method, findings, and theory in studies of visual masking · masking and the contributions of...

Psychological Bulletin1968, Vol. 70, No. 6, 404-425

METHOD, FINDINGS, AND THEORY IN STUDIES OFVISUAL MASKING1

DANIEL KAHNEMAN 2

Center for Cognitive Studies, Harvard University

The various paradigms in the study of visual masking are classified and re-lated to cases of interference among cotemporaneous stimuli. The dependentvariables in masking studies are described. A distinction between criterion con-tent and criterion level is introduced in the discussion of detection under mask-ing and metacontrast. Various conceptions of identification of forms undermasking and the contributions of masking effects to the study of psychologicaltime are reviewed.

The study of visual masking is currentlyone of the most active fields of experimentalpsychology; more studies of visual maskinghave appeared since the most recent reviewof the field (Raab, 1963) than were cited inthat review. Although the major theoreticalissues in visual masking are yet to be resolved,an additional review may be timely. Thepresent review is divided into five main parts.The first introduces terms and classifies vari-ous paradigms in the study of masking. Thesecond part is concerned with measurements ofthe strength of masking effects. The thirdand fourth, respectively, review the effects ofmasking on the detection of flashes and on theidentification of forms. Finally, theoreticalinterpretations of masking are briefly re-viewed.

Coverage of the literature has been selec-tive, and no attempt was made to compile acomprehensive list of titles in the field. Addi-tional references may be found in Raab(1963), where masking effects in othermodalities are also described. Alpern (1952)

1 Part of the content of this paper was presentedat the Conference on Temporal Factors in Vision,conducted by the Center for Visual Science at theUniversity of Rochester in June 1966. Preparationof the manuscript was supported in part by theNational Science Foundation, Contract No. GS-1153to Harvard University, Center for Cognitive Studies,and in part by the National Institute of MentalHealth, Grant No. 1 P01-MH 12623, to HarvardUniversity, Center for Cognitive Studies. An ex-cellent review of the field of masking (Norman,196S) facilitated the author's task. The very detailedcomments of D. Aaronson, R. M. Boynton, andJ. Norman are acknowledged with special thanks.

2 Now at the Department of Psychology, HebrewUniversity, Jerusalem.

reviewed early work on metacontrast, andSperling (1965) provides a comprehensive dis-cussion of the detection of light flashes undermasking.

PARADIGMS OF VISUAL MASKING

Terminology

The term "visual masking" appears to havebeen introduced by Pieron (1925) and wasrevived by Boynton and Kandel (1957). Itcovers the class of situations in which somemeasure of the effectiveness of a visual stimu-lus (the test stimulus, TS) is reduced by thepresentation of another (the masking stimu-lus, MS) in close temporal contiguity to it.

1. When MS follows TS, the situation isone of backward masking. Forward maskingis the case when MS precedes TS.

2. When TS and MS do not spatially over-lap, the cases of backward and forward mask-ing are, respectively, termed metacontrast andparacontrast (Stigler, 1910). However, theterm "metacontrast" is often applied gener-ically to masking by a nonoverlapping figure,regardless of the temporal order of TS andMS.

3. In the case of spatial overlap betweenTS and MS, a distinction is drawn betweenmasking by light and masking by pattern(Sperling, 1964). In masking by light, MSconsists of a flash of homogeneous illuminationover an area that completely contains thecontours of TS. The pattern in masking bypattern may be regular (Schiller & Wiener,1963), or else it may consist of a randomarray of white and dark areas (Sperling,1963). Kinsbourne and Warrington (1962a,

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METHOD, FINDINGS, AND THEORY IN STUDIES OF VISUAL MASKING 405

1962b) termed the latter condition "maskingby visual noise," and that usage is followedhere.

The presentations of TS and MS are usuallyseparated by an interval. The interstimulusinterval (ISI) is the interval between the endof the first stimulus and the onset of thesecond. Stimulus-onset asynchrony (SOA;Kulli, 1967) is used in the present paper torefer to the interval between the onsets ofthe two stimuli.

Some idiosyncratic usages should be men-tioned. "Conditioning stimulus" has beenused for MS by several investigators (e.g.,Battersby & Wagman, 1959; Boynton &Kandel, 1957), following Crawford (1947).In backward masking, of course, TS actuallyprecedes the "conditioning stimulus" thataffects its detectability.

The term "erasure" was introduced byAverbach and Coriell (1961) to describe back-ward masking of a letter (black on white) bya black ring surrounding its position. In thepresent terms, the paradigm is one of meta-contrast. More recently, Alpern and Rushton(1965) used "after-flash effect" to denote therise of absolute thresholds under conditionsof metacontrast.

Mayzner and his colleagues (Mayzner,Tresselt, Adrignolo, & Cohen, 1967; Mayzner,Tresselt, & Cohen, 1966; Mayzner, Tresselt,& Heifer, 1967) have studied a phenomenonthat they term "sequential blanking." Theletters that make up a word are successivelypresented in their appropriate positions on acomputer-controlled oscilloscope face, and thedisplay is repeated indefinitely. With certainsequences of letter exposures, some of theletters are not seen at all, whereas the othersremain relatively steady in the field; the wordCHAIR may be seen as C A R, and thispercept is maintained as long as the stimulussequence is shown. The sequential blankingeffect is treated in the present paper as aspecial case of metacontrast, since the inter-acting elements of the pattern appear inadjacent areas.

Lindsley (1961; Lindsley & Emmons, 1958)had previously introduced the term "blanking"for the masking of a patterned stimulus by aflash of light. He also used the highly sug-gestive "clearing time" to denote that interval

between onsets of TS and MS beyond whichmasking is no longer found. The temporalmasking function where TS threshold isplotted as a function of SOA has been calledan "on-response" (Boynton & Kandel, 1957;Boynton & Siegfried, 1962; Onley & Boynton,1962). The absence of any reference to mask-ing in the titles of Boynton's papers probablycaused them to be missed by many potentialreaders.8

Limiting Cases of Masking Paradigms

In the general limiting case of visual mask-ing, TS and MS are presented together for anindefinite period. Different limiting subcasesillustrate the great variety of situations thatare grouped under the common label of mask-ing:

1. The metacontrast and paracontrast para-digms reduce to conditions of simultaneouscontrast or to contour interference, dependingon whether the effect under study is a changeof apparent brightness or a reduction ofacuity for TS.

2. When TS is a flash of light and MS isan overlapping larger flash, their simultaneouspresentation is appropriate to a study ofbrightness discrimination.

3. When TS is a form and MS is a field ofhomogeneous illumination, their simultaneouspresentation reduces figure-ground contrastfor TS.

4. When TS is a form and MS is a fieldof visual noise, their joint presentation resultsin a general degrading of the image.

Masking paradigms are related to other de-signs in which visual stimulation is systemati-cally varied over time. Thus, temporal inte-gration may be viewed as a special case ofmasking—in which TS and MS are identicaland in which the usual result is facilitationrather than interference.4

Experimental conditions are identical instudies of forward masking by light and in

3 Psychological Abstracts, for example, failed tosee the connection, and do not list this work under"masking."

4 The Broca-Sulzer effect, in which a longerstimulus is less effective than a shorter stimulus ofequal luminance (Boynton, 1961; Raab, 1962), isnot discussed in this review.

406 DANIEL KAHNEMAN

investigations of the onset of dark adaptation.It was in a study of light adaptation thatCrawford (1947) discovered the "Crawfordeffect" of backward masking of a flash by asubsequent larger and more intense stimulus.Baker's work on masking (e.g., Baker, 1953,1955, 1963) is consistently presented as astudy of light and dark adaptation, and Bat-tersby and Wagman (1959), as well as Boyn-ton and Miller (1963), have used these terms.

Finally, the stimulus conditions which pro-duce metacontrast are closely related to thosewhich produce apparent motion (Fehrer,1966; Fehrer & Smith, 1962; Kahneman,1967b; Mayzner, Tresselt, & Cohen, 1966;Mayzner, Tresselt, & Heifer, 1967; Toch,1956). For example, apparent motion is seenwhen a pattern is shown first in one locationand then in another; metacontrast suppres-sion is obtained when the original pattern issimultaneously repeated in two different loca-tions on the second exposure.

There is an important theoretical conclusionto be drawn from the list of associations be-tween masking phenomena and other visualeffects. The variants of masking almost cer-tainly involve different mechanisms, as theirlimiting cases do; it is, therefore, highly un-likely that a single theoretical concept canaccount for the different cases of masking.

MEASUREMENT OF MASKING EFFECTS

The present section reviews dependent vari-ables which have been used as measures ofvisual masking.

