stimulus factors affecting the categorisation of faces and scrambled faces

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acta psychologica ELSEVIER Acta Psychologica 85 (1994) 219-234 Stimulus factors affecting the categorisation of faces and scrambled faces * N. Donnelly **Ta, G.W. Humphreys b, J. Sawyer b a Institute of Social and Applied Psychology, University of Kent at Canterbury, Canterbury, Kent CT2 7LZ, UK, b Cognitive Science Research Centre, School of Psychology University of Birmingham, Birmingham, UK (Accepted June 1993) Abstract Three experiments are reported which investigate the categorisation of faces and scrambled faces in a face/scrambled face decision task. Three kinds of stimuli were presented in upright and inverted orientations; faces, highly scrambled faces (all features out of position) and moderately scrambled faces (two features out of position). Experiment 1 demonstrated that faces and highly scrambled faces are categorised equally quickly and both types of stimulus were categorised faster than moderately scrambled faces. These results held for both upright and inverted presentations. It is argued that for both upright and inverted presentations, faces are categorised by being matched in parallel to a stored mental representation of a face. In contrast scrambled faces are categorised following a serial search of facial features which is probably self-terminating. Experiment 2 demon- strates that the results of Experiment 1 hold when facial features are replaced by other objects which retain the same global shape as facial features and suggest that faces are categorised using a coarsely coded visual description. Experiment 3 demonstrates the importance of stimulus outline on the categorisation of both moderately and highly scrambled faces but not real faces. The results are discussed in terms of the stimulus information used, and the effect of inversion, on face categorisation. 1. Introduction Face recognition is well known to be affected by inversion. Locally inverted facial features are difficult to perceive (the Thatcher illusion, Thompson, 19801, * The present series of experiments was supported by a grant from the Medical Research Council of Great Britain awarded to the second author. We should like to thank Jules Davidoff and one anonymous reviewer for their helpful comments. ** Corresponding author. Fax: +44 227 763674; E-mail: [email protected] OOOl-6918/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI OOOl-6918(93)E0038-4

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Page 1: Stimulus factors affecting the categorisation of faces and scrambled faces

acta psychologica

ELSEVIER Acta Psychologica 85 (1994) 219-234

Stimulus factors affecting the categorisation of faces and scrambled faces *

N. Donnelly **Ta, G.W. Humphreys b, J. Sawyer b

a Institute of Social and Applied Psychology, University of Kent at Canterbury, Canterbury, Kent CT2 7LZ, UK,

b Cognitive Science Research Centre, School of Psychology University of Birmingham, Birmingham, UK

(Accepted June 1993)

Abstract

Three experiments are reported which investigate the categorisation of faces and scrambled faces in a face/scrambled face decision task. Three kinds of stimuli were presented in upright and inverted orientations; faces, highly scrambled faces (all features out of position) and moderately scrambled faces (two features out of position). Experiment 1 demonstrated that faces and highly scrambled faces are categorised equally quickly and both types of stimulus were categorised faster than moderately scrambled faces. These results held for both upright and inverted presentations. It is argued that for both upright and inverted presentations, faces are categorised by being matched in parallel to a stored mental representation of a face. In contrast scrambled faces are categorised following a serial search of facial features which is probably self-terminating. Experiment 2 demon- strates that the results of Experiment 1 hold when facial features are replaced by other objects which retain the same global shape as facial features and suggest that faces are categorised using a coarsely coded visual description. Experiment 3 demonstrates the importance of stimulus outline on the categorisation of both moderately and highly scrambled faces but not real faces. The results are discussed in terms of the stimulus information used, and the effect of inversion, on face categorisation.

1. Introduction

Face recognition is well known to be affected by inversion. Locally inverted facial features are difficult to perceive (the Thatcher illusion, Thompson, 19801,

* The present series of experiments was supported by a grant from the Medical Research Council of Great Britain awarded to the second author. We should like to thank Jules Davidoff and one

anonymous reviewer for their helpful comments. ** Corresponding author. Fax: +44 227 763674; E-mail: [email protected]

OOOl-6918/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI OOOl-6918(93)E0038-4

Page 2: Stimulus factors affecting the categorisation of faces and scrambled faces

face recognition is slower and less accurate for inverted than for upright faces (Valentine, 19911, and sex judgements are slowed as a function of the degree of the angle of rotation from the upright (Sergent and Corballis, 1989).

