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A comparative perspective on perceptual, cognitive, and social development
Karyl B. Swartz Department of Psychological Sciences, Purdue University, West Lafayette, Indiana, U.S.A. Received 16February 1981 and accepted 3 November 1981 Keywords: social perception, perceptual development, information processing, habituation, comparative psychology.
A Comparative Perspective on Perceptual, Cognitive, and Social Development* With the exception of some early work by Fantz on visual preferences there has been a dearth of integrative comparative considerations of perceptual, cognitive, and social development in primate infants. A current trend in research with human infants has been an information processing approach to basic skills such as memory, attention, and concept acquisition. This empirical work has focused on a levels-ofprocessing analysis and has provided a theoretical understanding of the ontogeny of basic cognitive processes in the human infant. For example, experimental studies of visual habituation have shown that even very young infants demonstrate abstract conceptual understanding that increases in complexity and level of abstraction with age. Such an approach could be of tremendous value to comparative psychologists investigating similar processes in non-human primate infants. The understanding of the development of cognitive skills in non-human primate infants is fundamental to an understanding of other developmental processes such as the categorization and recognition of various social partners. A consideration of the literature on human cognitive development and the application of some of the conceptual and methodological strategies of the information processing approach will provide a broader understanding of primate perceptual, cognitive, and social development.
t
T h e developing n o n - h u m a n primate infant's perception o f its social world is based no only on its experience with social partners b u t also depends on the infant's level of perceptual a n d cognitive ability. F o r example, in order for the infant to distinguish familiar social partners (i.e., members of the immediate social group) f r o m unfamiliar partners (i.e., strangers or intruders into the group), the infant must have sufficient perceptual skills to make the appropriate discrimination between other individuals, a n d it must have sufficient m e m o r y capacity to r e m e m b e r which individuals are indeed those with w h o m it has previously interacted. Such capacities certainly play a role in the socially living primate's discrimination of other social group members, and, hence, its integration into the d o m i n a n c e hierarchy of the group. T h e y also play a role in m o r e immediate socially relevant discriminative tasks to the very y o u n g infant such as recognition of the m o t h e r and other genetic kin. R e c ~ 1 7 6 ~ the m ~ is a f u n d a m e n t a l f ~ ~ s~ differen" tiation which most primate infants achieve early in development. A m a j o r aspect o f the infant's a t t a c h m e n t to the m o t h e r during the first weeks of life is the increasing specificity of its response to the mother, such that with time she takes on a unique and exclusive function for the infant. Such selectivity has been demonstrated in studies of mother-infant separation a n d experimental studies of m o t h e r recognition in m a c a q u e s (Rosenblum & K a u f m a n , 1968; Rosenblum, 1971; R o s e n b l u m & Alpert, 1974, 1977). A l t h o u g h it is clear that social differentiation of specific individuals such as the above requires social experience, there m a y be some types of social differentiation that require *Paper presented at the symposium on Comparative Psychology in Primates, organized by J. T. Braggio, P. A. Bertacchinl and A. Tartabini within the VIIIth Congress of the International Primatological Society, in Corigliano, Italy, 3-5 July, 1980. Journal of Human Evolution (1982) 11, 315-320 0047-2484/82/040315+06 $03.00/0
9 1982 Academic Press Inc. (London) Limited
316
K.B. S'~VARTZ
little or no social experience. Previous research regarding differentiation and preference for social partners in infant macaques has suggested that some global categorical distinctions may be made early in tile infant's life and in the absence of specific social experience. Using the self-selection circus, a multiple choice apparatus that allows the subject selectively to orient toward and to approach live animal stimuli, Sackett (1970) found that partial isolate reared (i.e., visual and auditory contact but no tactual contact with Conspecific age peers) rhesus infants under nine months of age showed a preference for an adult female conspecific over adult females of two other macaque species. Further, partial isolates tess than 44 months preferred an adult female conspecific over an adult male conspecific. In contrast to these early appearing distinctions, partial isolate infants two to seven months of age showed no consistent gender preference with age peers, although infants nine to 1'6 months of age showed a preference for same gender peers. This preference shifted to opposite-gender peers when the animals reached adulthood. AS previously suggested, recognition of specific individuals may require some period of time