reported no effect of the number of enemies

in the neighbourhood on substitution errors, but did find an effect of the

number of enemies on addition errors to word-initial singleton consonants;

he argued that enemies in substitution errors had little phonological

coherence (because e.g., enemies of the word sit which do not start with

singleton /s/ do not reinforce any one competing consonant), but that most

enemies nonetheless have a singleton onset (e.g., for sit, 13 singleton-onset

enemies pit, tit, kit, bit, mitt, knit, fit, shit, chit, writ, lit, wit, hit vs. 3 cluster-

onset enemies slit, spit, skit) and so reinforce an output without a consonant

cluster (thereby making addition errors less likely). This appears to be the

case with the repetition effect as well.

I have shown that there is a repetition effect on both consonants and

vowels, that the effect is largely perseveratory, that it is specific to the

location of the target phoneme in the word/syllable, and that it affects both

whole segments and subsegmental features. The fact that the effects are

largely perseveratory does not match the statistics of errors in adult speech,

where anticipatory errors are much more common, but does match the

statistics of early child language, where perseverations are proportionately

more frequent (Stemberger, 1989). In Study 4, I examine whether the

repetition effect is present in speech errors in early childhood, whether it

affects whole segments more than features, and whether it is perseveratory vs.

anticipatory.

STUDY 4. REPETITION IN ONSET ERRORS IN CHILDLANGUAGE PRODUCTION: SUBSTITUTION ERRORS

Method

The speech error corpus. This study uses a different corpus of errors

made by young children, described in Stemberger (1989). Word-initial

consonant errors were identified and analysed as in Study 1. Vowels and

coda consonants were not analysed, because the number of such errors is too

small for the results to be statistically reliable. Stemberger (1989) argues that

similarity effects are based on the child’s actual pronunciation rather than on

the adult target; e.g., /k/ pronounced [t] functions just like /t/ in speech errors.

I will present the initial analysis in both ways (the adult vs. the child

pronunciation). Errors were similar to adult errors illustrated above.

Results

Results for substitution errors on word-initial consonants are presented in

Tables 10 and 11.


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Using the adult pronunciation, the mild elevation of the error rate after a

repeated phoneme does not reach significance. Using the child’s own

pronunciation, there is a significant elevation of the error rate after a

repeated phoneme, a non-significant elevation when surrounded by a

repeated phoneme, and no effect of a following repeated phoneme; the

effect size is much lower for children than for adults (cf. Table 1 above). Table

11 shows some perseveratory effect of repeated subsegmental features, at

least for place features; the effect size is similar to that in adult errors (cf.

Table 2 above).

Discussion

Results show that there is an anti-perseveration effect even in the speech of

young children, though it is of lower magnitude than in adult speech. This is

consistent with the fact that the rate of perseveratory errors is much higher

TABLE 10Child substitution errors on singleton onset consonants (n�144)

Context

Chance

repetition

Observed


Adult pronunciation Before .095 .132 1.45 2.30 ns

After .039 .042 1.07 0.02 ns

Both .007 .007 1.05 0.00 ns

Child pronunciation Before .114 .167 1.55 3.87 .05

After .045 .049 1.08 0.04 ns

Both .010 .014 1.39 0.22 ns

TABLE 11Subsegmental feature repetition in onset errors, child errors, child

pronunciations (N�144)


Chance

repetition

Observed


Before Place 63 .268 .381 1.68 4.13 .05

Manner 42 .260 .286 1.14 0.14 ns

Voicing 18 .314 .500 2.19 2.90 .10

After Place 63 .093 .095 1.02 0.00 ns

Manner 42 .113 .095 0.83 0.13 ns

Voicing 18 .142 .111 0.76 0.14 ns

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in child speech; the anti-perseveration mechanisms are still coming into

place. The lack of an anti-anticipation effect is consistent with other

evidence for less advance speech planning than in adult speech. It is

interesting that there is a perseveratory effect on the repetition of place of

articulation features (at least) which is of a comparable magnitude in child

and adult speech errors. However, the effect of repeated features is just as

great as the effect of repeated whole segments (unlike adult speech, where

whole-segment repetition has a greater effect). Stemberger (1989) shows that

errors involve features as independent elements to a much greater extent in

child errors than in adult errors (though Jaeger, 2004, argues that that is

more true of errors after the age of 2;0 than before). On the basis of these

data, we cannot rule out the possibility that the repetition effect in child

speech errors is based entirely at the feature level, with no special status for

whole-segment repetition; this is congruent with Stemberger’s (1989)

findings that between-word substitution and addition errors are less sensitive

to shared whole phonemes in the target and source words in child errors

than in adult errors.

