Invariant face expressions

8/7/2019 Invariant face expressions

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Abstraction of Invariant Face Expressions in InfancyAuthor(s): Rose F. Caron, Albert J. Caron, Rose S. MyersSource: Child Development, Vol. 53, No. 4 (Aug., 1982), pp. 1008-1015Published by: Blackwell Publishing on behalf of the Society for Research in Child DevelopmentStable URL: http://www.jstor.org/stable/1129141 .

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Abstraction of Invariant Face Expressionsin InfancyRose F. CaronGeorgeWashington UniversityAlbert J. CaronNationalInstituteof MentalHealthRose S. MyersGeorgeWashingtonUniversity

CARON, ROSEF.; CARON, ALBERT J.; and MYERS,ROSES. Abstractionf Invariant ace Ex-pressions n Infancy.CHILDDEVELOPMENT,982, 53, 1008-1015. To determinewhetherinfantscan abstract nvariant face expressionsacross differentpersons (i.e., can form face expressioncategories), groupsof 18-, 24-, and 30-week-oldinfants (18 boys and 18 girls per group) werehabituatedby the infant controlprocedure o photographsof 4 different emale faces all wear-ing an identical expression(happy or surprise). In an immediatelyfollowing test phase, cate-gorizationwas inferredfrom greatergeneralizationof habituation(less recoveryof fixation) to2 new female faces in the familiarizedexpressionthan to the same new faces in the altered(novel) expression.To rule out the possibilitythat generalizationat test might be due to fail-ure to discriminate he new persons,controlgroupsof 18 boys and 18 girls at each age sawthe same test faces following repeatedpresentationsof only 1 of the 4 habituationfaces. Theresultsindicatedthat not until 30 weeks could infantsdifferentiatehappy and surpriseexpres-sionson a categoricalbasis. At 24 weeks they could distinguisha surpriseexpression ollowinghabituation to happy faces, but could not do the reverse. At 18 weeks they could do neither.Overall,the performanceof girlswas superior o that of boys. The findingsare consistentwithrecent evidence suggesting that the ability to extractinvariantconfigural nformationrelativeto the humanface does not emerge until about 7 months of age.The ability to recognize and respond ap-propriately to the expressive behavior of othersis generally assumed to be an essential skill,one that mediates fundamental aspects of psy-chosocial and emotional development. Sensitiv-ity to nonverbal forms of affect such as facialexpression is thought to be particularly impor-tant in the elaboration of early social interac-tion. For example, students of the infant-care-

giver relationship emphasize the role of themother's facially conveyed affect in fosteringattachment, communication, and social learning(Ainsworth 1973; Bowlby 1969; Gewirtz 1969;Stern 1977; Trevarthen 1977). There is the im-plicit conviction, then, that the capacity to rec-ognize various facial expressions is present earlyin life. To date, however, we have little con-

clusive evidence as to when infants come to dis-criminate distinctive facial configurations uni-versally associated with such basic emotions ashappiness, sadness, anger, fear, and surprise(for reviews, see Charlesworth&Kreutzer 1973;Ekman & Oster 1979; and Oster 1981).The absence of definitive knowledge aboutinfant recognition of facial expression stems in

part from methodological problems. Earlierstudies (Ahrens 1954; Biuhler& Hetzer 1928;Spitz &Wolf 1946), which used live portrayalsof facial expressions as stimuli and on-the-spotjudgments of differential emotional respondingas a discriminative index, were flawed becauseof inconsisten'cyof stimulus presentation and un-reliability of response measures. Consequently,their failure to find clear signs of infant differ-This researchwas supportedby a Socialand BehavioralSciences ResearchGrantfrom theMarchof Dimes/BirthDefects Foundationand by a grantfromthe William T. GrantFounda-tion. An abbreviatedversionof the manuscriptwas presentedat the biennial meeting of theSociety for Research in Child Development, Boston, April 1981. Requestsfor reprintsshouldbe sent to Rose F. Caron,Infant ResearchLaboratory,10620 GeorgiaAvenue, Silver Spring,Maryland20902.[ChildDevelopment,982,53, 1008-1015.@1982by the Societyfor Research n ChildDevelopment,Inc.All rightsreserved.0009-3920/82/5304-0012$01.00]