and social experience. For example, in a study of the development of recognition of t h e mother in bonnet and pigtail macaque infants, Rosenblum & Alpert (1974, 1977) found that there was no consistent preference for the mother over an unfamiliar female in a two-choice situation until 12 weeks of age. This preference for the mother was demonstrated first and was strongest in bonnet females with a similar but weaker pattern of preference shown by bonnet males. Although pigtail infants did demonstrate a preference for the mother beginning at three months, the preference was weak compared to that demonstrated by bonnet infants, and, in fact, disappeared in pigtail male inthnts after 24 weeks of age. The suggestion from the data obtained in experimental studies of social choice is that there may be an ontogeny of social categorization ia infant macaques such that some social categories are differentiated earlier than others and that some of the early categorization or differentiation may occur independent of experience. Thus, global categorical distinctions may be based on perceptual and cognitive abilities of the infant regardless of social experience whereas distinctions at the individual level require social experience as well as certain perceptual/cognitive abilities. Independent of social experience it may also be the case that the more global categorical distinctions may require less in terms of cognitive and perceptual skills than do finer distinctions such as individual recognition. Thus, not only might each ontogenetically successive type of categorization be perceived as requiring a higher level of information processing but it may also be the case that some of the finer discriminations are built on the development of prior discriminations. For example, the infant differentiates adult female conspecifics from adult males long before it differentiates the mother from other adult females. T h e types of cues used to make the more global gender discrimination, i.e., to categorize an adult as belonging to one or the other gender group, may or may not be basic to discriminations within categories. One very global type of cue, e.g., size, may be sufficient to differentiate males from females whereas distinctions between individuals within a category may be based on a specific facial feature or some configurational combination of facial or other features. It i s proposed here that the development of social categorization (or social perception) in macaque infants may be conceptualized in much the same way that cognitive development has been conceptualized in the human infant, that is, in a levels-of-processing approach. Each subsequent level of social discrimination or categorization that the infant demonstrates may require a higher level of cognitive processing as well as some
P E R C E P T U A L , C O G N I T I V E AND SOCIAL D E V E L O P M E N T
317
prescribed level or amount of social experience. By systematically exploring the development of these proposed "levels" of social differentiation, an ontogeny of social perception in macaques could be developed. To date, no one has systematically explored the ontogeny of social differentiation in infant macaques. The purposes of the present paper are (1) to present an experimental paradigm that has been used successfully with h u m a n infant subjects to study early cognitive processes, and (2) to suggest the use of this paradigm in the study of social differentiation in macaque infants. Cohen (1977; Cohen & Caputo, 1978; Cohen & Strauss, 1979) has developed an experimental procedure which allows assessment of the human infant's acquisition of complex cotlcepts such as categories. This procedure is based on a simple visual habituationdishabituation paradigm which has proven to be quite useful in the study of infant visual information processing (see Jeffrey & Cohen, 1971 ; Clifton & Nelson, 1976). Briefly, the habituation-dishabituation paradigm is as follows. The infant is presented with a visual stimulus repeatedly for several discrete trials. During each stimulus presentation the length of time the infant fixates the stimulus is recorded. Over the course of several trials of stimulus presentation the infant's fixation time to tile stimulus decreases (habituation). At some pre-designated point during this period of response waning, a new stimulus is presented that differs from the original stimulus in some specifiable way. If the infant's fixation time to the changed stimulus increases to a level similar to its initial fixation time (dishabituation), it is inferred that the infant has discriminated the change. Several control conditions are also included in this paradigm to demonstrate that neither the response waning during habituation nor the response increase during the dishabituation trials is spurious. Usually a single habituation stimulus is presented during this procedure, and the dishabituation stimulus differs on a single definable perceptual characteristic. Such a manipulation allows the determination of very precise perceptual or cognitive capabilities of even very young infants. Cohen has expanded this paradigm to ask questions about the infant's perception of more complex abstract stimuli such as concepts. By presenting several exemplars of a concept (e.g., "stuffed animal") during the habituation phase