It is, however, difficult to establish why inversion has such profound effects. One suggestion, taken from studies of the recognition of disoriented objects, is that recognition involves the matching of a visual input to a stored mental representa- tion and that inverted objects require mental rotation prior to identification. For example, Jolicoeur (1985) demonstrated that the more disoriented objects were from upright, the slower they were recognised and that the relationship between increased RT and angle of disorientation was linear up to 120”. But there are at least three problems with extending the idea of the mental rotation of objects as the basis for an account of findings from experiments using inverted face stimuli. The first point is that studies of mental rotation have used objects drawn from categories where the category exemplars differ in both gross features and in the spatial arrangement of features, whereas faces share similar (though not identical) features in a similar, though highly significant, configural arrangement (see Ser- gent, 1984; Rhodes et al., 1993). As such, the processes involved in object recognition need not generalise to face recognition. Secondly, even with objects, it is not clear that the recognition of inverted stimuli requires mental rotation. In the study by Jolicoeur (1985) RTs to identify inverted objects were faster than expected from linear extrapolation from the effects of disorientation upto 120”. This suggests that, under conditions of inversion, some additional process may come into play, such as a top-bottom transformation (r-e-labelling features at the retinal top of the object as being at the bottom etc., see McMullen and Jolicoeur, 1990) And thirdly, the recognition of inverted objects is only marginally less accurate than that of upright objects (Jolicoeur, 1985) but the accuracy of face recognition is severely disrupted by inversion (Valentine, 1988). Therefore it is unclear whether mental rotation alone can provide an account of the effects of inversion on face processing. At present all we can say is that inversion affects both object and face recognition; it slows down the processes involved in identifying objects and it leads to both slow and inaccurate face recognition.

When considering face recognition specifically, some investigators (e.g. Rock, 1974; Diamond and Carey, 1986) have suggested another reason why inversion affects performance strongly. Firstly, they suggest that faces must be mentally rotated before a process of representation can begin. And secondly, they suggest that the process of individualisation is difficult for faces because important features are formed from fine differences in spatial arrangement. Inversion leads to poor face recognition because, when stimuli are inverted, subjects cannot represent the important distinguishing features (relative to some mental represen- tation of a face) necessary for individualisation of the person. If inversion leads to problems because of the difficulties with representing the distinguishing features necessary for individualisation, we might also expect inversion effects to be less on tasks not requiring the individualisation of faces - such as visual categorisation of a stimulus merely as a face. Such a result would suggest that the visual description

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N. Donnelly et al. /Acta Psychologica 85 (I 994) 219-234 221

for face categorisation judgements differs from that used for the recognition of an

individual person. There are in fact reasons for doubting that face categorisation and recognition

judgements are made from the same representation. In particular, face categorisa- tion is likely to be based on a coarser visual description of a stimulus than the description required for recognition. Such a suggestion is made implicitly by those who believe that the representations needed for face recognition are formed by scoring deviations from a generic face or norm (e.g. Rhodes et al., 1993) and would also fit with patients who have face recognition problems nevertheless being able to categorise faces as faces (Bodamer, 1947; Humphreys et al., 1992). Furthermore, realignment via mental rotation may not be needed to derive the coarser visual descriptions required for face categorisation. Data which support this position have been presented by both Perrett et al. (1988) and Davidoff and Landis (1990, Experiment 3). They both have shown that face categorisation was somewhat slowed by inversion but that faces were categorised faster than scrambled faces both when faces were upright and when they were inverted. This suggests that whatever the description of the visual input used for the categorisation of upright faces, it is also used for inverted faces. In contrast, as we have said above, the visual descriptions for face recognition seem to be profoundly disrupted by inversion.

It is not only the nature of the visual input used to categorise faces which is unclear. Another related issue is how matches are made between upright and inverted faces and the stored mental representation of a face held in long-term memory. In other words, what is the nature of the processes that map features of the visual input onto the appropriate stored mental representation; in particular are the features matched in serial or in parallel? Perrett et al. (1988) assumed that in normal subjects, face stimuli are matched as a configuration (and by definition in parallel) to a stored mental representation. In contrast, they assumed that scrambled face stimuli are tested for their ‘lack of faceness’ by a serial self- terminating search of features, presumably matching the feature list against the stored mental representation of a face. To support this assertion, Perrett et al. performed a face/scrambled face categorisation task in which subjects were shown a stimulus and asked to respond either ‘face’ or ‘scrambled face’. Subjects categorised faces faster than scrambled faces, enabling Perrett et al. to conclude that faces could not be categorised as a result of a serial self-terminating search as this would have led to scrambled faces being categorised faster than faces.

In the present experiments we address two issues concerning face categorisa- tion. Firstly, we wish to investigate whether upright faces and inverted faces are categorised using the same information. Secondly, we are interested in determin- ing the information crucial to categorising a stimulus as a face, and how can this process of face categorisation be distinguished from that involved in categorising a stimulus as a scrambled face. These issues were addressed in a face/scrambled face categorisation task with both upright and inverted stimulus presentations where the nature of the facial features was varied.

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21 N. Donrdiy et 01. /Actcr Psychologica CIS (1994) 219-234

2. General method

2.1. Subjects

In each of the following experiments 10 subjects were used. All subjects had normal or corrected-to-normal vision. Different subjects were used in each experi- ment.

2.2. Stimulus construction

Precise details varied for each experiment, however, some general principles applied.