Duration Thresholds

Duration thresholds have sometimes beenused in the measurement of metacontrast(e.g., Fehrer, 1966; Kolers, 1962) and mask-ing (e.g., Schiller, 1965b). The independentvariable in these studies was ISI, and dura-tion thresholds were plotted as a function ofthis variable.

When the duration of TS is varied, withISI constant, the SOA between TS and MSis completely confounded with the durationvariable. The implicit assumption in this de-sign is that ISI is the main determinant ofmasking effects, but the evidence indicates thatSOA is the more important variable in both

masking by light (Donchin, 1967) and meta-contrast (Kahneman, 1967b). The confound-ing of SOA with duration is, therefore, un-desirable.

Luminance Thresholds

In the standard design for investigations ofthe masking of light flashes, luminance thresh-olds for TS are determined as a function ofSOA (e.g., Baker, 1963; Boynton, 1961;Sperling, 1965). Luminance thresholds havealso been measured for the detection of aflash masked by metacontrast (Alpern, 1965;Alpern & Rushton, 1965). Schiller and Smith(1965) determined luminance thresholds forthe identification of letters under several con-ditions of masking.

The term "masking function" is used in thepresent paper for the display of threshold TSluminance against SOA (see Figure 1). De-tails of the shape of this function have beenof considerable theoretical interest. The peakof the masking function defines the maximalmasking effect of a particular MS.

In the masking function, TS luminance isplotted against the physical SOA betweenTS and MS. However, it should be noted thatconduction speed is usually a function ofstimulus intensity. Consequently, the asyn-chrony between the arrival times of the twosensory messages to the locus where they in-teract is not necessarily the same as SOA(Baker, 1963; Crawford, 1947; Donchin,1967). Variations of the luminance of TS andMS presumably affect the effective asynchronybetween them and, thereby, introduce a milderversion of the complexities which were previ-ously described in the context of durationthresholds (Boynton & Siegfried, 1962).

Probability of Success

Many experimenters have displayed mask-ing effects by plotting probability of detectionor identification of a target against SOA orISI. The method is very often used in studiesof the masking of letters and words. There isno confounding of dependent and independentvariables in this method. However, it suffersfrom the important drawback that the prob-ability variable is sensitive only over a narrowrange. The rule of thumb, in many visual situ-


ations, is that detection of a target varies fromchance to absolute certainty with a tenfoldvariation of target luminance. By the normalstandards of visual experimentation, this is asmall range. One study of letter recognition inwhich luminance thresholds were determined(Boynton & Miller, 1963) included conditionswhere the TS threshold varied by a factor of10,000. Thus, any masking effect which canbe measured by probability of success wouldbe considered minute by most visual scientists.

The physical characteristics of TS, includ-ing size, luminance, and duration, are usuallykept constant or treated as parameters whenmasking is measured by probability of suc-cess. Other variables, such as ISI or theluminance of MS, are manipulated. The in-vestigator faces the problem of selecting tar-gets for which probability of success will beneither as high as 100% nor as low as chanceperformance over the range of conditionswhich are to be studied. Normally, differentsubjects require different targets if this re-quirement is to be met.

In Eriksen's laboratory, the difficulty ofthe identification task is equated for differentsubjects by manipulating the duration of TS.For each subject a duration is found at whichhe is successful on, say, 80% of the trials inthe absence of masking. The duration of TSis maintained at that value under all condi-tions of masking, and the probability of suc-cess is most often plotted as a function of ISI(e.g., Eriksen & Collins, 1964; Eriksen &Lappin, 1964; Eriksen & Steffy, 1964). Inthis procedure, different subjects experiencedifferent values of SOA in precise correspon-dence with their original duration thresholds.The variable of duration is ignored whenprobability of success is finally displayedagainst ISI. However, Eriksen always ascer-tains that the variations of treatment for thedifferent subjects have no effect by testing thesignificance of the Subject X ISI interaction.

Kahneman (1966) used a similar procedurein a study of the effects of masking on acuity,but he adjusted target size rather than dura-tion of exposure to make the task equally dif-ficult for different subjects. The likelihood ofundesirable confounding may be somewhatsmaller in this case than when exposure dura-tions are allowed to vary.

Brightness Matching

Two studies measured metacontrast effectsin terms of the reduced brightness of TS.Alpern's subjects (1953) adjusted the lumi-nance of TS to make its brightness match astandard. Such a procedure introduces majorvariations in the strength of the masking ef-fect that is ostensibly under study (Fehrer &Smith, 1962). Schiller and Smith (1966) usedthe more appropriate method of manipulatingthe luminance of a comparison stimulus tomatch the changing brightness of TS at differ-ent values of SOA. However, they encounteredanother difficulty; in the most critical rangeof masking, TS is not seen, and its brightnesscannot be judged.

Rating Methods and Phenomenological Re-ports

Ratings have recently been used in studiesof metacontrast. Blanc-Garin (1965) obtainedratings of apparent contrast for a black targetwhich was masked by metacontrast. A differ-ent rating method has been introduced byKahneman (1967b). A perceptual effect, suchas metacontrast or apparent motion, is de-scribed to the subject in some detail. Thesubject is then shown various displays, andhe rates each of them for their correspondenceto the effect that was described to him. Thework of Mayzner's group, most recently re-viewed by Mayzner, Tresselt, and Heifer(1967), relies on a particularly simple typeof phenomenological report. A sequence ofadjacent symbols is repeatedly presented ona computer-controlled scope face, and the ob-server merely reports which symbols he cansee. Different observers show essentially per-fect agreement in their reports, and thesevary systematically with such parameters asexposure duration, SOA, and the order of ap-pearance of the symbols.

Acuity Measures

Visual acuity, the ability to resolve detailsof form, is the single most important dimen-sion of visual effectiveness. Acuity is opera-tionally defined by the minimal size of detailthat can still be resolved. Standard tests ofacuity are available which are equally ap-plicable to the calibration of stimulus condi-

408 DANIEL KAHNEMAN

tions and to the measurement of individualdifferences (Boynton, 1962; Luckiesh, 1944).

Since masking operations reduce subjects'ability to recognize forms, it is true by defini-tion that these procedures reduce visual acuity.However, because the reduction of acuity wasnot explicitly recognized as such, maskingeffects have never been assessed in terms ofcritical size.

The failure to conceptualize the masking ofform as a reduction of acuity has had an im-portant methodological consequence. It is stan-dard practice for each investigator to selectthe targets that are to be resolved under mask-ing and, thus, construct his own test of acuity.Consequently, results obtained in differentlaboratories are often difficult to compare. Inassessing the effects of a masking ring on letterrecognition, for example, one laboratory hasused the letters A, T, and U, presented at lowenergy (Eriksen & Collins, 1965; Eriksen,Collins, & Greenspoon, 1967); while anotherlaboratory (Weisstein, 1966; Weisstein &Haber, 1965) used the letters O and D, pre-sented at a higher stimulating energy. The twoconditions can hardly be considered equivalenttests of acuity.

The technical difficulties of preparing alarge number of stimuli which differ only insize may discourage the investigator fromusing target size as the dependent variable instudies of masking. However, it is sometimesadvisable to use a standard acuity target forall conditions and to measure masking bysome variable other than critical size. Boyn-ton and Miller (1963) used the Sloan-Snellenletters and measured the contrast thresholdfor correct identification of the letters undervarious masking conditions. Kahneman (1966)used a Landolt ring and determined the prob-ability of successful recognition with andwithout masking. The use of a standard acuitytarget, such as the Landolt ring or the SnellenE, has an important advantage. It providesan immediately interpretable index of theseverity of the basic viewing conditions. Werecognize at a glance that the conditionswhich produce an acuity of .04 are extremelysevere, whereas conditions are optimal if anacuity of 1.5 is attained. The present customof reporting letter size and viewing distancein inches is hardly a satisfactory substitute,

as it precludes adequate comparisons amongstudies.

DETECTION OF TARGET FLASHESUNDER MASKING

Masking by Light

The masking junction. A standard designhas been used with few modifications in alarge number of studies of flash detectionunder masking. TS is a brief flash which il-luminates a small circular patch. MS is amore intense flash, illuminating a larger area.The two flashes are often superposed on alarger adapting field. Detection thresholds areobtained for TS at a series of values of SOA,almost always by shortcut psychophysicalmethods. The parameters that are most oftenvaried in studies of masking are the lumi-nances of MS and the adapting field (Baker,1953,1955,1963; Battersby & Wagman, 1959,1962; Crawford, 1947; Onley & Boynton,1962; Sperling, 1965; Wagman & Battersby,1959). (For additional references, see Sper-ling, 1965.)