So far I have addressed the impact of the repetition of target phonemes on

error rates, but activation-based mechanisms may predict similar effects

when the source phoneme appears more than once in the context. In some

errors there is a possible source for the error on both sides of the target

phoneme, as when /b/ is mispronounced as /p/, in the target sequence /p. . .

b. . .p/. Stemberger (1989) notes that it is unclear which possible source

(before vs. after) is the actual source; he simply classifies such errors as

anticipations/perseverations, and notes that such errors are possibly simple

anticipations or perseverations in which there is by chance another copy of

the source nearby. However, in an activation-based account of the repetition

effect, the source token before the target presumably weakens the source

token after the target, possibly affecting the probability with which the token

after the target can in fact be the source of the error; and similarly, the source

token after the target presumably weakens the source token before the target.

Similar inhibitory interactions between tokens of the source may occur when

there are two tokens of the source before the target (e.g., /p. . .p. . .b/, in

perseveration errors) or after the target (/b. . .p. . .p/, in anticipation errors).

If the first token of /p/ weakens the second token of /p/ in /p. . .p. . .b/, does

this change the probability of an error in which /b/ is mispronounced as /p/?

If the second token of /p/ weakens the first token of /p/ in /b. . .p. . .p/, does

this change the probability of an error in which /b/ is mispronounced as /p/?

Investigating the effect of repeated source consonants in principle should

provide more information about the details of the mechanism underlying the

repetition effect.


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STUDY 5. REPETITION OF THE SOURCE:SUBSTITUTION ERRORS

Method

The speech error corpus. The corpus, and the methods used to determine

whether a phoneme was repeated in the preceding and/or following context,

was the same as in Study 1.

Estimating chance. For the test of whether there is an elevated rate of

errors where there is a possible source on both sides of the target word,

chance was estimated in the following fashion. First, exchange errors were

eliminated, because every source is also a target (and we already know that

repeated targets affect error rates). Errors were divided into perseverations,

anticipations (including incomplete anticipations), and anticipations/perse-

verations. The anticipation/perseveration errors were divided up as anticipa-

tions vs. perseverations in the same ratio as the unambiguous errors (under

the null hypothesis that the two tokens do not influence each other). Using

the chance rate of repetition from Study 1, it was determined how often a

perseveration error would also by chance have a copy of the source following

the target, and how often an anticipation error would also by chance have a

copy of the source preceding it; summing these two numbers yields a chance

estimate of how often errors would ambiguously have a copy of the source on

both sides of the target. For the tests involving two copies of the source

preceding or following the target, all exchange errors were eliminated

(because all sources are also targets), and all anticipation/perseveration

errors were eliminated (because for any given error we do not know whether

the source preceded or followed the target). Chance was calculated on the

basis of preceding context in perseverations vs. following context in

anticipations. Here are some examples of repeated sources for errors on

onset consonants (target consonants in bold font, source and error

consonants both in bold and in italics):

Source /t/ on both sides of target /k/; also two tokens of source /t/ after target /k/:

don’t touch your told � cold toe to me

Two tokens of source /l/ before target /w/:

My left arm’s a lot leaker � weaker than my right.

Results

Results for repeated contexts on word-initial consonants are presented in

Tables 12 and 13.