Degree of scrambling to produce scrambled faces Three sets of stimuli were made for each experiment. Thirty face stimuli, 15

scrambled face stimuli where two features were displaced (moderately scrambled) and 15 scrambled face stimuli where all three features were displaced (highly scrambled). For both moderately scrambled and highly scrambled stimuli all combinations of feature rearrangements were used. Stimulus displays covered approximately 4” visual angle.

Face features

The face features used were taken from the caricature faces of Homa et al. (1976). Homa et al. present 5 versions of each feature. In the present experiments, each face feature occurred equally often in faces and scrambled faces. Five separate stimuli were created for each stimulus type. The five faces were photo- copied 6 times to produce the thirty faces and five moderately scrambled faces were photocopied 3 times to produce 15 moderately scrambled. Likewise, five highly scrambled faces were photocopied 3 times to produce 15 highly scrambled.

Face features were surrounded by a face outline with ears. Stimulus orientation could be determined from the outline feature alone (see Fig. 1).

Orien ta tiori

The sixty stimuli noted above were attached to white cards so that they could be presented upright and inverted. In total each subject received 120 trials (3 stimulus types x 2 orientations), sixty faces, thirty moderately scrambled and thirty highly scrambled.

2.3. Apparatus

Stimuli were presented in an Electronics development 2-field tachistoscope. Responses were recorded by two response buttons and these were attached to a Birkbeck timer which recorded RTs in centiseconds. The allocation of response keys to stimulus type was balanced across subjects.

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N. Donnelly et al. /Acta Psychologica 85 (1994) 219-234 223

Fig. 1. Upper panel: Example face and scrambled face stimuli used in Experiment 1. Lower panel:

Range of features used in Experiment 1. (Adapted from ‘Perceptibility of schematic face stimuli:

Evidence for a perceptual Gestalt’ by Homa et al., 1976, Memory and Cognition 4, Fig. 1, p. 178.

Copyright held by the Psychonomic Society. Reprinted by permission.)

2.4. Procedure

The task was the same for all subjects in each experiment. They had to decide whether a stimulus that had been presented was a face or a scrambled face. Subjects were not required to distinguish between the two types of scrambled face. Each trial began with a 500 msecs. fixation cross which was followed immediately at offset by a stimulus presentation. The stimulus presentation lasted until a response was made.

3. Experiment 1

In Experiment 1 we were interested in both the effect of (1) orientation and (2) number of features displaced to produce a scrambled face on the speed with which a stimulus could be categorised as a face or scrambled face.

3.1. Results

RTs and arcsined errors were analysed in a stimulus type (3: Face vs. Moder- ately Scrambled vs. Highly Scrambled) X orientation (2: Upright vs. Inverted) ANOVA repeated over all factors. All post-hoc analyses were conducted using Newman-Keuls tests.

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224 N. Donnelly et al. /Acta kychologicu 85 (1994) 219-234

Table I RTs(““” ‘Id “““) and Q correct scores for Experiment 1

Upright Inverted

Face

Moderately scrambled

Highly scrambled

6497’ (99) 772”” (96)

750” (9.3) x55’” (XI)

6x737 (98) 744”” (96)

Reaction times

The main effects of stimulus type and orientation were both significant (F(2,18) = 7.07 and F(1,9) = 7.61 respectively, both ps < 0.01) (see Table 1). Moderately scrambled stimuli were discriminated slower than both faces and highly scrambled stimuli but there was no difference between faces and highly scrambled faces. Inverted stimuli took longer to categorise than upright stimuli. The interaction between stimulus type and orientation did not approach significance (F(2,18) = < 1).

Errors

The main effect of stimulus type was significant but orientation just failed to reach significance (F(2,18) = 18.69, p < 0.01 and F(1,9) = 3.99). More errors were made in categorising moderately scrambled stimuli than in categorising either faces or highly scrambled stimuli. There was no difference in categorising faces and highly scrambled faces. The interaction between stimulus type and orientation

failed to reach significance (F(2,18) = 2.03).

.?. 2. Discussion

Experiment 1 has shown significant effects of both stimulus orientation and stimulus type on the face/scrambled face categorisation task. Categorisation of all stimulus types is slower when the stimulus is inverted than when it is upright. And whilst there is no significant difference in RT or errors between faces and highly scrambled faces either when presented upright or inverted, longer RTs and more errors are found for moderately scrambled faces relative to both faces and highly scrambled faces when upright and inverted. These data show that it is easy to make decisions about whether a face, or a highly scrambled face, matches, or definitely does not match, a mental representation of a face. The decision is more difficult when partial matches are made (partial matches may be defined here as not just based on the presence of individual features, but also where one feature is in the correct location). The effect of stimulus type (intact face vs. highly and moderately scrambled faces) is additive with the effects of inversion (see also Davidoff and Landis, 1990, Experiment 3).