and then testing with a new exemplar of the same concept as well as an exemplar from a second concept (e.g., "rattle)", Cohen & Caputo (1978) were able to demonstrate developmental changes in the acquisition of the concept (23-week-old infants were inconsistent in responding to the new exemplar of the habituation concept whereas 28-week-old infants dishabituated only when an exemplar from a new concept was presented). This procedure can be applied to questions of social categorization such as those discussed in the previous sections of the present paper. T h a t is, by presenting groups of inthnts of varying ages with a series of habituation stimuli that are all exemplars of a social category (e.g., adult females) and then presenting a dishabituation (test) stimulus from another category (e.g., adult male), the age at which such categorical discriminations first occur can be determined. In a similar fashion, other more specific discriminations, such as individual recognition, cart be addressed. Further, once the age of onset of particular types of differentiation are determined, this procedure can be used to determine what visual cues underlie the discrimination. T h a t is, by systematically varying the types of visual cues that are changed during the dishabituation phase the investigator can determine which cues are important to the discrimination (i.e., which changes lead to dishabituation) and which cues are not involved in the discrimination (i.e., which changes do not lead to dishabituation). This procedure has been used recently in our laboratory to study perception of social
318
K.B. SWARTZ
facial cues (facial expression)-in h u m a n infants (Swartz & Keane, in preparation). A brief description of this study will provide an example of how the Cohen procedure can be applied to questions of social perception. T h e subjects were 24 three-month-old and 24 six-month-old h u m a n infants, balanced for sex in each age group and experimental condition. The procedure was an infant-controlled visual habituation procedure. The infant sat on a parent's lap facing a display screen. A blinking light, directly in front of the infant, signaled the onset of each trial. Once the infant looked at the light it went off and a slide was projected to the infant's right. Infants were allowed one unrestricted look at each slide. W h e n the infant looked away the slide was turned off and the light reappeared, beginning a new trial. Fixation times were recorded from a closed-circuit television monitor. The stimuli were photographs of four male college students portraying either a happy facial expression (smile) or sad expression (frown, downturned mouth). T h e habituation series consisted of pictures of three of the college students showing one of the facial expressions. T h e subjects were presented with these three faces, always with the same facial expression, repeated in the same order for four times, giving a series of 12 habituation trials. H a l f the subjects were habituated with h a p p y faces and half with sad faces, but the people pictured and the order of presentation were the same for all subjects. Following the 12 habituation trials, subjects in the Experimental group were presented with three test slides in balanced order: (1) a repetition of one of the habituation slides (nochange control), (2) a slide of one of the individuals pictured in the habituation trials but with the other facial expression (expression change), and (3) a slide of an unfamiliar individual showing the same facial expression demonstrated during the habituation trials (person change). Rather than being presented with changing test trials, the subjects in the Control group were presented with another repetition of the three habituation slides. T h e results showed, first, no effect of gender, age, or facial expression on habituation. All subjects showed a significant decrease in fixation time from the first three habituation trials to the last three trials of the habituation series, i.e., all subjects habituated to the series of slides. The results of major interest were the subjects' performance on the test trials (shown in Table 1). Subjects in the Experimental group showed a significant inTable 1
Mean fixation scores on habituation and test trials
Group
Type of trial
Control
Experimental
t
Mean
First three habituation trials Last three habituation trials "Test" trials
0.95 0-66 0.56
~" First three habituation trials Last three habituation trials No-change test Expression-change test Person-change test
0.81 0-52 0.46 0.54 0.58
* Transformed fixation times: X' = logl0(X + 1). crease in fixation time to both test trials in which there was a change, but no increase to the no-change test trial. As expected, the control subjects showed no increase during the fifth repetition of the habituation trials. The results of this study demonstrated that both the three- and six-month-old infants could recognize a change in individuals whom they had seen only a few times (i.e., they
PERCEPTUAL, COGNITIVE AND SOCIAL DEVELOPMENT
319