Table 12 shows that, at least for consonants, the rate of errors where there

is a possible source on both sides is basically that expected by chance (for

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both adults and children). There is some elevation of the error rate for

vowels, but this does not reach significance. Table 13 shows no significant

effects of having two tokens of the source on the same side of the target,

whether preceding or following. The only hint that there might be an effect

comes from the fact that, for consonants in adult errors, the nonsignificant

differences are for an increased error rate when both copies of the source

precede the target, but a decreased error rate when both copies of the source

follow the target phoneme. For word-initial onset errors, this interaction

reaches significance: adjusting chance before, x2(1)�3.99, pB.05; adjusting

chance after, x2(1)�4.40, pB.05. For word-final errors, this difference does

not reach significance: adjusting chance before, x2(1)�1.24, ns; adjusting

chance after: x2(1)�1.37, ns. For vowel errors and for onset errors in child

language, the interactions do not even approach significance.

Discussion

As far as can be reliably determined, there are no effects of repeated sources:

sources occur repeated at about the rate that would be expected by chance.

TABLE 12Repeated source: surrounding the target (A/P category)

Age Location # Targets

Chance

repetition

Observed


Adult Onset 447 .085 .083 0.97 0.04 ns

Coda 142 .135 .113 0.82 0.58 ns

Vowel 223 .134 .175 1.37 3.24 .10

Child Onset 124 .087 .065 0.72 0.81 ns

TABLE 13Repeated source: both on the same side of the target

Age Location # Targets

Chance

repetition

Observed


Onset (Adult) Before 114 .088 .123 1.45 1.72 ns

After 296 .072 .051 0.69 2.01 ns

Coda (Adult) Before 49 .115 .143 1.28 0.58 ns

After 77 .099 .065 0.63 1.10 ns

Vowel (Adult) Before 72 .185 .181 0.97 0.01 ns

After 112 .123 .134 1.10 0.43 ns

Onset (Child) Before 48 .130 .123 0.96 0.01 ns

After 71 .049 .056 1.15 0.08 ns


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There is a hint of an effect in an interaction, however: for consonants in adult

errors (significantly for onset errors, nonsignificantly for coda errors),

repetition of the source before the target consonant nonsignificantly increases

error rates, but repetition of the source after the target nonsignificantly

decreases error rates. In other words, in the sequence /p. . .p. . .b/, repetition

of the source /p/ increases the interference effect of the source on the target

phoneme, leading to more errors like /p...p...p/, where yet another token of

the source phoneme appears. In contrast, in /b...p...p/, the source phoneme

/p/ has a lowered ability to interfere with the target consonant, making the

anticipation of the source (in the error /p...p...p/) less likely. But, assuming

that these differences are reliable, it is unclear why vowels would not show

similar effects, and why such effects should also be absent from child

language for onset consonants. As noted below, it is also unclear exactly what

effects are predicted. These results must remain suggestive.

DISCUSSION AND CONCLUSIONS

There are clearly effects of the repetition of elements on the rate of

phonological errors. The second of two identical segments shows an increase

in error rate: double in adult speech for all positions in the syllable, one and a

half times in child speech. The first of two identical segments shows an

elevated error rate, of slightly smaller magnitude, only in onset position and

only in adult speech. Repeated subsegmental features are subject to an

increase in error rate, but the effect is small (between 1.3 and 1.5 times), only

perseveratory, only for onset consonants, and is found in both adult and

child speech. Preceding or following similar but non-identical segments have

no effect on error rate (separate from the effects of repeated features). There

is also a perseveratory effect for consonants to be lost from consonant

clusters, only when the preceding token of the lost consonant is part of a

cluster, not when it is a singleton consonant, but regardless of whether the

preceding cluster is identical to the target cluster. Addition errors (e.g., /p/0/pr/) are not increased by a preceding identical target consonant. The locus of

the repetition effect is primarily on segments and secondarily on features,

and is not on larger units such as onsets (and is incompatible with the notion

that clusters such as /pr/ are made up of two individual phonemes).

There are two basic ways to approach the repetition effect that have been

proposed in the literature. (1) The effect arises during the access of

phonological elements during speech planning, due to intrinsic character-

istics of processing. (2) The effect arises at a later stage, due to the actions of

a pre-articulatory output editor which flags suspicious (but correct)

repetition and resolves these putative ‘errors’ by making a change that

creates a true error. I discuss each approach in turn.