Of course, it may be the case that the difficulty with categorising a moderately scrambled face occurs because of the combination of inversion and feature position swapping. For example, in the case where the eyes and the mouth are switched, to create a moderately scrambled face, and then the whole stimulus is inverted, it might be thought that this inverted moderately scrambled face stimulus

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N. Donnelly et al. /Acta Psychologica 85 (1994) 219-234 225

is similar to an upright face. In the present experiment there are too few trials per condition to test the hypothesis that some scrambled feature combinations led to more difficult categorisations than other combinations (i.e. 5 trials per condition minus any errors that were made). However, for this to be a valid explanation subjects would have had to ignore the orientation of both the internal facial features and the external feature. Later experiments will show why it is possible that this occurred.

In all respects, the results of Experiment 1 replicate those of previous studies but with the advantage that they do so in a single experiment. A face advantage over moderately scrambled faces is present for both upright and inverted faces (Perrett et al., 1988). And the advantage for face categorisation over scrambled face categorisation was present only when compared to moderately scrambled faces and not to highly scrambled faces (cf. Young et al., 1985). It should be noted here that, on the basis of Young et al.3 definition, all of the scrambled face stimuli used in this experiment would be listed as moderately scrambled. The basic point of similarity between Experiment 1 and their study is that, as scrambled faces share more features in the appropriate locations for faces, so a face advantage becomes apparent in categorisation.

The ease of categorising highly scrambled faces relative to faces is of more interest than is immediately apparent. This is because it counters the argument that subjects simply have a generalised response bias to ‘face’ and therefore produce an artifactual face advantage over moderately scrambled faces. We can also rule out explanations that face categorisation was based on the serial self- terminating coding of simple facial features. By such a model, the order of classification, in terms of RTs, should have led to highly scrambled faces being classified faster then moderately scrambled faces, which in turn should have been classified faster than faces. This is simply because of the number of single feature checks that would have to be performed to categorise the stimuli accurately. Clearly this was not the pattern of data found. It should be noted however, that we cannot rule out the possibility that scrambled faces are categorised using a serial self-terminating strategy.

There remain two ways to explain the faster discrimination of faces over moderately scrambled stimuli. First, it is possible that subjects are sensitive to the configural properties of faces which are matched in parallel to a stored mental representation. If a configuration is not detected, then subjects might employ a serial self-terminating strategy which operates at a high cost to both RTs and errors when a stimulus is moderately scrambled but not when it is highly scram- bled. Second, discriminations might be based on one of two sources of information which are processed simultaneously. The first source is the information produced by matching features in parallel to a stored mental representation of a face and the second source is information produced by matching features in serial. As only faces have a stored mental representation to map onto, only faces could be categorised by matches resulting from the parallel system. A subject’s response might be based on a race between the two sources of information, with a response driven by the first system to reach a decision. From the data, it would seem that

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226 N. Donnelly et ul. /ACM Psychologica X.5 (1994) 219-234

this horse-race account is best supported. This is because if the serial self-terminat- ing search only began on the failure to find a face configuration, discriminations of highly scrambled stimuli should have been slower than discriminations of faces. There is no significant RT face advantage over highly scrambled faces.

We now turn to the additive relations between inversion and the face superior- ity effect relative to moderately scrambled faces. Additive factors logic (Sternberg, 1969) suggests that the effects of inversion occur independently of this face advantage. It follows that the process underlying the face advantage is the same whether faces are inverted or upright. For instance, the parallel matching of faces to stored mental representations might take place for inverted as well as for upright faces. If, for example, inverted faces could not be matched in parallel to a stored mental representation then inversion should have affected the categorisa- tion of faces more than that of moderately scrambled faces. This was not what was found. One way an additive result could have arisen would be if inversion degrades facial features being processed in parallel, perhaps because such parallel process- ing depends on the representation of the features relative to an upright frame of reference. Subsequent matching to a stored mental representation will be slowed. An alternative is that there is some form of mental rotation prior to categorisation taking place (Davidoff and Landis, 1990). For reasons we present in the General discussion we prefer the first alternative.

Although the effects of inversion on the face advantage relative to moderately scrambled faces were additive (e.g. the effect of inversion produced a 123 msec increment for faces and a 105 msecs increment on scrambled faces), there was a trend for a reduced inversion effect with highly scrambled faces (inversion pro- duced only a 57 msec increment on these stimuli). Similar results are also apparent in Experiments 2 and 3 here. This reduced inversion effect for highly scrambled faces fits with a horse-race account of performance. For instance, if parallel matching to stored face representations is slowed by inversion more than serial checking of face features, then any inversion effect will tend to be most pro- nounced on stimuli where performance is strongly determined by parallel match- ing, namely on faces and moderately scrambled faces.

Experiment 1 suggests that faces must be categorised by some mechanism which allows categorisation of faces via a parallel match to a mental representation of a face. Moreover, the categorisation of faces by matching in parallel to a stored mental representation seems possible for both upright and inverted presentations. In Experiment 2 we address the issue of whether parallel matching to a stored mental representation still occurs when faces are replaced by stimuli (‘object-faces’) with non-facial features placed in the normal locations for facial features. Further- more, we address the issue of whether the information used to categorise upright object-faces also serves when the stimuli are inverted.