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entiation of positive and negative emotional ex-pressions before 5 or 6 months of age could notbe taken as established fact (see Charlesworth& Kreutzer 1973; Oster 1981; Young-Browne,Rosenfeld, & Horowitz 1977).More recent investigations, on the otherhand, using better controlled stimuli (films orslides, often of "standardized" facial expres-sions [Ekman & Friesen 1975; Izard 1971])and more objective measures of response (visu-al fixation), have obtained positive findings foryounger infants. For example, LaBarbera,Izard, Vietze, and Parisi (1976), using a visualpreference procedure, reported that 4-month-olds looked significantly longer at a joy expres-sion than at either anger or neutral expressions(no difference between anger and neutral).Likewise, Young-Browne et al. (1977), whosemethod involved the more sensitive infant-con-trol habituation paradigm, found that 3-month-olds could distinguish a happy from a surpriseexpression, though not happy from sad (sur-prise was discriminated following habituationto sad, but not the reverse). Finally, Barreraand Maurer (1981) described two infant-con-trol habituation experiments in which groupsof 3-month-olds differentiated mother's smilefrom frown and stranger's smile from frown.These studies, then, though differing in manyaspects and often inconsistent in outcome, af-firmthe infant's ability to discriminate a smilingfrom a nonsmiling expression as early as 3-4months of age.

Despite the increased rigor of the fore-going research, one might still have reserva-tions about attributing to very young infantsthe capacity to discriminate facial expression.This is particularly the case because compari-sons were always made between different ex-pressions posed by a single model. Consequent-ly, one cannot determine whether infants recog-nized a given configuration related to adults'judgments of emotional expression or whetherthey simply responded to differences in someunusual or isolated facial feature. For example,the presence or absence of a "toothy" mouthcould have accounted for the smile-frown dif-ferences obtained by Barrera et al. (1981),and, similarly, the contrast between wide andnarrow eyes may have contributed to thehappy-surprise effects of Young-Browne et al.(1977).One way to approach the question ofwhether infants perceive the facial pattern rele-vant to affective expression is to see if theyrecognize similarity of expression across entirely

Caron, Caron, and Myers 1009different people, so that when habituated topersons A, B, and C wearing a given expres-sion, they generalize habituation to person Din the same expression but not to person D ina novel expression. This conceptual responsepattern (little or no recovery to D in the sameexpression, significant recovery to D in thenovel expression) would constitute stronger evi-dence that discrimination had been based on achange in some kind of facial gestalt ratherthan in a single isolated element. Since numer-ous studies have shown that infants as youngas 3 months can abstract the invariant struc-tural properties of a wide variety of visualstimuli (e.g., Caron, Caron, & Carlson 1979;Gibson, Owsley, & Johnston 1978; Milewski1979), a failure on the part of young infantsto generalize across persons could not be at-tributed to a capacity deficit.

Recently a study was conducted using avariant of this technique (Nelson, Morse, &Levitt 1979). Seven-month-old infants weretested in a familiarization-novelty paradigmwith the expressions happy and fear. Two mod-els posing a given expression were shown dur-ing the familiarization period, and a thirdmodel posing the familiar and novel expres-sions was shown during test. Although signifi-cant discrimination was obtained, the develop-mental implications of the study are unfortu-nately clouded because no younger age groupswere tested and the effect occurred in only onedirection (happy to fear but not fear to happy),due apparently to insufficient habituation to thefear expression.In the present research, three age groupswere used--4, 51, and 7 months-and the in-fant-control habituation-recoveryprocedure wasemployed with all subjects habituated to the

same criterion. During the habituation phase,half the infants at each age were exposed tofour persons displaying the same expression(happy or surprise), and, to determine whetherthe habituation and test faces were discrimi-nable, the other half were exposed repeatedlyto a single person displaying the given expres-sion. In an immediately following test phase,concept acquisition was inferred from general-ization of habituation to two new persons inthe familiar expression and recovery of respond-ing to the same new persons in a novel expres-sion. The happy and surprise expressions wereused because they yielded the most clear-cutfindings in the 3-month-old group employedby Young-Browne et al. (1977), where an in-fant control procedure was also utilized.