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Repetition effect inherent in the access of elements

The repetition effect could arise due to intrinsic characteristics of activation

in the access of elements during language production. As shown in Figure 1,

it is assumed that there is a distribution in the final activation of an element

(with low activation on the left, high activation on the right). Errors arise

when activation falls too low to guarantee accurate access of the target

element. I indicate two thresholds. Threshold A is the point below which

activation is so low that almost every token is in error. Threshold B is the

point above which almost every token is correct. Between Threshold A and

Threshold B is a region in which errors occur, but only when there is

competition from another word (or from elsewhere in the same word). The

two thresholds are far enough apart that most errors in adult speech are due

to interference from competing words (e.g., Stemberger, 1989). Repetition of

an element leads to a decrease in activation levels, shifting the activation

distribution to the left. Because a larger proportion of trials fall between the

two thresholds, the rate of contextual speech errors (anticipations, perse-

verations, and exchanges) increases. Previously activated elements induce a

large shift (a perseveratory effect or refractory period). Following elements

are generally less activated; only following onset consonants, which are

processed earliest in the incremental activation of elements in a word, are

activated enough to cause a leftward shift of the activation function that is

large enough to lead to a detectable effect on error rates. Whole-segment

information becomes available earlier in processing than feature information;

Figure 1. Activation distribution and errors (with hypothetical thresholds A and B).


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as a result, effects are greater with whole segments, and repetition of features

leads only to perseveratory effects.

The shift in activation levels is not itself enough to understand the effect.

It is also necessary to understand the role of the source. Previous researchhas argued that onset (word-initial) consonants are more highly activated

than segments later in the word (whether coda consonants, vowels, or the

second consonant in an onset cluster); i.e., that the distribution is shifted to

the right. However, this upward shift of activation does not lead to a

decreased rate of errors on onset consonants, but to an increased error rate.

If we assume that errors are more likely when the source segment is at some

particular range of the activation function, the upward shift would also place

more tokens of potential source segments in that range, possibly leading tomore utterances in which a target segment in the right activation range

(between Threshold A and Threshold B) occurs with a source segment in the

right activation range, creating the conditions for a contextual phonological

error. With the repetition effect, the activation range of the target segment is

shifted to the left (downwards) while the activation range of the source

segment is unaffected, leading to an increased number of tokens in which the

target and source segments are both in the right activation ranges to cause a

contextual phonological error. It is important to note that, had the activationfunction of the source consonant also shifted downwards, the repetition

effect would not necessarily have been observable, because the proportion of

source segments in the right range for a contextual error may also have

decreased.

There are several aspects of the findings above that could be attributed to

a tailoring of the repetition effect to the statistics of error, but all have other

explanations as well. The larger perseveratory effect than anticipatory effect

may reflect a lower degree of pre-planning, as noted, but may also be alegacy of the initially high rates of perseveration errors in child language. The

fact that the anticipatory effect is found only with onset consonants could

also be rooted in error statistics: anticipation errors are especially pre-

dominant in onsets. Contrasting anticipations (plus exchanges and incom-

plete anticipations, which, as discussed below, have been argued to start out

as anticipation errors) with perseverations (and eliminating errors that are

ambiguous between anticipations and perseverations), anticipations are

more predominant for onset errors (73.9% in the data in Table 1) than incodas (63.7%) or vowels (62.9%). It is thus reasonable to find a stronger anti-

anticipation mechanism for onsets. Lastly, the repetition effect is stronger for

place of articulation features than for manner or voicing features, and errors

involving just place of articulation are greatly overrepresented in speech

errors. Stemberger (1989) reported that errors involving only a change in

place features constituted 69% of adult phonological errors and 53% of child

errors. Stoel-Gammon and Stemberger (1994), in a study of consonant

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harmony (long-distance assimilation between consonants) in child language,

demonstrate that harmony involving place of articulation is far more

frequent than harmony involving manner or voicing features (as had long

been suggested in non-quantitative studies). If anti-repetition mechanismsare tailored to correct more likely errors, we would expect to see more of a

repetition effect on place features, as observed. However, these results can be

attributed to other factors as well (see below). At this point, it is unclear

whether the repetition effect is tailored to be stronger for types of errors that

are more common.