4. Experiment 2

In all respects this was as for Experiment 1, except for certain stimulus changes. Davidoff (1986) noted how a face superiority effect (e.g. Homa et al., 19761 could

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N. Donnelly et al. /Acta Psychologica 85 (1994) 219-234 221

Fig. 2. Upper panel: Example face and scrambled face stimuli used in Experiment 2. Lower panel: Range of features used in Experiment 2. (Adapted from ‘The mental representation of faces: Spatial

and temporal factors’ by J. Davidoff, 1986, Perception and Psychophysics 40, Fig. 1. p. 392. Copyright

held by the Psychonomic Society. Reprinted by permission.)

still be found if facial features were replaced by objects. Subjects were informed that the eyes in faces were replaced by telephones, noses replaced by flowers and mouths replaced by cars (see Fig. 2). These features retained the basic dimensions and proportions of the facial features used in Experiment 1.

In Experiment 2, stimuli were constructed in the same way as for Experiment 1 but using the object features of Davidoff (1986).

4.1. Method and procedure

As for Experiment 1, subjects were asked to categorise the stimuli as faces or scrambled faces. Even though the stimuli did not contain facial features subjects learnt to perform the task very easily during the short block of practice trials provided.

4.2. Results

The results were analysed as in Experiment 1.

Table 2 Mean RTs(““‘~ ‘XL error1 and % correct scores for Experiment 2

Upright

Face 56344 (94)

Moderately scrambled 693” (95)

Highly scrambled 59852 (96)

Inverted

689h’ (98) 798’” (93)

657” (97)

Page 10: Stimulus factors affecting the categorisation of faces and scrambled faces

Reaction times The main effects of stimulus type and degree of orientation were both signifi-

cant (F(2,18) = 21.75 and F(l,Y) = 18.94 respectively, both ps < 0.01) (see Table 2). Moderately scrambled stimuli were discriminated slower than faces or highly scrambled stimuli, but there was no difference between faces and highly scrambled stimuli. Inverted stimuli were responded to slower than upright stimuli (126 ms vs. 105 ms vs. 59 ms for the faces, moderately scrambled faces and highly scrambled faces respectively).

Stimulus type and degree of orientation interacted (F(2,18) = 4.77, p < 0.05). Inversion slowed down the categorisation of faces, moderately scrambled and highly scrambled faces, however, this effect was less marked with highly scrambled faces.

Error3 The main effect of stimulus type just failed to reach significance (F(2,18) = 3.35)

and degree of orientation was not significant (F(1,9) = < 1). The interaction between stimulus type and degree of orientation also failed to reach significance

(F(2,lX) = 1.09).

4.3. Discussion

The results of Experiment 2 essentially replicated all aspects of Experiment 1. Firstly, there was no overall difference in either RTs or errors between face and highly scrambled stimuli, removing any possibility of a generalised bias to respond ‘face’. Secondly, faces were again categorised faster than moderately scrambled faces, showing that the use of object features instead of facial features did not inhibit faces being matched in parallel to a stored mental representation of a face (see also Davidoff, 1986). Thirdly, the advantage for true object-faces over moder- ately scrambled object-faces was additive with the effects of inversion.

Experiment 2 has demonstrated that facial features can be replaced by objects which share only the global dimensions of real facial features and faces will still be categorised by a parallel analysis of features. Moreover, faces are matched to a mental representation of a face in parallel whether upright or inverted. These data show that it does not matter particularly if physically accurate feature information is presented for both upright and inverted presentations. We conclude that precise facial feature information is not required for faces to be categorised as faces, both when they arc inverted and when they arc upright.

In Experiment 2, the effects of inversion were reliably less on highly scrambled object-faces relative to real and moderately scrambled object-faces. This same trend was not significant in Experiment 1, but it was clearly apparent nevertheless. It can be attributed to the categorisation of highly scrambled object-faces being dependent on serial feature-checking more than on parallel matching to a stored representation. Inversion affects parallel matching more than serial feature-check- ing, and hence exerts a stronger effect on real ‘object-faces’ and moderately scrambled ‘object-faces’, for which categorisation is more strongly dependent on parallel matching.

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N. Donnelly et al. /Acta Psychologica 85 (1994) 219-234 229

Given that faces are discriminated from moderately scrambled faces on the basis of processes that can be applied to both upright and inverted faces, it may be possible to begin to address the issue of what information underlies the parallel computation of a match to a stored mental representation. One likely candidate is the information that helps determine the orientation of a stimulus. In the present context, the main information determining stimulus orientation, aside from the orientation of local features, is the stimulus outline. It was noted earlier how the stimulus outline could help determine stimulus orientation by virtue of the location of the ears and neck relative to the head outline (see Figs. 1 and 2). In Experiment 3 we tested this by removing the outline.

5. Experiment 3

5.1. Method

As in the General method except that subjects performed the face/scrambled face categorisation task with two sets of stimuli. Items in set one were like those used in Experiment 1 and those in set two were like those used in Experiment 2 except that in both cases the stimulus outline was removed. The order in which the two tasks were performed was counterbalanced.