Figure 1 shows a schematic masking func-tion which illustrates the main features thathave been reported from many experiments inthe past. When TS precedes MS by 100-150milliseconds, its threshold is paradoxicallylowered. The phenomenon of "backward sensi-tization" has been described by Sperling(1965); backward sensitization is also ap-parent in the data of Boynton and Miller

-ZOO 0

STIMULUS

400 600

ASYNCHRONY

Fio. 1. Schematic masking function. The thresholdfor detection of a brief TS is plotted against SOA.The duration of MS is 500 milliseconds. Plottedvalues are characteristic but arbitrary.


(1963). When TS precedes MS by less than50-100 milliseconds, the threshold for its de-tection rises steeply. This is backward mask-ing, also known in this context as the Craw-ford effect. Masking effects are usually mostsevere at SOA = 0, and the threshold for de-tection of TS gradually declines with furtherincreases in SOA, until it finally reaches astable light-adapted value (Baker, 1963). Aslight rise in threshold is sometimes reportedwhen TS just precedes the termination of MS,or coincides with it (Battersby & Wagman,1962).

Masking as a measure of on-response. Boyn-ton (1958, 1961; Boynton & Kandel, 1957;Boynton & Siegfried, 1962; Onley & Boyn-ton, 1962) was impressed by the similaritybetween the masking function shown in Figure1 and the temporal course of the on-dischargeto a sudden increase of illumination. In hisview, MS occasions a massive discharge; thevisual system is overloaded by this responseand, therefore, fails to respond to the smallerand weaker TS. The masking function (Figure1) is a measure of the on-response to MS.Baker (1955, 1963) has outlined a similarposition.

In an elegant derivation from this theory,Boynton (1958; Boynton & Kandel, 1957)reasoned that preadaptation to light shoulddeplete the store of available photochemicalmaterial and, thereby, reduce the on-responseto MS and its ability to interfere with TS.Thus, a high state of light adaptation shouldresult in a lower threshold for the detectionof TS under masking. This prediction wasconfirmed. The luminance threshold for TSat SOA = 0 was the measure of the on-re-sponse (to MS) in these analyses. Thresholdswere obtained under three conditions:

BO = threshold for TS under dark adapta-tion, without masking;

Bp = threshold for TS under light adapta-tion, without masking; and

Bmp = threshold for TS under light adapta-tion, with masking.

The net masking effect of MS, with adapta-tion effects removed, is: Bm = Bmp (B0/BP),where the expression within parentheses isassumed to measure the reduction of visualsensitivity by light adaptation. Related sub-

traction procedures were employed by Bat-tersby and Wagman (1959) and by Sperling(1965).

In subsequent studies of the on-response,Boynton and Siegfried (1962) showed essen-tially identical masking effects of two briefstimuli that differed in luminance and dura-tion, but were equal in total energy. The twoon-responses differed only in latency; theresponse to the briefer and more intense stimu-lus was faster. Onley and Boynton (1962)used different levels of light adaptation tovary the apparent brightness of the maskingflash independently of its luminance. Theyconcluded that masking flashes which areequal in brightness often have identical mask-ing effects.

Sperling (1965) defines the masking effectas (Bmp —Bp) . In several sets of data, includ-ing those of Onley and Boynton (1962),Weber's law applies. The difference (Bmp - Bp)is linearly related to the energy of MS and isindependent of the adapting luminance onwhich TS and MS are superimposed.

Contour effects. Battersby and Wagman(1959) used a subtraction procedure to esti-mate the relative contribution of photochemi-cal and neural factors to the masking effect.They agreed with other investigators (Baker,1963; Boynton, 1961; Sperling, 1965) thatmasking effects are mainly neural, but wentfurther than others in suggesting that muchof the interaction of TS and MS may occurat a central locus. The suggestion is sup-ported by the occurrence of backward mask-ing at SOAs of 50-100 milliseconds. At theseSOAs the sensory message elicited by TS hasalready reached the cortex when MS is shown.Further support for central effects in maskingis derived from studies of dichoptic masking,where TS is shown to one eye and MS to theother (Battersby, Oesterreich, & Sturr, 1964;Battersby & Wagman, 1962; Boynton, 1961;Wagman & Battersby, 1959). Dichoptic mask-ing is pronounced only when the contours ofTS and MS are adjacent, which also definesthe condition for severe binocular rivalry(Kahneman, 1967a). The evidence indicatesthat dichoptic interaction is almost exclu-sively a contour effect. Adjacent contours alsofavor masking in the monoptic case, but lessdramatically.

410 DANIEL KAHNEMAN

Robinson (1966) has reported a phenome-non of disinhibition in which contour processesare probably involved. TS is a .23° circularpatch, 5 millilamberts in luminance and 20milliseconds in duration. MS, which followsafter an ISI of 25 milliseconds, is a circularpatch of similar luminance and duration, .46°in diameter. TS is never seen. However, whenMS is followed in turn by a larger flash (.92°diameter), the first circle is seen on 80% oftrials, and the second is never seen! The dis-inhibition effect was subsequently confirmedby Dember and Purcell (1967).

Observer's criterion and the masking junc-tion. Sperling (1965) offers an analysis of themasking effect which illustrates the centralimportance of the observer's criterion, al-though Sperling himself does not emphasizethe term. An explication of the concept ofcriterion may facilitate the discussion ofSperling's views and will be useful later in thepresent author's description of metacontrast.

The term "criterion" has at least two dis-tinct meanings. Specification of criterion levelanswers a quantitative question: How reluc-tant is the subject to give a particular re-sponse? "Criterion" in this sense is used in de-tection theory. Experiments within that theo-retical framework have repeatedly shown thatcriterion level may be made to vary over awide range by suitable manipulations of pay-offs and expectancies (Green & Swets, 1966;Swets, 1964). When the subject is allowedonly two responses, "yes" and "no," the cri-terion is said to be low if "yes" responses arerelatively frequent. In specifying criterion con-tent, one answers such questions as: "Canthe subject maintain a stable criterion injudging the brightness of flashes that vary induration?" or "What criterion does the sub-ject apply when he reports a failure to see TSin a metacontrast display?" These questionsconcern phenomenology. They request a fullerdescription of the code that the subject usesin mapping his private experience onto re-sponses to the experimenter's questions.

Detection theory has emphasized the labilityof criterion level and has generally ignoredthe issue of criterion content. However, animportant fact must be noted: in many situa-tions, different subjects apparently gravitatetowards the use of criteria that are highly

similar in both content and level, and eachsubject appears capable of maintaining sucha criterion over many sets of observations. Inthe following discussion, a criterion that meetsthe requirements of within-subject reliabilityand intersubject agreement is called a naturalcriterion.

The data reviewed by Sperling (1965) dem-onstrate that a natural criterion exists instudies of the masking of one flash by another.Different observers in different laboratories,all using yes-no responses as a semantic indi-cator (Goldiamond, 1958), provide essentiallyidentical masking functions (Baker, 1963;Battersby & Wagman, 1959; Crawford, 1947;Onley & Boynton, 1962). Further, the levelof this natural criterion is low: Sperling(1965) confirmed this conclusion by requiringconfidence ratings in series of trials, includingcatch trials on which TS was not shown.However, the content of the criterion appearsto vary systematically with SOA in the mask-ing experiment.

Sperling's analysis of the masking functionin Figure 1 relies heavily on hypothesizedvariations of criterion content. Thus, thethreshold is highest at SOA = 0, because theobserver is forced to rely exclusively onspatial contrast between the brightness of thetarget, which is illuminated by both TS andMS, and that of the surround, which is il-luminated by MS alone. However, the subjectis capable of using temporal contrast as wellwhen MS precedes TS: the central area in-creases in brightness when TS is superimposedon it, then becomes dimmer again. This ad-ditional information leads to lower thresholds.The backward masking effect, when TS pre-cedes MS, is explained by a lack of temporalresolution. Backward sensitization, the slightdecrease of threshold for TS when it precedesMS by 100 milliseconds (see Figure 1), isexplained as follows: even a sub threshold TSmay noticeably reduce the brightness of MS,and observers use this information in report-ing that TS has been presented. Finally,Sperling reanalyzes the data of an earlier ex-periment (Battersby & Wagman, 1959) andnotes that a 500 millisecond MS is much moreeffective than a 50 millisecond MS of equalluminance. Sperling believes that the longerflash more effectively prevents the perception


of an afterimage of TS (Sperling, 1960b).Similarly, he interprets the paradoxical de-crease of thresholds with increases of back-ground luminance (Boynton & Kandel, 195 7)as due to the elimination of afterimages of MS.