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1010 Child DevelopmentMethodSubjectsThe subjects were 216 full-term, healthyinfants divided equally among three agegroups: 18 weeks (median: 125 days; range:119-133), 24 weeks (median: 168 days; range:161-175), and 30 weeks (median: 209 days;range: 203-217). Thirty-six infants at each age(18 boys and 18 girls) were tested in the four-exemplar condition and 36 at each age (18boys and 18 girls) in the single-exemplar con-dition. Thirty-one additional infants (13, 10,and 8 at the three respective ages) were elimi-nated from the four exemplar groups and 22(7, 8, and 9, respectively) were eliminatedfrom the single-exemplar group because of poorstate or failure to meet minimal fixation criteria(at least 10 sec of overall looking during ha-bituation).Stimuli and DesignThe stimuli were photographs of motherswho had come to the laboratory for otherstudies and who were given the Ekman photos(Ekman & Friesen 1975) to use as a guide tofacial expression. In the four-exemplar condi-tion, four faces displaying the same expression(happy or surprise) were presented sequen-

tially during the habituation phase as shownin the first row of figure 1 (i.e., F1, F2, F3, F4,F1, F2,. . .). In an immediately following testphase, two new faces were exposed, first in thefamiliarized expression and then in the novelexpression, as shown in the second row offigure 1. In the single-exemplar conditions, thetest phase was identical but the habituationphase consisted of repeated presentations ofonly one of the faces used in the four-exem-plar condition (each woman being shown to ap-proximately one-quarter of the infants at eachage). Eighteen infants (9 boys and 9 girls) ateach age in each exemplar group were habitu-ated to happy faces and 18 were habituatedto surprise faces. Since pilot data indicatedthere were no effects due to the order of ex-pressions at test, no attempt was made to coun-terbalance test order. The order used (familiarexpressions followed by novel) was thoughtto be the more conservative because fixation ofthe novel expression would be less likely to beenhanced by novelty of person (the new per-sons having just been seen on the two previoustest trials).ProcedureThe stimuli were rear projected onto an18 x 18-cm rigid lenscreen window located 15

FIG. 1.-Happy and surprisefaces used in the study


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degrees to the right of midline about 36 cmfrom the infant, who was seated in a slightlyreclining infant seat. The mother stood just be-hind and to one side of the child so that shewas readily available for reassurance, if needed;otherwise she was instructed to remain silent.Fixation was scored by the standard cornealreflection technique from a TV monitor. Triallength was under infant control (being termi-nated by a 0.5-sec look-away), and all subjectswere habituated to the same criterial level (a50% decline from an initial three-trial base).A blinking light at midline held the infant'sattention during a 4-sec intertrial interval. Trialonset and termination, slide advance, determi-nation of criterion, and printout of trial datawere computer controlled. Scorers were un-aware of the stimuli being scored. Scorer agree-ment as to trial length has consistently averagedbetween 92%and 96% for these procedures inour laboratory.ResultsTest DataTest performance was evaluated in termsof two interrelated measures-one assessinggeneralization of habituation to the new in-stances of the familiar expression, the otherassessing discrimination of the novel expression.Both would have to occur if a face expressioncategory had been formed during habituation.Generalization was scored as the amount of re-sponding to the familiar expression relative tothe terminal habituation level; it was calculatedby dividing total fixation on the two familiartest expression trials by overall fixation on thesetwo trials plus the last two habituation trials.A score of 50%or less would indicate that gen-eralization (no response recovery) had occurredto the two familiar expressions relative to thelast two habituation trials. Discrimination ofthe novel expression was calculated by dividingtotal fixation on the two novel expression trialsby overall fixation at test (i.e., by fixation onthe two novel and the two familiar expressiontrials combined). A score greater than 50% onthis measure would indicate that the novel ex-pression had been discriminated.

Generalization.-The generalization scoreswere subjected to a 3 (age) x 2 (number ofexemplars) x 2 (stimulus) x 2 (sex) factorialANOVA. The major finding (fig. 2) was ahighly significant difference between exemplarconditions, F(1,192) = 37.60, p


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1012 Child DevelopmentFOUREXEMPLAR ROUPS

8070-60O" 5040 NOVELSURPRISE NOVELHAPPY COMBINEDFACE FACE FACES

SINGLE EXEMPLAR GROUPS70-

z 60

NOVEL SURPRISE NOVELHAPPY COMBINEDFACE FACE FACES30

S18 WKSS24 WKS

S30 WKSFIG. 3.-Percent fixationof the two novel rela-tive to the two familiar face expressionsfor theage x stimulusX number of exemplar subgroups.Scores significantlyin excess of 50% ndicate dis-criminationof the novel expression.