This approach predicts that there would be effects of repeated sources as

well, because the repeated tokens would induce shifts in the activation

distributions of the source segments, affecting the proportion of tokens inwhich the potential source is in the right activation range for a contextual

phonological error to occur. Consider first the situation with two tokens of a

source following the target (e.g., /b...p...p/). Because we do not find an

anticipatory repetition effect on errors except with onsets in adult speech, we

would not expect to see an effect of following repeated sources for vowels or

codas, or in child speech. For onsets in adult speech, the second token of /p/

should shift the activation function of the first token of /p/ to the left,

potentially decreasing the amount of interference of that /p/ on the preceding/b/ and lowering the error rate. We do indeed find a lower error rate, but this

does not approach significance, and we find a nonsignificant difference of a

similar magnitude with coda consonants (where it is not expected). We

cannot rule out that such an effect is there, but it is certainly of much lower

magnitude than the effect of repeated targets. Next consider the situation

with two tokens of a source preceding the target (e.g., /p...p...b/); we expect to

find perseveratory repetition effects in all parts of the syllable and in child

language, in which e.g., the first token of /p/ weakens the second token of /p/(and vice versa), potentially decreasing the level of interference from each

token of /p/, and so decreasing the error rate on the target /b/. Again we see

no significant effects, and no hint of an effect in child language and for

vowels in adult language, but there is a nonsignificant increase in error rate in

onset consonants in adult language and (to a lesser degree) in coda

consonants. Perhaps the combined effects of the two tokens of /p/ (either

one of which would individually be able to replace the following target /b/ in

an error) increases the level of interference on the target /b/ and henceincreases the error rate, despite the weakening that arises from repetition; but

I have no explanation for why this would not also hold true for vowels in

adult speech or for onsets in child language. The increase in error rate when

the repeated sources preceded the target vs. the lowering when they followed

the target did reach significance for errors on onset consonants in adult

language, suggesting that there is something going on here. But it is difficult

to hang any conclusions on an interaction in only one of the four sets of


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errors, in the absence of any main effects. Predictions will depend on details

of the activation functions and the model of the error process that are

currently unknown and would probably require computational modelling

(which is beyond the scope of this paper). While I predict that relating error

rates to activation functions should lead to effects for repeated sources, it is

unclear what those results should be, and what the magnitude of the effects

should be. I leave that for later research.

Similarly, there is no significant effect when the target is in the middle of

two tokens of the source (e.g., /p...b...p/), though the a/b ratio is mildly

elevated for vowels and mildly depressed for onsets in child language. Again,

it is unclear what predictions are made here. It might be expected that the

two tokens would combine to interfere more with the target segment (thereby

increasing the error rate), but that the repetition would shift the activation

functions of both tokens of the source downwards (thereby decreasing the

error rate). Again, exploration of this potential interaction of sources must

be left for the future.

One class of connectionist model has some difficulty with the data

presented here. Dell, Juliano, and Govindjee (1993) is a recurrent connec-

tionist model that outputs one phoneme at a time as a distributed pattern

across all output units. The model cannot in fact produce between-word

contextual phonological errors of the sort analysed here, but there is

another fundamental problem. That model has no way to represent the

notion of segmental identity, e.g., that the phoneme /b/ has been repeated.

A phoneme is a higher-level element that binds together smaller parts

(subsegmental features), but this model represents only the lower-level units.

There may be a way to represent identity at higher levels, but no way has yet

been forthcoming; Marcus (2001) has taken this to be a fundamental flaw in

connectionist models. The data discussed here add more evidence that a

mechanism for identity of high-level units must be developed. Of course,

there are also symbolic models that do not represent segmental identity,

including Browman and Goldstein (1986), Studdert-Kennedy and Goodell

(1995), and Pouplier (2007), and these data are problematic for such

approaches as well. On the other side of things, the fact that there is some

effect of repeated subsegmental features is also problematic for the model of

Shattuck-Hufnagel and Klatt (1979), in which features are not independent

elements. The data support a model in which whole segments are present

early in processing, with subsegmental features present later in processing.

Repetition effect external to the access of elements

Shattuck-Hufnagel (1979) presents an alternative approach. There is no

effect of repetition on the actual occurrence of errors in speech planning.