The long list of variations of criterion con-tent is not the main point of Sperling's paper.Rather, Sperling relies on these variations tojustify his exclusive concern with the peak ofthe masking function and to explain aberrantfindings. His main intent is to show that thethreshold at SOA = 0 is a pure contrast dis-crimination and to show that the maskingeffect (Brap - Bp) follows Weber's law and islargely independent of background luminanceand of MS brightness. These conclusions arecontrary to Boynton's analysis. In addition,it may be noted that the entire approach tothe masking function differs sharply fromBoynton's interpretation of that function asan on-response to MS. The phenomenologicaleffects that Sperling describes deserves muchfurther study.

Detection under Metacontrast

Metacontrast paradigms. In metacontrast(Alpern, 19S2; Stigler, 1910), the brightnessof TS is reduced when its presentation isfollowed by that of MS to an adjacent area.Under many conditions, the effect of meta-contrast is more radical than mere dimming;TS is reported to be phenomenally absent(Alpern, 1952,1965; Alpern & Rushton, 1965;Fehrer & Biederman, 1962; Fehrer & Raab,1962; Kahneman, 1967b; Kolers, 1962; Ko-lers & Rosner, 1960; Schiller, 1965a; Toch,1956; Werner, 1935). This phenomenal dis-appearance of TS will be termed metacontrastsuppression.

Metacontrast suppression has been demon-strated under a variety of spatial arrangementsof TS and MS (Alpern, 1952; Mayzner, Tres-selt, Adrignolo, & Cohen, 1967; Mayzner,Tresselt, & Cohen, 1966; Mayzner, Tresselt,& Heifer, 1967; Toch, 1956; Werner, 1935,1940). However, most research has been donewith two particular displays. These are closelyrelated to well-known displays which producetwo types of apparent motion (Kahneman,1967b):

1. In the three-object display, a pattern isshown, and its presentation is followed by

that of two similar patterns which flank it oneither side (Alpern, 1953; Fehrer & Bieder-man, 1962; Fehrer & Raab, 1962; Fry, 1934;Kahneman, 1967b). All one need do in orderto produce beta motion is remove one of theflanking objects. The complex displays thatMayzner and his colleagues have investigatedbelong to this group.

2. In the disk-ring display, TS is a diskand MS is a ring; the inner contour of thisring is adjacent to, or coincident with, theouter contour of the disk (Heckenmueller &Dember, 1965a, 1965b; Kolers, 1962; Kolers& Rosner, 1960; Schiller, 1965a; Schiller &Smith, 1966; Werner, 1935). The display isrelated to one in which a disk followed byanother larger disk produces expansion—motion.

Masking junctions in metacontrast. Twotypes of masking functions which have beenobtained in metacontrast experiments are il-lustrated in Figure 2. The ordinate of Figure 2is labeled "magnitude of masking effect" withdeliberate ambiguity, because the two func-tions are usually obtained by different opera-tions of measurement. The two types of mask-ing functions have been called Type A andType B by Kolers (1962). In Type A, thepeak masking effect occurs at SOA = 0, andthere is much forward masking (Battersby,Oesterreich, & Sturr, 1964; Fehrer & Smith,1962; Kietzman, 1962). In Type B, there islittle or no masking at SOA = 0, forwardmasking is weak or absent, and the peakmasking effect is found at an SOA of 50-100milliseconds (Alpern, 1953; Fehrer & Smith,1962; Kahneman, 1967b; Kolers, 1962;Mayzner, Tresselt, Adrignolo, & Cohen, 1967;Mayzner, Tresselt, & Cohen, 1966; Schiller &Smith, 1966; Toch, 1956; Weisstein, 1968;Werner, 1935).

The conditions that determine whethermasking follows a function of Type A orType B are reasonably well understood. Amost important determinant is the observer'scriterion. Metacontrast suppression can beobtained even when TS and MS are identicalin intensity and duration (Kahneman, 1967b);it then follows a Type B function. Most ob-servers agree that in a three-object display,there is nothing where the central object

412 DANIEL KAHNEMAN

-200 0 200

ONSET ASYNCHRONY

FIG. 2. Two types of masking functionsin metacontrast.

ought to be. However, the appearance of thedisplay is easily discriminable from the presen-tation of MS alone (Fehrer & Raab, 1962;Fehrer & Smith, 1962; Schiller, 1965a;Schiller & Smith, 1966). The two objects thatare seen in the three-object display are oftenseen to move away from the center. That im-pression of motion is, of course, absent whenthe two flanking objects are not preceded byMS, and the difference is easily available tothe observer if he is to discriminate between athree-object and a two-object display. Simi-larly, Mayzner's display in which some let-ters of a word are never "seen" is easily dis-tinguished from a display in which these let-ters are, in fact, not shown. The use of asemantic indicator will, therefore, indicate thatmasking occurs under conditions in which anaccuracy indicator would show perfect dis-crimination.

The conclusion follows that the level ofthe natural criterion is high when a semanticindicator is used in metacontrast. Criterioncontent is such that the subject says "yes"only when a well-formed image of definitecontour is visible in the center of the display.The motion of the masking figures is ignored,since the observer confines his report to thepresence or absence of the central object.

Type A masking functions can be obtainedeven with a semantic indicator, but only whenMS is a much stronger visual stimulus thanTS in terms of duration, luminance, or figure-ground contrast (Fehrer & Smith, 1962;Kolers, 1962). The dependent variable inthese studies was the probability of reportingseeing MS, which was plotted as a function ofISI. Type A functions are also obtained when

the detection threshold of MS is determinedby varying its luminance (Battersby, Oester-reich, & Sturr, 1964; Kietzman, 1962). Thelow criterion level that this type of experi-mental situation tends to induce necessarilyresults in a very marked imbalance betweenthe energy levels of TS and MS.

The conclusion of this section must be thatType B masking functions do not representabsolute detection thresholds, in any sensibleuse of that controversial term. The suppres-sion of TS in Type B functions is not com-plete. Thus, the well-known experiment ofFehrer and Raab (1962) demonstrated thatsubjects in a simple reaction-time situationrespond to the initial TS, which they never"perceive." This important result has beenreplicated several times (Fehrer & Biederman,1962; Harrison & Fox, 1966; Schiller &Smith, 1966). In addition, Schiller and Chor-over (1966) have shown that the evokedpotential to TS is essentially intact in meta-contrast, even when TS appears extremelydim or fails altogether to be seen. The situa-tion is different in the masking of a small flashby a larger overlapping flash; in that case,the correlation between the perceived bright-ness of TS and the evoked potential is quiteclose (Donchin &Lindsley, 196Sb).

It is also significant in this context thatType A and Type B metacontrast can be ob-tained dichoptically. Battersby, Oesterreich,and Sturr (1964) presented an illuminateddisk to one eye and a ring to the other anddetermined the masking function for the de-tection of the disk. The function is verysimilar to that of masking by light (Figure1). Schiller (196Sa) obtained a Type A func-tion in dichoptic metacontrast with blacktargets. Werner (1940), Toch (1956), Kolersand Rosner (1960), and Mayzner, Tresselt,Adrignolo, and Cohen (1967) obtained TypeB metacontrast dichoptically with several dif-ferent stimulus arrangements. In summary,the evidence is not consistent with an inter-pretation of metacontrast suppression in termsof retinal interaction (Alpern, 1965; Alpern& Rushton, 1965). Such an interpretation isparticularly inappropriate to data which ap-parently conform to a Type B function, sincethis function is typically associated with ahigh criterion level.


Metacontrast suppression—two types orone? The preceding section described the twotypes of results that may be obtained in ametacontrast experiment: the Type B func-tion, which occurs when TS and MS aresimilar in energy and which can only be ob-tained with a semantic indicator and at ahigh criterion level, and the Type A func-tion, which is characteristically observedwhen MS is markedly longer or more intensethan TS. Do the two functions represent dif-ferent types of suppression processes? Is therea simple comprehensive theory that may ex-plain the two?

Several partial theories have recently beenadvanced to account for Type B functions.One theory was first proposed by Fehrer(1965, 1966; Fehrer & Smith, 1962) andfurther developed by Kahneman (1967b). Theessential idea in this view is that Type Bfunctions are closely related to the phenome-non of apparent motion, whereas Type Afunctions are not. The similarity of the TypeB masking function to the temporal param-eters of apparent motion was noted inciden-tally by several authors (Fehrer & Raab,1962; Kolers & Rosner, 1960; Schiller &Smith, 1966; Toch, 1956; Werner, 1935). Infact, the temporal parameters of the two ef-fects are virtually identical (Kahneman,1967b). In that study, complete metacontrastsuppression occurred at an SOA of about 100milliseconds, which also produced most re-ports of optimal motion. Kahneman (1967b)noted that the three-object display providescues for an "impossible motion" of the cen-tral object in two directions at once, much inthe way that the drawings of Penrose andPenrose (1958) provide cues for impossiblethree-dimensionality. The disk-ring sequenceis similarly impossible in that the disk ismade to grow and disappear at the same time.These observations suggest that metacontrastsuppression may result from a failure of per-ceptual synthesis.