68.8%,the two older groups scoring significant-ly above chance-p < .01 and p < .001, re-spectively), but at no age was the novel ex-pression discriminable in the single-exemplarcondition (47.7%, 53.0%,and 49.1%,respective-ly). Performance improved significantly at eachsuccessive age in the four-exemplar condition(18- vs. 24-week-olds, p < .05; 24- vs. 30-week-olds, p < .01, by Newman-Keuls tests).As can be seen in the two left-hand por-tions of figure 3, there was also an overallstimulus effect, F (1,192) = 55.36, p < .001,indicating that it was generally easier to detectthe surprise expression following happy thanthe happy expression following surprise. In thefour-exemplar condition, the 24- and 30-week-old infants each significantly discriminated thenovel surprise expression (64.9%and 75.4%,re-spectively, p < .001, in each case), but onlythe 30-week-olds could discriminate the novelhappy expression (62.3%, p


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was also, as might be expected, a significantage effect for habituationtime, F(2,192) =17.61, p < .001--younger subjects fixatinglonger-but, interestingly,not for numberofhabituation rials.There wereno exemplar,agex exemplar,or sex effects for either variable,indicating that our major test findings couldnot be attributed o differencesn habituation.Discussion

The results ndicate thatonly at 30 weeksare infants able to differentiatehappyand sur-

70a-0 60C,r-z 40-z 30-zU 20-

10-

FOUR ONEEXEMPLARS EXEMPLAR

BOYSGIRLS

FIG. 5.-Percentage of boys and girls in eachexemplar group who both generalized to the fa-miliar expressionsand discriminated he novel ex-pressions.

Caron, Caron, and Myers 1013prise facial expressionson a conceptualbasis(i.e., only at this age could more than half thesample both generalize to new instances of thefamiliarexpressionand discriminate he novelexpression).That the performanceof the 30-week-olds was indeed conceptual and not sim-ply a failureto discriminate he new person nthe familiar expression was evident in thesingle-exemplar condition where 30-week-olds(as well as the two youngerage groups)clear-ly detected person change but not expressionchange. The timetablesuggestedby our data,furthermore, is consistent with what is alreadyknown regarding the developmental course ofinfant face perception. Thus, the failure of 18-week-olds to abstract invariant expressionswould be expected given that the full-faceconfiguration seems not to be discriminableuntil about 21 weeks (Caron, Caron, Caldwell,& Weiss 1973). Likewise, the 30-week-old'sfacility in categorizing expressions conformsnicely with recent demonstrations that abstrac-tion of other facial invariants (the same personacross orientations, female face across persons)does not occur until 7 months (Cohen & Strauss1979;Fagan1976).The data arepatentlydiscrepantwith evi-dence that successful discrimination f facialexpression is achieved as early as 3-4 months(Barrera & Maurer 1981; LaBarbera et al.1976; Young-Browne et al. 1977). As notedpreviously, the experiments associated withthese findings had in common the use of asingle modelposing differentexpressions,husleaving open the questionof whether infantshad in fact perceiveddifferences n expressionor haddifferentiated omeisolatedfeaturehav-ing no affectiverelevance.Indeed, in a study

TABLE 1MEAN NUMBER OF HABITUATIONTRIALS AND MEAN LOG TOTALFIXATIONTIME DURINGHABITUATIONOF THEAGE X STIMULUSX NUMBER OF EXEMPLARSUBGROUPS

FOUR EXEMPLAR SINGLE EXEMPLAR ALLAGE Happy Surprise Combined Happy Surprise CombinedSUBJECTSTrials:18weeks... 6.8 9.9 8.4 8.0 7.5 7.8 8.124weeks... 8.7 9.3 9.0 8.5 11.9 10.2 9.630weeks... 7.0 8.4 7.7 7.6 8.1 7.8 7.8All subjects. 7.5 9.2 8.4 8.0 9.2 8.6 8.5Logatotal fixationtime:18 weeks... 4.33 4.31 4.32 4.09 4.41 4.25 4.2824weeks... 3.98 4.17 4.08 3.61 4.10 3.85 3.9630weeks... 3.36 3.81 3.58 3.76 3.46 3.61 3.60Allsubjects. 3.89 4.10 3.99 3.82 3.99 3.90 3.95

a Common ogarithms.