dishabituated to the person-change test stimulus). This finding has implications for short-term m e m o r y in these infants, as well as suggesting the limits of familiarity and unfamiliarity in very young infants. T h a t these infants recognized a change in individuals w h o m they had seen presented for only four trials suggests that it requires very little experience for a face to become familiar to a three- or a six-month-old h u m a n infant. The developmental progression of facial recognition in h u m a n infants remains to be determined. More importantly, the results of the study demonstrated that the infants could recognize a change in facial expression even when they were presented with multiple exemplars of that expression. This ability to abstract a common element from multiple specific exemplars of a category and to recognize a change in that abstract feature even when not all the features change (i.e., the person pictured did not change even though the facial expression did) can at least be called an instance of discrimination of facial expressions, if not categorization. Whether this finding can be called an instance of concept acquisition as Cohen (1977) has defined it, is not clear. I f the infants have acquired a concept, then it is a social concept. This definitional issue does not detract from the finding that infants as young as three months old could discriminate a change in facial expression as well as a change in the individuals portraying the expressions. This multiple exemplar habituationdishabituation procedure could be used quite effectively to study other social stimuli than facial expression, including the social categories such as species, gender, and age discussed previously. Although this specific procedure has not been used with macaque subjects, H u m p h r e y (1972, 1974; H u m p h r e y & Keeble, 1976) has used a variant of the habituation-dishabituation paradigm with adult rhesus to study differentiation of social and non-social pictorial stimuli. Further, a visual fixation response has been used to study visual acuity in young macaque infants (Teller et al., 1978), and Gunderson (personal communication) has recently employed a visual fixation novelty preference paradigm (originally developed with h u m a n infant subjects; see Fantz et al., 1975) to study recognition memory in young infant macaques. I t should be possible, using this multiple exemplar visual habituation procedure in conjunction with more global experimental procedures such as the selfselection circus, to develop a theoretical understanding of the ontogeny of social perception in macaques, including specification of the visual cues contributing to early social differentiation. Systematic manipulation of the perceptual and cognitive demands of the experimental paradigm can also allow determination of the role of these basic processes in the development of social perception.
References
Clifton, R. K. & Nelson, M. N. (1976). Developmental study of habituation in infants: The importance of paradigm, response system, and state. In (T.J. Tighe, R. N. Leaton, Eds) Habituation, pp. 159-205. Hillsdale, N. J.: Lawrence Erlbaum. Cohen, L. B. (1977). Concept acquisition in the human infant. Paper presented at Society for Research in Child Development Meeting, New Orleans. Cohen, L. B. & Caputo, N. F. (1978), Instructing infants to respond to perceptual categories. Paper presented at the Midwestern Psychological Association Convention, Chicago. Cohen, L. B. & Strauss, M. S. (1979). Concept acquisition in the human infant. Child Development 50, 419-424. Fantz, R. L., Fagan, J. F. I I I & Miranda, S. B. (1975). Early visual selectivity. In (L. B. Cohen & P. Salapatek, Eds) Infant perception : From sensation to cognition, vol. I, pp. 249-345. New York: Academic Press.
320
K. B, SWARTZ
Humphrey, N. K. (1972). "Interest" and "pleasure" : Two determinants of a monkey's visual preferences. Perception 1, 395-416. Humphrey, N. K. (1974). Species and individuals in the perceptual world of monkeys. Perception 3, 105114. Humphrey, N. K. & Keeble, G. R. (1976). How monkeys acquire a new way of seeing. Perception 5, 51-56. Jeffrey, W. E. & Cohen, L. B. (1971). Habituation in the h u m a n infant. In (H. W. Reese, Ed.) Advances in child development and behavior, vol. 1, pp. 63-97. New York: Academic Press. Rosenblum, L. A. (1971). Infant attachment in monkeys. In (H. R. Schaffer, Ed.) The origins of human social relations, pp. 85-113. New York: Academic Press. Rosenblum, L. A. & Alpert, S. (1974). J~lear of strangers and specificity of attachment in monkeys. In (M. Lewis & L. A. Rosenblum, Eds) The origins of fear, pp. 165-193. New York: Wiley. Rosenblum, L. A. & Alpert, S. (1977). Response to mother and stranger: A first step in socialization. In (S. Chevalier-Skolnikoff & F. Poirier, Eds) Primate bio-social development, pp. 463-477. New'York: Garland. Rosenblum, L. A. & Kaufman, I. C. (1968). Variations in infant development and response to maternal loss in monkeys. American Journal of Orthopsychiatry 38, 418-426. Sackett, G. P. (1970). Unlearned responses, differential rearing experiences and the development of soclal attachments by rhesus monkeys. I n (L. A. Rosenblum, Ed.) Primate behavior, vol. 1, pp. 112-140. New York: Academic Press. Swartz, K. B. & Keane, T. (in preparation) Visual habituation to facial expressions in three- and sixmonth-old h u m a n infants. Teller, D. Y., Regal, D. M., Videen, T. O. & Pulos, E. (1978). Development of visual acuity in infant monkeys (Macaca nemestrina) during the early postnatal weeks. Vision Research 18, 561-566.