The most compelling evidence of a link be-tween metacontrast suppression and impos-sible motion has been offered by Mayznerand his colleagues (Mayzner, Tresselt, Adrig-nolo, & Cohen, 1967; Mayzner, Tresselt, &Cohen, 1966), although they prefer anotherinterpretation (Mayzner, Tresselt, & Heifer,

1967). When the letters of a word are shownin irregular succession in their computer-con-trolled display, the letters that are presentedfirst fail to be seen. Even the two extremeletters of a word may be suppressed when theycannot be incorporated in a coherent perceptof motion. The suppression is invariably aU-shaped function of presentation speed. Onthe other hand, all letters are seen in themany different sequences that permit theperception of a regular flow of motion.

There are other indications that backwardmasking and apparent motion may be closelyrelated. Kahneman (1967b; Kahneman &Wolman, 1968) has noted that optimal motioninvolves an apparently retroactive suppressionand modification of the percept of the firstobject to be shown. In addition, the effects ofa value of SOA that is too short for optimalmotion (or suppression) are strikingly similarin the two-object and three-object displays:the first object is seen as dimmer than theothers. The dimming effect was noted byWertheimer (1912) in the motion display; itdepends very little on whether TS is followedby one object or by two.

Another theory of Type B masking func-tions was proposed by Mayzner, Tresselt, andHeifer (1967). They note that the effects ofa stimulus (Si) are particularly susceptible tointerference by a subsequent stimulus (82)that follows Si after an SOA of 100 millisec-onds. They suggest that the visual input isdelayed at a gate prior to entry into subjec-tive experience and is especially vulnerable atthe instant of entry. The model is constructedspecifically to account for the large quantity ofdata collected by these authors on varioustypes of sequential blanking and displacement.

The partial theories that have been listedso far are obviously inapplicable to Type Amasking functions. Alternative mechanismshave been mentioned to account for the latterresult. Fehrer and Smith (1962) and Kahne-man (1967b) note that those conditions ofimbalance between TS and MS which produceType A functions also favor the occurrenceof lateral inhibition and simultaneous con-trast. It is, therefore, possible that the twotypes of masking functions are due to twodistinct suppression processes: Type A is akinto masking by overlapping light (Kolers,

414 DANIEL KAHNEMAN

1962; Schiller, 1965a) and involves lateralinhibition; Type B is an interaction that canoccur only between successive stimuli, as isthe case in apparent motion.

There has been one attempt to deal withboth types of masking functions by a single setof processes (Weisstein, 1968). Weisstein hasshown that a process of lateral inhibition canproduce Type B masking functions. She as-sumes that the detection of TS depends onwhether, at any time, the algebraic sum of anexcitatory process (generated by TS) and ofan inhibitory process (generated by MS) issufficient to activate a central decision unit.She further assumes that the inhibitory ac-tivity produced by MS has a shorter latencythan the excitatory activity due to TS whenthe two stimuli are equal in duration andluminance. Therefore, if the two stimuli arepresented simultaneously (SOA = 0) inhibi-tion will be past its maximum when excita-tion builds up to its peak; inhibition thus"misses" excitation and there is little mask-ing. The presentation of TS must conse-quently be delayed in order to increase thetemporal overlap between the excitation andinhibition processes; this yields a Type Bmasking function. Now consider the effect ofmaking MS longer than TS; at SOA = 0, therise of inhibition still precedes that of ex-citation, but inhibition is now maintained dur-ing the entire excitatory response to TS, andtotal suppression occurs. The masking func-tion now belongs to Type A, since TS isseen only when the SOA is so long that theexcitation produced by TS occurs earlier thanthe inhibition produced by MS. Similar con-siderations explain the effect of the luminanceratio on the shape of the masking function.

Weisstein (1968) constructed a five-neuronmodel incorporating this basic idea and simu-lated on a computer the results of a numberof metacontrast studies. The model elaboratesLandahPs (1967) conception of neural nets.It is generally successful in separating condi-tions leading to Type A and to Type B func-tions, but the success of predictions concern-ing the shape of masking functions is less im-pressive.

Weisstein's formulation has an advantage ofspecificity and parsimony over all its prede-cessors. However, the relation between Type

B metacontrast and apparent motion is toocompelling to be ignored, as it is in her theory.It now appears likely that a comprehensivetheory may be able to account for both theseeffects by the operation of a single mechanism.Weisstein's work indicates that variations ofquantitative parameters of that mechanismmay yield Type A functions on some occa-sions and Type B functions on others. Hermodel would have to be further elaborated toaccount for the fact that "creative" effectssuch as beta motion are sometimes the counter-part of the suppressive effects of metacontrast.The function of perceptual synthesis shouldbe added to the functions of excitation and in-hibition that the model presently considers.

A Plea for Phenomenological Description

The studies that were reviewed in precedingsections illustrate the importance of phe-nomenological reports. Even informal reportsmay serve to illuminate complex results, as inSperling's (1965) analysis of the maskingfunction. Further, reports of subjective experi-ence may become a source of data, as in thestudies of metacontrast by Mayzner's groupand by Kahneman. The high level of within-subject and between-subject reliability that iseasily attained by naive observers justifies theuse of this approach in studies of masking.

In the context of more "objective" studiesof detection thresholds, the crucial role of theobserver's criterion deserves special emphasis.Detection theory has emphasized one aspectof the problem, the necessity for control andspecification of criterion level. The presenttreatment emphasizes the necessity of ade-quate specification of criterion content. Theliterature is notably deficient in this respect,perhaps because the description of criterioncontent involves phenomenological reportsand sometimes requires detailed introspection.Whether we like it or not, however, the con-tent of the subject's criterion determines hisperformance, and should, therefore, be de-scribed with the same care that is customarilydevoted to specifying the stimulus. It is com-mon practice in visual science to devote pagesof text to the independent variable and todismiss the dependent variable in a sentence.

The literature on masking consists verylargely of threshold studies, where a low cri-


terion level is presumably desirable. It shouldbe emphasized, however, that a high criterionlevel may be quite valuable, provided onlythat it is a natural criterion. Type B meta-contrast is just as real as Type A, even if itis obtained only when criterion is high. Infact, a low criterion level may produce resultswhich are hard to interpret. Sperling's (1965)analysis of the masking of flashes suggeststhat a criterion level which is consistently lowmay be associated with a criterion contentwhich is highly variable over conditions; theobserver uses all available cues in attemptingto maintain a high rate of detection.

IDENTIFICATION OF FORMS UNDER MASKING

Identification of forms has been studiedunder three modes of masking: by an over-lapping field of homogeneous luminance(masking by light), by a contour-rich over-lapping field (masking by noise), and by acontour surrounding the target (masking bymetacontrast). The three paradigms are ex-amined in the subsequent section.

Masking oj Forms by Light

The masking junction. Boynton and Miller(1963) report the only study in which astandard masking function was obtained foran identification task. They determined thecontrast required for the identification ofSloan-Snellen letters of four different sizes,projected in positive contrast, under severalconditions of sudden increase or decrease ofbackground illumination. The masking func-tions that they obtained for identification areidentical in shape to the functions reported fordetection (Figure 1). In fact, Boynton andMiller report that the same contrast is re-quired for a letter to be read and for a circularpatch of similar size to be detected. The mostplausible interpretation of this important re-sult may be that the detection of the lightpatch was itself limited by contour processesunder the conditions that Boynton and Millerstudied.

Two characteristics of the masking functionhave been confirmed in a number of experi-ments:

1. A masking flash interferes with the iden-tification of TS, both when it precedes TS

(forward masking) and when it follows TS(backward masking).

2. Forward and backward masking effectsare both most severe when TS and MS followone another immediately, at ISI = 0 (e.g.,Eriksen & Hoffman, 1963; Eriksen & Lappin,1964; Schiller, 196Sb, 1966; Schiller & Smith,1965).

Eriksen and Lappin (1964) found that thedependence of masking on ISI is very similarin forward and in backward masking. Mow-bray and Durr (1964) masked target wordsby a black rectangle and also found forwardand backward masking to be similar. However,other evidence indicates that the temporalrange of forward masking is larger than thatof backward masking (Kietzman, 1962;Schiller & Smith, 1965; Smith & Schiller,1966).