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However, after the phonological form has been planned, but before it is

actually articulated, a monitor examines the representation for various

purposes that include deciding on whether the representation is well-formed

or contains errors. At this stage, the monitor detects errors, which are then

corrected. If the detection process detects more errors of some kinds than of

others, statistical skewings are introduced into the output. Shattuck-

Hufnagel proposes that the monitor uses repetition of phonemes as a

potential cue that an error has occurred. This catches true errors of

anticipation and perseveration (but not exchanges), but at the cost of

flagging correct chance repetition of phonemes as potential errors. She goes

further to suggest that the monitor also has editing capabilities, and can

correct the error directly (though this often leads to the creation of an error,

in the case of phonemes that are repeated by chance). Note that this

mechanism is not sensitive to the presence of repeated source consonants,

and so is compatible with the failure to find (main) effects of repeated

sources in Study 5.

One might expect that a mechanism of this sort would be tailored to the

statistics of errors, ‘correcting’ chance repetitions in such a way as to more

often correct the most likely error. Since anticipations (and exchanges) are

more common than perseverations, that should lead to the first phoneme

being corrected more often, with the prediction that the repetition effect

should be largely anticipatory (an increased error rate due to a following

identical phoneme). As noted above, there are possibly some elements of

tailoring to error statistics (e.g., the anticipatory effect only on onsets), but

those effects also have timing explanations. It is perhaps reasonable to view

the greater perseveratory effect as a legacy of high rates of perseveration

errors in early childhood. There is no explanation, however, for the

differences between onset consonants vs. coda consonants and vowels.

However, it is reasonable for the monitor/editor to be sensitive to the chance

probability of repetition of whole segments vs. of features, and to flag

repetition more often when chance repetition is lower (because repetition is

proportionately more likely to be the result of an error if repetition occurs

infrequently in correct sentences).

There is in principle no obstacle to tailoring the monitor/editor to match

the observed statistics. However, I would like to note that there is no inherent

reason why the monitor/editor should show the particular statistical

characteristics that I report in this study. It is likely that the characteristics

of the monitor/editor will at least partly have to be stipulated (and attributed

to arbitrary innate characteristics of the system). This approach can be fit to

the observed results, but cannot provide a non-arbitrary explanation nor

make novel predictions about so-far unexplored factors.


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Ramifications for other characteristics of language processing

The mechanism behind the repetition effect may have consequences for other

aspects of speech errors, in particular for the rate of exchange errors.

According to Shattuck-Hufnagel (1979) and to Dell (1986), an exchange

error starts with an anticipation; it converts into an exchange when the

earlier target phoneme (having erroneously failed to appear in its target

location) perseverates to replace the later source phoneme (which now

erroneously appears at the earlier location), resulting in the reversal of the

original order of the two phonemes. In his simulations, Dell (1986) argued

that the rate of exchange errors partly relates to the magnitude of the

tendency to make perseveration errors relative to the magnitude of the

tendency to make anticipation errors: the greater the tendency towards

perseveration, the greater the proportion of anticipation errors that are

converted into exchange errors. I propose a variation on Dell’s analysis: the

probability of an exchange error is not related to the probability of

perseveration, but rather to the magnitude of the perseveratory repetition

effect. In a target sequence such as /p...b/, a simple anticipation creates a

whole-segment repetition (/b...b/); the greater the leftward shift of the

activation function for /b/, the greater the likelihood that the second token

of /b/ (the /b/ in its original location) will be inaccessible, so that it will be

replaced by a competing segment (the activated but so-far unused /p/ that

was displaced in the anticipation error). It follows that a large effect size for

the repetition effect (so that repetition is heavily disfavoured) will lead to a

high rate of exchange errors; a low effect size will lead to a low rate of

exchange errors.

There is in principle an optimal effect size for the repetition effect,

determined by its cost�benefit ratio. If the magnitude of the effect is too

small, it will not do an adequate job of preventing perseveration; there will

be insufficient benefit. If the magnitude of the effect is too large, it will cause

problems; the cost will be too high. The more efficient an anti-repetition

mechanism gets, the more likely it will be to induce errors in the processing of

elements that are repeated by chance. The optimal magnitude of the

repetition effect is thus at a level that will maximise the prevention of errors

of perseveration while minimising the induction of errors of other sorts.