5.2. Results

The results were analysed as in Experiment 1, but with the additional factor of feature type (2: face vs. objects>.

Reaction times The main effects of stimulus type and orientation were both significant (F(2,181

= 19.69 and F(1,9) = 10.80 respectively, both ps < 0.01) (see Table 3). Faces and object-faces were responded to faster than highly scrambled faces and highly

Table 3 Mean RTSW’h \kl error, , and c/c correct scores for Experiment 3

Face features

Face

Moderately scrambled

Highly scrambled

Upright

531?7 (98)

672”” (96)

664j3 (93)

Inverted

656.77 (91)

749h’ (85) 634j’ (99)

Face Moderately scrambled

Highly scrambled

Object features

Upright

62@’ (98)

8097H (89) 708”s (97)

Inverted

740h’ (97)

82072 (89) 709”” (99)

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scrambled object-faces. Both of these stimulus types were responded to faster than moderately scrambled faces and object-faces. Inverted stimuli were responded to slower than upright stimuli. Type of feature was not significant (F(1,9) = 2.68).

Stimulus type and degree of orientation interacted (F(2,18) = 8.04, p < 0.01). Faces were categorised faster when upright than when inverted but the speed of categorisation for moderately scrambled and highly scrambled faces was unaf- fected by inversion.

No other interaction approached significance (all F(2,18) = < 1).

Errors

The main effect of stimulus type was significant (F(2,18) = 18.54, p < 0.01).

More errors were made to moderately scrambled faces than to either faces or highly scrambled faces. There was no difference in the number of errors made to faces and highly scrambled faces. No other main effect approached significance.

Stimulus type interacted with degree of orientation (F(2,18) = 43.05, p < 0.01). The only other interaction to approach significance was the type of feature, stimulus type and degree of orientation (F(2,18) = 2.76).

5.3. Discussion

The absence of a stimulus outline affected subjects’ performance in important ways (cf. Fraser and Parker, 1986). Removing the outline from both moderately scrambled faces and highly scrambled faces reduced any effects of inversion on their categorisation (and indeed, any effects were eliminated with highly scrambled faces; see Table 3). We can conclude from this that in Experiments 1 and 2, the facial outlines provide a cue to a frame of reference for scrambled faces within which to perform a serial self-terminating search that we infer is involved in rejecting scrambled faces. For instance, the frame provided by the face outline may help direct the order of the search process (e.g. always starting at the top of the head, and thus at the bottom of an inverted stimulus). Removing the outline leads to a common search pattern being adopted for upright and inverted stimuli.

In contrast to the null effect of inversion on highly scrambled faces, there was a reliable inversion effect on faces. Indeed, this effect did not differ in size relative to that observed in Experiment 1 (for faces) or 2 (for object-faces) (F < 1.0, for the relative inversion effects comparing across the relevent experiments). This suggests that the external features of faces are not necessary to determine the orientation of any frame of reference used to match facial features in parallel to stored face representations. Rather it seems to be based on the relative positions of the internal features. Providing the internal features are in the correct relative posi- tions (whether upright or inverted), parallel face-matching seems to be induced. This is so even if categorisation is relatively slowed by faces being inverted.

Responses to moderately scrambled faces seem to involve a mixture of parallel matching and serial feature-search trials. When the external feature cues to orientation are removed, serial search, unconstrained by the frame of reference, may determine performance more strongly than parallel matching. For instance,

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the search need no longer start at the top of the head, and so encounter a partial match when the eyes are met. This would reduce any effects of inversion on performance, since RTs would be determined more by the serial search than by the parallel matching process.

6. General discussion

Experiments 1-3 have shown that faces are categorised using a different procedure from that used with scrambled faces in a face/scrambled face categori- sation task. For instance, face categorisation tends to be more strongly affected by inversion than the categorisation of highly scrambled faces (Experiments 1 and 2). This difference is particularly strong when external features are removed from faces and scrambled faces (Experiment 3). In addition, faces are categorised faster than moderately scrambled faces. Therefore a serial self-terminating search of simple facial features cannot account for face categorisation, though it can account for the categorisation of scrambled faces. The data suggest that the mechanisms that allow ‘face categorisation are slowed but not altered by inversion and are unaffected by the presence or absence of external facial features. For scrambled faces however, the process of categorisation is affected by the orientation indicated by the presence of external features. In fact, this orientation information may sometimes be detrimental to performance, slowing categorisation times because the search may then begin with features (e.g. at the top of the head) that do not discriminate scrambled faces from faces (Experiment 3). Removing the external features can improve performance, relatively, with inverted stimuli.