Masking of forms by an unpatterned MScannot be obtained dichoptically (Mowbray& Durr, 1964; Schiller, 1965a; Schiller &Wiener, 1963; Smith & Schiller, 1966). Thisresult is to be expected on the general prin-ciple that dichoptic effects in masking cor-respond to dominance effects in binocularrivalry (Kahneman, 1967a). When contoursare presented to one eye, and a homogeneousfield of light is presented to the other, thecontours are always perceived with little orno interference.

The time-intensity reciprocity law does notapply to the resolution of form under condi-tions of masking by light; a brief and in-tense exposure of TS is less effective than onethat is weaker and prolonged (Kahneman,1966; Kaswan & Young, 1963; Scharf, Za-mansky, & Brightbill, 1966). The interpreta-tion of this result must be qualified byDonchin's comment (Donchin, 1967) that du-ration of exposure is confounded with SOAin these experiments.

Interpretations oj masking by light. Con-sider the limiting case of masking by light inwhich MS (a homogeneous field of light) andTS (a contoured pattern) are presented simul-taneously; light from MS is spread over boththe figure and the background of TS and re-duces the contrast between them. Eriksen andhis students (Eriksen, 1966; Eriksen & Hoff-man, 1963; Eriksen & Lappin, 1964; Eriksen

416 DANIEL KAHNEMAN

& Steffy, 1964; Thompson, 1966) have pre-sented a theory of masking by light which re-duces that effect to its limiting case of reducedfigure-ground contrast. The physical asyn-chrony of TS and MS is assumed to bebridged by temporal integration of luminance.This is the luminance-summation theory ofmasking by light.

Luminance-summation theory entails thatthere should be little difference between for-ward and backward masking: temporal in-tegration occurs in either case. In fact, thesimilarity between functions for forward andbackward masking is often quite impressive(Eriksen & Lappin, 1964; Kahneman, 1966;Mowbray & Durr, 1964; Schiller & Smith,1965). Eriksen (1966) has presented supportfor several other qualitative predictions de-rived from the theory, and Thompson (1966)has shown that the interfering effects of mask-ing by light and of contrast reduction aregenerally similar. There is no reason to doubtthat luminance integration of TS and MSreduces the contrast of TS and, thereby, im-pairs its resolution. However, there are effectsof masking by light for which luminance-sum-mation theory cannot account. Other factorsare involved.

A study by Kahneman (1965) indicatesthat masking by light reduces the apparentcontrast of a figure in two ways: directly, byluminance summation; and indirectly, by pre-venting or retarding the formation of bound-ing contours. A matching method was used tomeasure the apparent contrast of a blacksquare that was presented under standardtachistoscopic conditions, that is, with pre-and postexposure fields matching the lumi-nance of the background on which the squarewas shown. Luminance-summation theory en-tails that apparent contrast should be a linearfunction of exposure duration in this situa-tion, reaching maximal contrast at the criticalduration for brightness. In fact, the resultsindicate the existence of a definite durationthreshold for the appearance of bounding con-tours, which is as high as 40 milliseconds atvery low luminances. Only flicker is seen be-low that threshold, but apparent contrastrises rapidly when contours are finally seenand soon rejoins the function predicted byluminance summation. The presentation of a

thin outline of the target square in the adapt-ing field, which provides the target with pre-fabricated contours, yields a good fit to thelinear function predicted by luminance-sum-mation theory.

Contour effects may be responsible for otherresults that luminance-summation theory can-not explain: (a) the disinhibition effects re-ported by Robinson (1966) and by Demberand Purcell (1967); (b) the superior acuityfor a target that is superimposed on a steadyadapting field, compared to the case in whichthe adapting field is briefly turned off duringtarget presentation (Kahneman, 1966); and(c) the transient reduction of acuity whentarget and mask are presented simultaneously,compared to the light-adapted acuity whenMS precedes TS, which is later superimposedon it (Boynton & Miller, 1963).

Transient "on" and "off" responses occurin all these instances (Boynton, 1961; Dem-ber & Purcell, 1967; Schiller, 1968). The tech-nique of making contours of TS available inthe adapting field could be used to discoverwhether transient interference acts directlyon the apparent brightness and contrast ofTS, or whether the effect is mediated by re-tardation of contour formation.

Masking by Noise

Masking by noise is much more effective inreducing form recognition than is masking bylight (Scharf, Zamansky, & Brightbill, 1966;Schiller, 1965b; Schiller & Wiener, 1963;Smith & Schiller, 1966; Sperling, 1963). Themost important difference between the twotypes of masking is that masking by noise orpattern occurs in dichoptic presentation (Kins-bourne & Warrington, 1962b; Schiller, 1965b;Schiller & Wiener, 1963; Smith & Schiller,1966), whereas masking by light occurs onlywhen TS and MS are both shown to thesame eye.

All investigators agree that masking bypattern is most severe at ISI = 0, but theyusually fail to include a case of temporal over-lap of TS and MS (SOA = 0). Kinsbourneand Warrington (1962a, 1962b) reportedevidence for a simple relation between thresh-old TS duration and ISI in masking bynoise: Duration X ISI = Constant. Surpris-ingly, there have been no further investigations


of this result. The same function applied toboth forward and backward masking, butforward masking was consistently more severe.The greater severity of forward masking hasbeen confirmed for the monoptic case (Schiller,1966; Schiller & Smith, 1965; Smith &Schiller, 1966). However, Smith and Schiller(1966) and Greenspoon and Eriksen (1968)report that dichoptic forward masking bypattern is quite weak.

Interpretations of masking by noise. Twocompeting conceptions have been proposedfor masking by noise. Following Baxt (1871),Sperling (Averbach & Sperling, 1960; Sper-ling, 1963, 1967) concluded that the presenta-tion of MS interrupts the process of readingout parts of the primary visual image onto amore permanent storage. On the other hand,Kinsbourne and Warrington (1962a, 1962b)concluded that MS and TS are effectivelysimultaneous in masking by noise; the reduc-tion of performance is due to a degradation ofthe primary visual stimulus. Their interpreta-tion of masking by noise is precisely analogousto Eriksen's interpretation of masking by light.

Sperling's treatment is restricted to thecase of backward masking. Two lines of evi-dence support it:

1. When an array of letters was shown, thenumber of items retrieved was a linear func-tion of exposure duration prior to masking(Baxt, 1871; Sperling, 1963). Most com-pelling, the number of items retrieved wasindependent of the number of items shown:for several sizes of array, letters were ac-quired at a rate of one letter for every 10milliseconds of exposure up to a maximumof about 4.5 letters, which represents a limita-tion of short-term memory. Such resultsstrongly support a process of sequential read-ing out from a visual image, and the maskingexperiment was proposed (Sperling, 1963) asa means of measuring reading rate. A similarinterpretation was advanced by Averbach(1963) to account for the linear rise of thespan of apprehension with exposure durationunder conditions of masking by noise. Sper-ling (1967) subsequently amended his viewand proposed that items may be read in paral-lel from the visual image, but he did notmodify his interpretation of masking by noise.

2. The reduction of visual persistence bymasking is quite obvious, in both masking bylight (Dodge, 1907; Sperling, 1960a) and bynoise. The observer often reports the im-pression that a clear visual stimulus was thereto be read, but time did not suffice. Sperling(1967) has attempted to quantify directlythis effect of masking by noise on visual per-sistence. Subjects were required to adjust thetiming of clicks to apparent simultaneousnesswith the onset or disappearance of a visualtarget, which was preceded by light and fol-lowed by a field of visual noise. By this mea-sure, the apparent duration of the visualstimulus under masking by noise is onlyslightly longer than its physical duration, andthe correlation between physical and ap-parent duration is close, even in the range20-100 milliseconds.

The arguments for the interpretation ofmasking by noise in terms of interruptedprocessing of TS appear strong. However, thecounterarguments are equally compelling.

1. Sperling's theory rests on the assumptionthat a complete visual representation of TS isbriefly available for processing under condi-tions of masking by noise. This crucial as-sumption is never tested directly. The basicfinding, that the number of items retrievedrises with exposure duration prior to masking,is more parsimoniously explained on the claimthat exposure duration controls the quality ofthe visual image that the subject is requiredto read and not only the time in which hemay read it (Eriksen & Hoffman, 1963; Erik-sen & Steffy, 1964; Kinsbourne & Warring-ton, 1962a, 1962b).

2. Sperling's (1963) finding that the num-ber of items read is independent of the num-ber shown was not obtained by Kinsbourneand Warrington (1962a), who reported thatall the letters in an array become legible atabout the same SOA between TS and MS.The specific conditions under which one orthe other effect is obtained have not beendetermined.