A Comparative Perspective on Perceptual, Cognitive, and Social Development* With the exception of some early work by Fantz on visual preferences there has been a dearth of integrative comparative considerations of perceptual, cognitive, and social development in primate infants. A current trend in research with human infants has been an information processing approach to basic skills such as memory, attention, and concept acquisition. This empirical work has focused on a levels-ofprocessing analysis and has provided a theoretical understanding of the ontogeny of basic cognitive processes in the human infant. For example, experimental studies of visual habituation have shown that even very young infants demonstrate abstract conceptual understanding that increases in complexity and level of abstraction with age. Such an approach could be of tremendous value to comparative psychologists investigating similar processes in non-human primate infants. The understanding of the development of cognitive skills in non-human primate infants is fundamental to an understanding of other developmental processes such as the categorization and recognition of various social partners. A consideration of the literature on human cognitive development and the application of some of the conceptual and methodological strategies of the information processing approach will provide a broader understanding of primate perceptual, cognitive, and social development.
t
T h e developing n o n - h u m a n primate infant's perception o f its social world is based no only on its experience with social partners b u t also depends on the infant's level of perceptual a n d cognitive ability. F o r example, in order for the infant to distinguish familiar social partners (i.e., members of the immediate social group) f r o m unfamiliar partners (i.e., strangers or intruders into the group), the infant must have sufficient perceptual skills to make the appropriate discrimination between other individuals, a n d it must have sufficient m e m o r y capacity to r e m e m b e r which individuals are indeed those with w h o m it has previously interacted. Such capacities certainly play a role in the socially living primate's discrimination of other social group members, and, hence, its integration into the d o m i n a n c e hierarchy of the group. T h e y also play a role in m o r e immediate socially relevant discriminative tasks to the very y o u n g infant such as recognition of the m o t h e r and other genetic kin. R e c ~ 1 7 6 ~ the m ~ is a f u n d a m e n t a l f ~ ~ s~ differen" tiation which most primate infants achieve early in development. A m a j o r aspect o f the infant's a t t a c h m e n t to the m o t h e r during the first weeks of life is the increasing specificity of its response to the mother, such that with time she takes on a unique and exclusive function for the infant. Such selectivity has been demonstrated in studies of mother-infant separation a n d experimental studies of m o t h e r recognition in m a c a q u e s (Rosenblum & K a u f m a n , 1968; Rosenblum, 1971; R o s e n b l u m & Alpert, 1974, 1977). A l t h o u g h it is clear that social differentiation of specific individuals such as the above requires social experience, there m a y be some types of social differentiation that require *Paper presented at the symposium on Comparative Psychology in Primates, organized by J. T. Braggio, P. A. Bertacchinl and A. Tartabini within the VIIIth Congress of the International Primatological Society, in Corigliano, Italy, 3-5 July, 1980. Journal of Human Evolution (1982) 11, 315-320 0047-2484/82/040315+06 $03.00/0
9 1982 Academic Press Inc. (London) Limited
316
K.B. S'~VARTZ
little or no social experience. Previous research regarding differentiation and preference for social partners in infant macaques has suggested that some global categorical distinctions may be made early in tile infant's life and in the absence of specific social experience. Using the self-selection circus, a multiple choice apparatus that allows the subject selectively to orient toward and to approach live animal stimuli, Sackett (1970) found that partial isolate reared (i.e., visual and auditory contact but no tactual contact with Conspecific age peers) rhesus infants under nine months of age showed a preference for an adult female conspecific over adult females of two other macaque species. Further, partial isolates tess than 44 months preferred an adult female conspecific over an adult male conspecific. In contrast to these early appearing distinctions, partial isolate infants two to seven months of age showed no consistent gender preference with age peers, although infants nine to 1'6 months of age showed a preference for same gender peers. This preference shifted to opposite-gender peers when the animals reached adulthood. AS previously suggested, recognition of specific individuals may require some period of time and social experience. For example, in a study of the development of recognition of t h e mother in bonnet and pigtail macaque infants, Rosenblum & Alpert (1974, 1977) found that there was no consistent preference for the mother over an unfamiliar female in a two-choice situation until 12 weeks of age. This preference for the mother was demonstrated first and was strongest in bonnet females with a similar but weaker pattern of preference shown by bonnet males. Although pigtail infants did demonstrate a preference for the mother beginning at three months, the preference was weak compared to that demonstrated by bonnet infants, and, in