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1014 Child Developmentthat has just come to our attention, Oster andEwy (cited in Oster 1981) reported that 4-month-olds fixated photographs of toothy smileslonger than those of sad faces but showed nopreference for closed-mouth smiles over sadfaces. The fact that the infants in this studyalso failed to discriminate the toothy and sadfaces when they were presented upside downindicates that purely physical aspects of tooth-iness (e.g., brightness contrast) were not re-sponsible for the obtained preference. How-ever, since happiness resides as much in theeyes as in the mouth, it is still not clearthat an expressive gestalt was distinguished.While it can be argued that our older subjects,too, might simply have abstracted toothy oropen mouths rather than happy or surprise ex-pressions, it is nevertheless the case that our4- and 5,-month-olds could not do so. It re-mains for future research, using a much greatervariety of instances of the same expression(e.g., toothy and closed-mouth smiles) as wellas contrasting expressions that overlap in detail(e.g., toothy and compressed-mouth anger), todetermine unequivocally whether expressions orfeatures have been discriminated.

Given the nature of the stimuli employed,a question may be raised about the generalityof the present findings. A photograph of afacial expression is, after all, merely a distil-lation from a complex expressive reaction, andit is conceivable that more "realistic" stimuli(e.g., animated faces coupled with vocal andgestural cues) would be easier to discriminateat younger ages than faces in arrested anima-tion. Indeed, it has been argued from an eco-logical perspective that the perceptual systemis attuned at the outset to higher-order in-variants, multimodally perceived, and that de-graded facets of such dynamic patterns (e.g.,static facial expressions) are differentiatedmuch later in development (Gibson 1979).However, when Kreutzer and Charlesworth(Note 1) confronted infants with live portray-als of positive and negative facial expressionsaccompanied by appropriate vocalizations andbody movements, they found that 4-month-oldsresponded indiscriminately to the various emo-tions whereas 6-month-olds exhibited differen-tiation (as measured by visual fixation andaffective responses). While this study was notwithout methodological shortcomings, its re-sults, taken together with those of earlier natu-ralistic studies (Ahrens 1954; Biihler & Hetzer1928; Spitz &Wolf 1946), suggest that the useof more ecologically valid stimuli may not yielda developmental timetable much different fromthat obtained in the present research. It goes

without saying, however, that more systematicinvestigation, preferably using the conceptualhabituation paradigm with animated faces, issorely needed in this area.Finally, the finding that across all agesgirls were superior to boys in abstracting facialexpressions invites consideration. Although re-ports of sex-linked differences in performanceare not uncommon in infant research using thehabituation paradigm, superiorperformance hasgenerally been attributed to boys, not girls (seeTighe & Powlison [1978] for review). Boyshave been shown to habituate faster than girls,a finding assumed to reflect greater efficiencyof information processing on their part. Otherperformance measures, however, suggest that

girls may be processing stimulus input morecompletely or in more detail. For example, girlsshow greater target differentiation in their fix-ation latencies, so that they respond progres-sively faster during habituation to a complexstimulus (24 x 24 checkerboard) than to a lesscomplex one (2 x 2 checkerboard) (Cohen,DeLoache, &Rissman 1975). They also exhibitstronger response recovery to discrepancy fromthe habituated standard than males (e.g., Caron& Caron 1969). The performance of the girlsin the present study is in keeping with theforegoing in that their superiority lay not inspeed of processing (there was no differencein rate of habituation between the sexes) butin the content processed. While girls apparentlyresponded to the expressive aspects of the face,boys seemed to have attended mostly to thoseparticular details that characterized an individ-ual face. It may be tempting to conclude, assome have, that girls have a more developedface "schema"than boys (Kagan, Henker, Hen-Tov, & Lewis 1966). At present, however, thereis no hard evidence for such an interpretationand, given the generally mixed findings withrespect to sex-linked differences in infancy, ourresults must be viewed as highly tentative.Reference Note1. Kreutzer,M. A., & Charlesworth,W. R. In-fants'reactions o differentexpressions f emo-tions. Paperpresentedto the biennialmeetingof the Societyfor Research n Child Develop-ment, Philadelphia,March1973.ReferencesAhrens, R. Beitrag zur Entwicklungdes Physiog-nomie-undMimikerkennens.Zeitschrift iir ex-perimentelle und angewandte Psychologie,1954, 2, 412-454.