3. Sperling's (1967) study of the durationof the visual image is also open to question.In informal attempts to apply Sperling's tech-nique, the present author has found that anobserver can adopt one of two different sets:

418 DANIEL KAHNEMAN

the click may be matched either to the disap-pearance of the letters or to the sudden ap-pearance of the masking pattern. The resultsthat Sperling reported are easily obtainedwhen the latter set is adopted, but the sub-ject, himself, is often unsure of whether theonset of the noise and the disappearance ofthe letters are, in fact, simultaneous.

4. Finally, the similarity of results forforward and for backward masking appears tobe a serious embarrassment for the interrup-tion theory and provides the strongest supportfor the simultaneity theory proposed by Kins-bourne and Warrington (1962b). However,further discussion below will show that aninterruption theory of backward masking maybe compatible with a simultaneity theory offorward masking.

The interruption theory promotes maskingby noise as the tool of choice with which tostudy the speed of perceptual events and,thereby, makes masking a central topic incognitive psychology (Haber, 1966; Neisser,1967). However, the evidence at hand is notsufficient to conclude that interruption ofprocessing is always the limiting factor when-ever masking by noise is used. There may beconditions of very high luminance, largetarget size, and high contrast where a legibleimage is formed that the subject has too littletime to read; under other conditions, maskingmay preclude identification by disrupting theformation of a legible image. Finally, thehypothesis may be considered that the imageitself is formed by a rapid sequential process B

instead of instantaneously, as Sperling (1960a,1963, 1967) suggests. This hypothesis sug-gests a distinction between "seeing" items andreading them, both sequential processes. Theconditions under which these various effectsmay occur are yet to be studied in detail.

Masking by Metacontrast

Averbach and Coriell (1961) presented anarray of letters and shortly thereafter a ringsurrounding the position of one of the letters.The ring was originally intended to serve asan indicator of which letter to report. In-stead, presentation of the ring at certain ISIs

8 P. Liss; R. N. Haber, personal communication,1967.

resulted in complete masking of the letter.The phenomenon was termed "erasure." Otherstudies have confirmed the effectiveness of aring as a masking stimulus, but there is noagreement on the shape of the masking func-tion.

We noted earlier that the detection of atarget under metacontrast may follow eithera Type A or a Type B masking function (Fig-ure 2). The two types of function have alsobeen obtained with identification as criterialresponse. Averbach and Coriell (1961) re-ported that backward masking by a ring is aU-shaped function of ISI (Type B). Weis-stein (1966; Weisstein & Haber, 196S) con-firmed this result. Fraisse (1966) and Mayz-ner and his colleagues (Mayzner, Tresselt,Adrignolo, & Cohen, 1967; Mayzner, Tresselt,& Cohen, 1966) have also reported U-shapedfunctions when different parts of a word arepresented in sequence. Other studies (Eriksen& Collins, 1964, 196S; Eriksen, Collins, &Greenspoon, 1967; Norman, 1965; Schiller &Smith, 196S) report functions in which mask-ing decreases monotonically with increasingISI. In several of these studies forward mask-ing by the ring is also observed (Eriksen &Collins, 1965; Schiller & Smith, 1965).

The review of metacontrast effects on thedetection threshold in an earlier section sug-gested that two distinct processes are in-volved under different experimental situations(Fehrer & Smith, 1962; Kahneman, 1967b;Kolers, 1962). The presence of contours inclose proximity to an acuity target stronglyinterferes with the resolution of that target,in both monoptic (Flom, Weymouth, &Kahneman, 1963) and dichoptic presentation(Flom, Heath, & Takahashi, 1963), a lateralinhibition effect. In addition, the suggestionwas made that impossible motion is sometimesinvolved. The relation between suppressionand motion is clearly illustrated in studiesof sequential blanking (Mayzner, Tresselt,Adrignolo, & Cohen, 1967; Mayzner, Tresselt,& Cohen, 1966). With the appropriate se-quencing of the presentations of individualletters, the word CHAIR is normally read asC A R , even when the presentation is re-peated indefinitely. Certainly, any measure ofacuity would reveal impaired performance inthe two critical positions. Fraisse (1966) has


used a related mode of presentation in whichtwo interlaced halves of a six-letter word arepresented in succession. Recognition was im-paired for the three letters that were shownfirst, and the masking function was U-shaped.

The conditions under which masking by asurrounding ring will produce masking func-tions of Type A or Type B remain unclear. Itis a plausible conjecture that investigatorshave obtained Type B functions by inad-vertently optimizing conditions for a motioneffect, but the parameters of that effect areobscure. So far, no study of target recogni-tion has been reported in which a U-shapedfunction is reliably obtained under some con-ditions but not under others. Eriksen, Collins,and Greenspoon (1967) noted several flaws inthe design of Weisstein's (1966) experiment,but their comments do not explain why sheobtained a U-shaped function and they didnot. It is probably significant, however, thatTS was presented at a higher energy in studieswhich yielded U-shaped functions (Averbach& Coriell, 1961; Weisstein, 1966; Weisstein& Haber, 196S) than in studies which yieldedmonotone functions (Eriksen & Collins, 1964,1965; Eriksen, Collins, & Greenspoon, 1967;Norman, 196S; Schiller & Smith, 196S).

Averbach and Coriell (1961) obtainedmasking by a ring only when several letterswere simultaneously presented in the targetarray. Weisstein (1966) obtained Type Bmasking functions even with a single target,but masking effects were more pronouncedwith larger arrays; also, the peak of themasking function was displaced to higherISIs when the array was large. Eriksen,Collins, and Greenspoon (1967) confirmedthe finding that masking is more severe whenthe array is larger with a Type A maskingfunction.

Effects of array size may require differentinterpretations depending on the shape of themasking function. A relevant observation,where motion is involved, is that apparent mo-tion may be inhibited by an "analytic" at-titude (Neuhaus, 1930) when attention issharply focused on the initial object shown.Informal observations of the present authorsuggest that apparent motion is most com-pelling when one part of a cluttered array isset into motion. This would account for the

failure of Averbach and Coriell (1961) toobtain masking of a single letter by a sur-rounding ring.

In the data of Eriksen, Collins, and Green-spoon (1967), the masking functions aremonotonic. The effect of array size on maskingis slight at short ISIs and increases steadilythereafter. The authors' interpretation is inthe spirit of earlier work (Averbach & Coriell,1961; Sperling, 1960a, 1963). When the sizeof the array exceeds the subject's capacity,items must be processed serially from a de-caying perceptual trace. At any time afterexposure, the probability that a given item hasbeen processed is inversely related to thenumber of items in the array. The ring can-not be effective as an indicator of which itemto report if it indicates an item which hasnot been processed and has already decayed.In addition, the ring has a masking effect.The weaker items in the larger array are morevulnerable to this masking effect, and theirrelative vulnerability increases with the decayof the perceptual trace.

THEORY

All current theories of backward maskingshare a central idea: the visual response to abrief stimulus lasts much longer than did thestimulus that caused it; consequently, theresponses to two successive stimuli may over-lap in time. Further, there is general agree-ment that overtake effects may contribute tomasking: when MS is more intense than TS,the difference in latencies increases the tem-poral overlap of the responses (Baker, 1963;Crawford, 1947; Donchin, 1967; Donchin& Lindsley, 196Sa, 196Sb). However, perfectoverlap is not a ncessary condition for theoccurrence of masking, since all varieties ofmasking have been observed with TS and MSequal in itensity (e.g., Averbach & Sperling,1960; Boynton, 1961; Greenspoon & Eriksen,1968; Kahneman, 1966, 1967b; Schiller,1968)8 Because of temporal overlap of re-sponses, backward masking is not a retro-active effect; it is an interaction betweenresponses that are at least partly concurrent.While agreeing on this general position,

a Partial overlap is a necessary condition for theoccurrence of apparent motion and Type B meta-contrast (Kahneman, 1967b; Weisstein, 1968).

420 DANIEL KABNZMAN

theories of masking diverge on the nature ofthe interaction between the response to TSand to MS. Two modes of interaction havebeen described most often: integration andinterruption.

Integration Theories

The simplest version of integration theoryassumes that TS and MS are linearly summedand that the response to their presentation insequence is the same as would be evoked bytheir joint simultaneous presentation. Visualmasking becomes a special case of temporalsummation of heterogeneous stimuli. Thetemporal range of masking corresponds to therange of temporal summation.