It should be noted at this point that there are quite large variations in baseline RTs between experiments and it might be thought that the differences between the conditions are simply the result of chance variation. In fact, analyses across the appropriate conditions of Experiments 1 and 3 and 2 and 3 reveals that only the faces in Experiment 1 are responded to significantly slower than the faces in Experiment 3. Nevertheless to the extent that there is variation in baselines between experiments we believe it does not represent a problem for the interpreta- tion of the data. We suggest that it is more important to emphasize the consistency of the size of the inversion effect across experiments. For example, inversion leads to an increased RT for real faces in Experiments 1 and 3 of 123 ms and 12.5 ms respectively and with object features in Experiments 2 and 3 of 126 ms and 112 ms respectively. We maintain that such consistency, even with some baseline shift, represents a reliable finding. In contrast, whilst the effect of inversion for both moderately and highly scrambled faces is consistent across Experiments 1 and 2 (105 ms vs. 105 ms and 57 ms vs. 55 ms respectively), it was reduced in Experiment 3 (71 ms vs. -30 ms and 11 vs. 1 for facial features and object features respectively). Inversion effects are reduced selectively for scrambled faces in Experiment 3.

These differences between the categorisation of real and scrambled faces suggest that at least two mechanisms can mediate categorisation decisions: parallel

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matching of features to stored face representations, and serial checking of individ- ual features. Parallel matching of features largely determines the categorisation of real faces. When faces are inverted, the features processed in this way seem to be degraded, slowing categorisation performance. Serial checking of features may take place simultaneously, and it can lead to relatively quick rejection of highly scrambled faces.

Given our supposition that face categorisation depends on parallel matching of features to stored mental representations, and that this process operates also with inverted faces, we may ask why the effects of inversion emerge. One possibility is that ‘face templates’ exist that enhance the processing of features encoded in parallel, but that they exist only for upright faces. When faces are inverted, and so fail to match the templates, the features are degraded. A good account of what we mean by the degradation of facial features comes from the work of LaBerge and his colleagues (LaBerge, 1983; LaBerge and Brown, 1989) on word recognition. They have argued that with real words visual attention is distributed across the stimulus to enhance feature processing relative to other locations which flank the stimulus. Hence, words are attended to rather than letters but non-words are attended to as sequences of letters. The difference between words and non-words occurs because the attentional system attempts to match everything passed through a filter to a stored representation and in the example from the work of LaBerge and his colleagues we have stored representations of words but not non-words. In a similar manner, it could be that upright face templates serve this purpose in face categorisation as stored representations of words serve in word recognition.

Interestingly, the kind of templates that might be involved seem to be coded relatively coarsely. In Experiment 2 here, changing the nature of the internal features (from facial features to objects) failed to affect performance fundamen- tally. This suggests location rather than the identity of facial features is important for categorisation. This proposal is supported by other findings in the literature. For instance, the Margaret Thatcher illusion is dependent on the features in the distorted face maintaining their appropriate relative locations, even though their local orientations are altered in the upright and inverted faces. Maintenance of the relative positions of the features seems sufficient for the visual system to categorise the inverted distorted features as being those of a normal face and thereby invoking procedures to encode further the relationships present in the face (Davidoff and Donnelly, 1990). Further evidence for categorisation decisions being made from a coarse visual code comes from cases of prosopagnosia where some prosopagnosics seem able to encode the parts of objects, but they seem relatively insensitive to the identities of the parts occupying those positions (e.g. Humphreys ct al., 1992). Interestingly, such patients remain able to categorise faces as faces. We suggest that this is because face categorisation is dependent on the rapid and spatially parallel assimilation of facial features onto a coarsely coded upright face template. In the terms introduced by Pomerantz (19831, the template codes type P (position) but not type N (identity) information about the local features present.

The results from Experiment 3, that face categorisation is unaffected by the presence of outline features, meshes with studies on face identification. Identifica-

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N. Donnelly et al. /Acta Psychologicu X5 (1994) 219-234 233

tion studies have shown that familiar faces are identified using internal rather than external features (Ellis et al., 1979). This may be because internal features induce a rapid categorisation process and activate a face template leading to enhanced processing of internal relative to external features.

The coarse coding of visual input to enable a match to a face template and the lack of importance of outline information for faces may explain another aspect of the data from Experiments 1 and 3 when face features are used. As stated earlier, we think it is likely that the increased error rate on inverted moderately scrambled faces is caused by certain feature switches (e.g. eyes-mouth) leading to confusions with upright face stimuli. In other words it might be that switches of the eyes and mouth in an inverted moderately scrambled face lead to the stimulus being similar to an upright face. For this to be true, subjects would have to be performing the categorisation task by taking no account of either the orientation of the outline feature or of the local features. In other words an inverted moderately scrambled face might match to a face template if, firstly, the orientation of the outline feature is ignored and, secondly, if perturbations in the local relationships between the nose and the eyes and mouth (caused by the local orientations of features) do not affect the match to a ‘face-template’. This second fact is exactly what happens in the Margaret Thatcher illusion (Thompson, 1980).