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Caron, Caron,and Myers 1015Ainsworth,M. D. S. The development of infant-motherattachment. n B. M. Caldwell& H. N.Ricciuti (Eds.), Review of child developmentresearch.Vol. 3. Chicago: Universityof Chi-cago Press, 1973.Barrera,M. E., & Maurer,D. The perceptionoffacial expressions by the three-month-old.Child Development,1981, 52, 203-206.Bowlby, J. Attachmentand loss. Vol. 1. Attach-ment. New York:Basic, 1969.Biihler, C., &Hetzer, H. Das ersteVerstdindnisiirAusdruckm erstenLebensjahr.Zeitschrift iirPsychologie,1928, 107, 50-61.Caron, A. J.; Caron, R. F.; Caldwell, R. C.; &Weiss, S. J. Infantperceptionof the structuralpropertiesof the face. DevelopmentalPsychol-ogy, 1973, 9, 385-399.Caron,A. J.; Caron,R. F.; &Carlson,V. R. Infantperceptionof the invariantshape of objectsvarying in slant. Child Development, 1979,50, 716-721.Caron, R. F., & Caron, A. J. Degree of stimuluscomplexity and habituationof visual fixationin infants. Psychonomic Science, 1969, 14,78-79.Charlesworth,W. R., &Kreutzer,M. A. Facial ex-pressionsof infantsand children.In P. Ekman(Ed.), Darwin and facial expression:a cen-tury of research in review. New York: Aca-demic Press, 1973.Cohen, L. B.; DeLoache, J. S.; & Rissman,M. W.The effect of stimulus complexity on infantvisual attention and habituation.Child Devel-

opment, 1975, 46, 611-617.Cohen, L. B., & Strauss,M. S. Concept acquisitionin the human nfant.ChildDevelopment,1979,50, 419-424.Ekman, P., & Friesen, N. V. Unmasking he face.Englewood Cliffs, N.J.: Prentice-Hall,1975.Ekman,P., &Oster,H. Facial expressionsof emo-tion. AnnualReview of Psychology,1979, 30,527-534.Fagan, J. G. Infants' recognitionof invariantfea-tures of faces. Child Development, 1976, 47,627-638.Gewirtz,J. Mechanismsof social learning.In D. A.Goslin(Ed.), Handbookof socializationheoryand research.Chicago: Rand McNally, 1969.

Gibson,E. J.; Owsley,C. J.; &Johnston,J. Percep-tion of invariantsby 5-month-old nfants: dif-ferentiationof two types of motion. Develop-mental Psychology,1978, 14, 407-415.Gibson,J. J. The ecologicalapproach o visualper-ception.Boston:HoughtonMifflin,1979.Izard,C. E. The face of emotion.New York:Ap-pleton-Century-Crofts,971.Kagan,J.; Henker,B. A.; Hen-Tov,A.; &Lewis, M.Infants' differentialreactionsto familiar anddistorted faces. Child Development,1966, 37,519-532.LaBarbera,J. D.; Izard,C. E.; Vietze, P.; &Parisi,S. A. Four- and six-month-old nfants' visual

responses to joy, anger, and neutral expres-sions. Child Development,1976, 47, 535-538.Milewski, A. E. Visual discriminationand detec-tion of configurationalnvariancein 3-monthinfants. Developmental Psychology, 1979, 15,357-363.Nelson, C. A.; Morse,P. A.; & Leavitt, L. A. Rec-ognition of facial expressionsby seven-month-old infants. Child Development, 1979, 56,1239-1242.Oster,H. "Recognition" f emotionalexpression ninfancy? In M. E. Lamb & L. R. Sherrod(Eds.), Infant social cognition:empiricalandtheoretical considerations.Hillsdale, N.J.: Erl-

baum, 1981.Spitz, R. A., & Wolf, K. M. The smilingresponse:a contribution o the orthogenesisof social re-lations.GeneticPsychologyMonographs, 946,34, 57-125.Stern, D. N. The first relationship: infant andmother. Cambridge, Mass.: Harvard Univer-sity Press, 1977.Tighe, T. J., & Powlison, L. B. Sex differences ninfanthabituationresearch:a surveyand somehypotheses. Bulletin of the PsychonomicSo-ciety, 1978, 12, 337-340.Trevarthen,C. Descriptive analysesof infant com-municativebehavior.In H. R. Schaffer(Ed.),Studies n mother-infantnteraction.New York:AcademicPress, 1977.Young-Browne,G.; Rosenfeld,H. M.; & Horowitz,F. D. Infant discriminationof facial expres-sions. Child Development, 1977, 48, 555-567.

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Invariant face expressions