Kinsbourne and Warrington (1962a; 1962b)first proposed that TS and MS are effectivelysimultaneous in masking by noise. Eriksen(1966; Eriksen & Hoffman, 1963) has beena vigorous champion of this view in explainingmasking by light in his luminance-summationtheory. The theory accounts for forwardmasking as well as for backward masking. Italso fits the general observation that dichopticinteractions follow the same rules for simul-taneous and for successive interactions.

Boynton's theory of masking (Boynton,1961; Boynton & Kandel, 1957) may also bedescribed as a summation theory. However, itassumes a nonlinear summation of responsesrather than a linear summation of stimuli.To the extent that the system is overloaded bythe massive response to MS, it is less capableof conveying information about TS, and amore intense TS is required to pierce thisbarrier. The masking function (Figure 1)describes the time course of the response toMS. Schiller (1968) has provided a strikingillustration of this concept in single-cell re-cordings in the lateral geniculate nucleus ofthe cat: cells that respond at their maximallevel to MS fail to register the earlier presen-tation of TS. The concept of response sum-mation provides an elegant explanation of themasking function; it also accounts for sometransient effects of masking that simple stim-ulus integration cannot explain.

Weisstein's (1968) theory of metacontrastrepresents a further level of complexity. Theresponses to the two stimuli are brokendown into excitatory and inhibitory com-

ponents, each with a time course of its own,and these components are continously summedby a "decision" neuron. The theory ex-plains both Type A and Type B functions inmetacontrast. It also offers promise of anexplanation for the disinhibition effects re-ported by Robinson (1966) and by DemberandPurcell (1967).

Interruption Theories

Interruption theories emphasize the ideathat the normal perception of a TS requirestime and that the process may be stopped bya stimulus presented during that time. Thereare different versions of what requires timeand of what is interrupted.

One version of interruption theory simplystates that MS interrupts the consolidationof the percept of TS (Lindsley, 1961; Lindsley& Emmons, 1958; von Noorden & Burian,1960). A rather different conception assumesthat a visual image of TS is formed, but theobserver is interrupted before he can read theinformation contained in that image into amore permanent store (Averbach, 1963; Aver-bach & Coriell, 1961; Averbach & Sperling,1960; Haber, 1966; Smith & Carey, 1966;Sperling, 1960a, 1963,1967; Weisstein, 1966).The concept of erasure (Averbach & Coriell,1961) implies that the presentation of MSimmediately relegates TS to the psychologi-cal past. Similarly, Sperling (1963, 1967)equates the duration of the visual image withthe SOA between TS and MS. The maindifference between the two versions of inter-ruption theory is that, in Lindsley's view, thepercept of TS is not formed under masking,whereas it is formed and quickly destroyedaccording to Averbach and to Sperling.

Interruption theories do not account forforward masking, but this is not a fatal flaw.MS is a more intense visual stimulus thanTS in most studies of masking. It is, therefore,conceivable that TS fails to be perceived inbackward masking, because MS interruptsthe process; TS fails to be seen in forwardmasking, because it is too weak a stimulus tointerrupt the continued processing of MS. Aninterruption theory of backward masking isnot incompatible with an integration theoryof forward masking.

When coupled with a view of reading out as


a serial process, interruption theory entailsthat the number of items read should be in-dependent of the number shown (Sperling,1963). However, Sperling (1967) recentlyconcluded from an analysis of error data thatvarious items may be processed in parallel,although at different rates. This hypothesis nolonger entails the result that Sperling (1963)obtained earlier. It is not obvious, in fact, thatan interruption theory is capable of generatingspecific hypotheses about target identificationwithout the additional assumption of serialprocessing.

Interruption theory accounts for a com-pelling experience that observers sometimesreport under conditions of masking. Manydifferent operations cause failures of identifica-tion, but masking may be unique in producingthe conviction that "given more time, I cer-tainly could have read it!" However, back-ward masking does not always yield thatexperience, and the conditions under whichit occurs have not been isolated. Even whenit occurs, of course, such an experience doesnot necessarily indicate that a lack of proc-essing time actually caused the failure ofidentification.

Masking and Psychological Time

It is generally agreed that the temporalsequence of perceptual events does not cor-respond strictly to the temporal sequence ofstimulation. Perception lags after stimulationand integrates successive stimuli into com-posite chunks, or moments (Boynton, 1961;Eriksen & Collins, 1967; Stroud, 1956;White, 1963), in which some elements may beobliterated and others altered. The rules thatrelate the sequence of percepts to the sequenceof stimuli define the problem of psychologicaltime, and it has long been hoped (Monje",1928; Pie"ron, 1925) that masking methodscould contribute to its solution. The fulfil-ment of this hope is tardy, but some promisingconcepts have emerged.

Clearing time (Lindsley, 1961) and per-ception time (von Noorden & Burian, 1960)are defined by the range of delays over whichbackward masking can be obtained. An eventthat has already occurred cannot be prevented,and the range of the masking function, there-fore, corresponds to the time required for the

consolidation of the percept of TS. An un-equivocal measurement of perception time re-quires that TS and MS be equal in intensityand spatial extent, so as to preclude overtakeeffects. Perception time is at least 130-150milliseconds long, by this measure (Mayzner,Tresselt, & Heifer, 1967): some metacontrasteffects only reach their peak at SO A =100milliseconds.

The significance of the high value of per-ception time is that we "live in the past" atleast to its extent (Efron, 1966). This factmay have important implications for the func-tion of subjective visual experience. Mayzner,Tresselt, and Heifer (1967) consider that thedelay of visual experience is consistent withthe value of simple reaction time to light.However, the more radical conclusion appearswarranted that subjective visual experience isnot causally involved in triggering a simpleresponse, since the two occur at about thesame time. Indeed, a response can occur whenthe stimulus that triggered it is not rep-resented at all in experience (Fehrer & Raab,1962).

Another significant parameter of maskingis the duration of the exposure of the maskedtarget. What is the exposure duration beyondwhich a target may not be masked by astimulus of equal intensity? There is a conver-gence of evidence that a state of stimulationwhich is maintained for more than 100 milli-seconds without any significant change willbe registered as a separate perceptual event.Mayzner, Tresselt, and Heifer (1967) andKahneman (1967b) have noted that meta-contrast suppression is not obtained withlonger durations of exposure. Kahneman(1967b; Kahneman & Wolman, 1968) hasdescribed changes in the experience of ap-parent motion when the first object is shownfor more than 100-120 milliseconds: withbrief exposures, the object is seen to be movingas soon as it appears; with longer exposures,the object is seen as stationary before itmoves. Michotte (1963) reported that inter-ruptions of real motion for less than 100milliseconds are seen as "stumbling" within acontinuous movement. Longer interruptionsproduce two separate impressions of motion.Similarly, the causality effect is no longerexperienced when the launching figure and its

422 DANIEL KAHNEMAN

target are stationary side by side for morethan 100 milliseconds (Michotte, 1963).Again, two distinct and unrelated movementsare seen.

There are other indications of a discon-tinuity in the effects of exposure duration at100 milliseconds. Kahneman (1967b; Kahne-man & Wolman, 1968) has suggested that theduration of the visual response begins to rep-resent the duration of the stimulus only whenthe latter exceeds 100-120 milliseconds. Allbriefer stimuli are treated as if they were 100milliseconds long. The assumption explains thestrict dependence of several perceptual effectson SOA, rather than on the duration of TSor on ISI. This rule applies to masking bylight (Donchin, 1967), to metacontrast, andto apparent motion with brief identical stimuli(Kahneman, 1967b; Kolers, 1962; Mayzner,Tresselt, & Heifer, 1967) and to the identifica-tion of letters that are presented in rapidsuccession on the same spot (Haber & Nathan-son).7 For motion, an abrupt transition to astrict dependence on ISI occurs at a durationof 100 milliseconds (Kahneman & Wolman,1968). This pattern of results is to be ex-pected if seemingly retroactive effects in per-ception depend on temporal overlap of re-sponses: the duration of TS is irrelevant tomasking if it is not represented in the dura-tion of the response, and it has been assumedthat it is not.

There appears to be a convergence of evi-dence for the conclusion that only brief stimulican be masked and that all stimuli below 100milliseconds may be considered brief. Mayz-ner, Tresselt, and Heifer (1967) singled outthe same value in their estimates of thecentral component of perceptual delay and ofthe duration of an instant of subjective visualexperience. The idea that psychological timeis quantized at a basic rate of 10/second is,of course, not new (Stroud, 1956). It is en-couraging that this parameter appears signifi-cant in studies of masking which were notavailable to Stroud.

7 Study entitled "Processing time versus stimulusduration—which predicts recognition best? (1968, inpreparation).

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(Received June 16, 1967)

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