Operationally, the optimal magnitude is one that, for speech during a given

period of time, prevents the largest possible number of perseveration errors,

while inducing the smallest possible number of other errors. Given a

particular effect size for the repetition effect, the number of induced errors

in a given period of time will be a function of how often repetition occurs by

chance during that time period: if chance repetition is rare, there will be few

errors; if chance repetition is high, there will be many errors. It follows that

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the repetition effect will optimally be stronger if chance repetition is low, and

weaker if chance repetition is high.

The chance rate of repetition is not the same for all types of elements in

human language, and so, if the system is flexible enough, the optimal effect

size of the repetition effect will vary for different types of elements. This

should then be reflected in the relative rates of exchange errors for different

types of elements. Different types of elements do indeed show different rates

of exchange errors. Garrett (1975, 1976) proposed that errors at the word

sequencing level were heavily biased towards full exchange errors, and

Stemberger (1989) argued that exchanges made up 64.9% of the word

ordering errors in his adult error corpus, and 55.8% in his child error corpus.

Shattuck-Hufnagel (1979) gave special emphasis to phonological exchange

errors, under the assumption that it would make sense for phonological and

syntactic processing to be similar. Subsequent research failed to support the

expected high rate of phonological exchange errors (Dell & Reich, 1981;

Jaeger, 2004; Stemberger, 1989); Stemberger (1989) argued that exchange

errors make up only about 15% of whole-segment phonological substitution

errors for adults and for children after age 4;0. Phonological errors involving

the exchange of subsegmental features are of very low frequency in adult

errors (Shattuck-Hufnagel & Klatt, 1979; Stemberger, 1989), but somewhat

more common in child errors (Jaeger, 2004; Stemberger, 1989). There are

corresponding differences in the rate of chance repetition at the different

levels. I do not know of any report of the rate at which an open-class

morpheme is repeated within a clause (e.g., a red car bumped into a blue car),

but it is probably less than 1�2% of clauses. In this paper, I’ve noted that the

chance repetition of whole segments is higher (9% for onset consonants, 15%

for vowels and coda consonants), while the rate of chance repetition of

subsegmental features is quite high (ranging from 18% to 57% in Tables 2�4,

depending on the contrast). Because chance repetition is infrequent at the

word level, anti-repetition mechanisms can be quite strong; and so more than

half of all word sequencing errors are exchanges. Because chance repetition

of phonemes is much more frequent (10�15% of tokens), anti-perseveratory

mechanisms must be weaker, and so only 15% of errors result in exchanges.

Because chance repetition of subsegmental features is very high (over 50%

for some features), anti-perseveratory mechanisms must be quite weak, and

so feature exchanges are uncommon. Thus, the findings reported above that

the effect size of the repetition effect is much larger for whole segments than

for subsegmental features has a functional explanation, and is congruent

with the greater rate of exchange errors involving whole segments than

involving features. It would appear that differences in the probability of

chance repetition at different levels may lead to different strengths of anti-

repetition mechanisms, and so to different error statistics.


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There was a perseveratory effect of feature repetition in onset consonants,

but not in vowel or coda consonants. It is also the case that chance repetition

of features is lower for onsets for both adults (.211 for place features) and

children (.268 for place features) than it is for codas and vowels (all around

.330 or higher), and that we thus predict a larger effect size on onset

consonants. It is possible that when the frequency of chance repetition is

above 30% of tokens, anti-perseveratory effects become too weak to detect

(at this sample size). Similarly, repetition effects involving place of articula-

tion features were easier to detect than for manner features, for both adults

and children. This may be due to the fact that the chance repetition rate for

place features is lower than for manner and voicing features.