The idea that true faces are discriminated using different mechanisms to scrambled faces can be found elsewhere in the existing literature. Firstly, Purcell and Stewart (1986) claimed that the detection threshold for faces is reduced compared to that from scrambled faces. While contrary findings to this have been reported (Boucart and Bonnet, 1990), a face detection effect would certainly not be inconsistent with the present findings. Secondly, work using single cell record- ings with monkeys and with the recording of evoked potentials from humans shows evidence for the rapid categorisation of faces and concur that cells in the temporal cortex are sensitive to presentations of faces, but not scrambled faces (Perrett et al., 1988; Jeffreys, 1989).

References

Bodamer, J.. 1947. Die Prosopagnosie. Archiv fiir Psychiatric und Zeitschrift fiir Neurologie 179. 6-54.

Boucart. M. and C. Bonnet, 1990. Only stimulus energy affects the detectability of visual forms and

objects. Bulletin of the Psychonomic Society 28. 415-417. Davidoff, J.. 1986. The mental representation of faces: Spatial and temporal factors. Perception and

Psychophysics 40. 391-400.

Davidoff. J. and N. Donnelly, 1990. Object superiority effects: Complete versus part probes. Acta Psychologica 73, 225-243.

Davidoff, J. and T. Landis, 1990. Recognition of unfamiliar faces. Neuropsychologia 28. 1141-I 161.

Diamond, R. and S. Carey, 1986. Why faces are not special: An effect of expertise. Journal of Experimental Psychology: General 115, 107-177.

Ellis, H.D., J. Shepherd and G.M. Davis, 1979. Identification of familiar and unfamiliar faces from

internal and external features: Some implications for theories of face recognition. Perception 8. 431-439.

Page 16: Stimulus factors affecting the categorisation of faces and scrambled faces

234 N. Dortnrlly 1’1 al. /Actu P,sychologica 8S (1994) 219-234

Fraser, I.H. and D.M. Parker, 1986. ‘Reaction time measures of feature saliency in a perceptual

integration task’. In: H. Ellis, M. Jeeves, F. Newcombe and A. Young (Eds.), Aspects of face processing. Dordrecht: Martinus Nijhoff.

Homa. D.. B. Haver and T. Schwartz. 1976. Perceptibility of schematic face stimuli: Evidence for a perceptual Gestalt. Memory and Cognition 4. 175-185.

Humphreys. G.W., M.J. Riddoch, P.T. Quinlan, C.J. Price and N. Donnelly, 1992. Parallel pattern

processing and visual agnosia. Canadian Journal of Psychology 46, 377-416.

Jeffreys, D.A., 1989. A face-responsive potential recorded from the human scalp. Experimental Brain Research 78, 193-202.

Jolicoeur, P.. 1985. The time to name disoriented natural objects. Memory and Cognition 13, 2899303. LaBerge, D., 1983. The spatial extent of attention to letters and words. Journal of Experimental

Psychology: Human Perception and Performance 9, 371-349. LaBerge, D. and V. Brown. 1989. Theory of attentional operations in shape identification. Psychologi-

cal Review 96. 101-124.

McMullen, P.A. and P. Jolicoeur, 1990. The spatial frame of reference in object naming and discrimination of left-right reflections. Memory and Cognition 18, 99-115.

Perrett. D.I.. A.J. Mistlin, A.J. Chitty, M.H. Harries, F. Newcombe and E.H.F. de Haan. 1088.

‘Neuronal mechanisms of face perception and their pathology’. In: C. Kennard and F. Clifford Rose

(Eds.). Physiological aspects of clinical neuro-opthalomology. London: Chapman and Hall.

Pomerantz, J.R., 1983. Global and local precedence: Selective attention in form and motion perception. Journal of Experimental Psychology: General 112. 516-540.

Purcell, D.G. and A.L. Stewart. 1986. The face-detection effect. Bulletin of the Psychonomic Society 24, 1 IX-120.

Rhodes, G.. S. Brake and A. Atkinson. 1993. What’s lost in inverted faces. Cognition 47. 25-57.

Rock. I., 1974. The perception of disoriented figures. Scientific American 230, 78-85.

Sergent, J., 1984. An investigation into component and configural processed underlying face perception.

British Journal of Psychology 75, 221-247.

Sergent, J. and M. Corballis, 1989. Categorisation of disoriented faces in the cerebral hemispheres of

normal and commissurotomized subjects. Journal of Experimental Psychology: Human Perception

and Performance 15. 701-710.

Sternberg, S., 1969. The discovery of processing stages: extensions of Donder’s method. Acta Psycholog-

ica 30, 276-315.

Thompson. P.. 1980. Margaret Thatcher: A new illusion. Perception 9, 483-484.

Valentine, T.. 1988. Upside-down faces: A review of the effect of inversion upon face recognition. British Journal of Psychology 79. 471-491.

Valentine. T,, 1991. A unified account of the effects of distinctiveness. inversion and race in face

recognition. Quarterly Journal of Experimental Psychology 43A. 161-204.

Young. A.W.. D.C. Hay and K.H. McWeeny. 19X5. Right cerebral hemisphere superiority for construct-

ing facial representations. Neuropsychologia 23. 195-202.