CONCLUSION

One fundamental challenge for any cognitive system is how to prevent

contextually appropriate states from maintaining themselves or recurring

over a short period of time. The data examined here demonstrate that there is

a mechanism that prevents such perseveration of phonological elements,

revealing itself in the ‘overapplication’ of the mechanism to prevent the

correct recurrence of elements. The anti-repetition mechanism primarily

suppresses phonological information that is available relatively early in

processing (whole segments). It is of equal magnitude for substitution errors

in all parts of the syllable (onset, vowel, or coda), but is of greater magnitude

for the loss of whole segments in the second position of word-initial

consonant clusters (perhaps reflecting the high rate of speech errors in which

consonants are erroneously added in that position). It is present from an early

age (but is of lower magnitude in the speech of young children). The anti-

repetition mechanism is also detectable, at a lower magnitude, on phonolo-

gical information that is available slightly later in processing: for word-initial

consonants, segments that are repeated in earlier words (anti-anticipation)

and subsegmental features that are repeated in earlier words (anti-persevera-

tion). Phonological information that is accessed even later is not subject to the

anti-repetition mechanism: for vowels and coda consonants, subsegmental

features anywhere and whole segments in later words.

The exact nature of the anti-repetition mechanism is unclear. Had the

effect been only perseveratory, it would have been straightforward to posit a

refractory period, during which the phonological elements are somewhat less

sensitive to activation for some period after use; but this is complicated by

the (limited) presence of an anti-anticipatory effect. Effects are dealt with if

we assume that the final activation function of activated elements is shifted

slightly downward (as in Figure 1), leading to more tokens falling into an

activation range in which they are vulnerable to disruption from competition

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from other activated elements. However, the possibility cannot be ruled out

that the effects arise as part of an error-correction mechanism within a pre-

articulatory output editor � mostly because the details of such a mechanism

at this point are so vague that the mechanism does not make detailed testablepredictions. Clearly, such mechanisms must be elaborated more clearly and

tested against the data presented here.

The finding that the effect primarily involves whole segments reinforces

previous research that has argued for an early stage in processing in which the

identity of whole segments is known, but where the constituent subsegmental

features are not yet known to the system. This suggests that systems that

do not allow for such a stage in processing (whether connectionist models

such as Dell et al., 1993, or symbolic articulatory phonology models suchas Browman & Goldstein, 1986, Studdert-Kennedy & Goodell, 1995, or

Pouplier, 2007) are missing a fundamental characteristic of the cognition of

speech. While segmental representations can simply be added to symbolic

models such as articulatory phonology, connectionist models must await a

fundamental breakthrough for a way to represent identity of larger elements

that are made up of many smaller elements � a break-through in which /b/ will

be treated as a unit, but /bl/ will not. The data reinforce the conclusions of

previous studies (e.g., Stemberger & Treiman, 1986) that sequences ofsegments in consonant clusters (such as /pl/) involve representations that

are fundamentally different from single units with subsegmental features; i.e.,

that /pl/ is in fact a sequence of two basic elements, and not one element made

up of subsegmental features that are similar to those in /p/ and in /l/. The

finding that there is some effect of the repetition of subsegmental features, if

only in word-initial consonants, joins with other evidence that argues that

subsegmental features also constitute an important type of element in

language processing.I have argued that the anti-repetition mechanism must in principle be

set up to be sensitive to the rate of chance (correct) repetition. If chance

(correct) repetition is too frequent, then strong anti-repetition mechanisms

will cause more errors than they prevent. I have argued that differences in

chance repetition for elements at different levels of representation (very

low for words, intermediate for segments, and very high for subsegmental

features) should lead to anti-repetition mechanisms of different strengths,

and may also underlie the relative proportion of exchange errors (high forwords, intermediate for segments, very low for features). The hope of

some researchers that exchange errors should be the basic type of error at

all linguistic levels (e.g., Shattuck-Hufnagel, 1979) may not be well-

founded.

This paper addresses anti-repetition mechanisms in natural speech, for

neurologically intact adult native speakers (of English) and (in somewhat

less detail) for young typically developing children. We will gain a more


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detailed picture of anti-repetition in human language by extending this

sort of research to other populations (to second-language speakers, to

children who are not typically developing, and to adults with neurogenic

disorders).

Manuscript received March 2008

Revised Manuscript received February 2009

First published online May 2009

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preventing perseveration in language production

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