fact, disappeared in pigtail male inthnts after 24 weeks of age. The suggestion from the data obtained in experimental studies of social choice is that there may be an ontogeny of social categorization ia infant macaques such that some social categories are differentiated earlier than others and that some of the early categorization or differentiation may occur independent of experience. Thus, global categorical distinctions may be based on perceptual and cognitive abilities of the infant regardless of social experience whereas distinctions at the individual level require social experience as well as certain perceptual/cognitive abilities. Independent of social experience it may also be the case that the more global categorical distinctions may require less in terms of cognitive and perceptual skills than do finer distinctions such as individual recognition. Thus, not only might each ontogenetically successive type of categorization be perceived as requiring a higher level of information processing but it may also be the case that some of the finer discriminations are built on the development of prior discriminations. For example, the infant differentiates adult female conspecifics from adult males long before it differentiates the mother from other adult females. T h e types of cues used to make the more global gender discrimination, i.e., to categorize an adult as belonging to one or the other gender group, may or may not be basic to discriminations within categories. One very global type of cue, e.g., size, may be sufficient to differentiate males from females whereas distinctions between individuals within a category may be based on a specific facial feature or some configurational combination of facial or other features. It i s proposed here that the development of social categorization (or social perception) in macaque infants may be conceptualized in much the same way that cognitive development has been conceptualized in the human infant, that is, in a levels-of-processing approach. Each subsequent level of social discrimination or categorization that the infant demonstrates may require a higher level of cognitive processing as well as some
P E R C E P T U A L , C O G N I T I V E AND SOCIAL D E V E L O P M E N T
317
prescribed level or amount of social experience. By systematically exploring the development of these proposed "levels" of social differentiation, an ontogeny of social perception in macaques could be developed. To date, no one has systematically explored the ontogeny of social differentiation in infant macaques. The purposes of the present paper are (1) to present an experimental paradigm that has been used successfully with h u m a n infant subjects to study early cognitive processes, and (2) to suggest the use of this paradigm in the study of social differentiation in macaque infants. Cohen (1977; Cohen & Caputo, 1978; Cohen & Strauss, 1979) has developed an experimental procedure which allows assessment of the human infant's acquisition of complex cotlcepts such as categories. This procedure is based on a simple visual habituationdishabituation paradigm which has proven to be quite useful in the study of infant visual information processing (see Jeffrey & Cohen, 1971 ; Clifton & Nelson, 1976). Briefly, the habituation-dishabituation paradigm is as follows. The infant is presented with a visual stimulus repeatedly for several discrete trials. During each stimulus presentation the length of time the infant fixates the stimulus is recorded. Over the course of several trials of stimulus presentation the infant's fixation time to tile stimulus decreases (habituation). At some pre-designated point during this period of response waning, a new stimulus is presented that differs from the original stimulus in some specifiable way. If the infant's fixation time to the changed stimulus increases to a level similar to its initial fixation time (dishabituation), it is inferred that the infant has discriminated the change. Several control conditions are also included in this paradigm to demonstrate that neither the response waning during habituation nor the response increase during the dishabituation trials is spurious. Usually a single habituation stimulus is presented during this procedure, and the dishabituation stimulus differs on a single definable perceptual characteristic. Such a manipulation allows the determination of very precise perceptual or cognitive capabilities of even very young infants. Cohen has expanded this paradigm to ask questions about the infant's perception of more complex abstract stimuli such as concepts. By presenting several exemplars of a concept (e.g., "stuffed animal") during the habituation phase and then testing with a new exemplar of the same concept as well as an exemplar from a second concept (e.g., "rattle)", Cohen & Caputo (1978) were able to demonstrate developmental changes in the acquisition of the concept (23-week-old infants were inconsistent in responding to the new exemplar of the habituation concept whereas 28-week-old infants dishabituated only when an exemplar from a new concept was presented). This procedure can be applied to questions of social categorization such as those discussed in the previous sections of the present paper. T h a t is, by presenting groups of inthnts of varying ages with a series of habituation stimuli that are all exemplars of a social category (e.g., adult females) and then presenting a dishabituation (test) stimulus from another category (e.g., adult male), the age at which such categorical discriminations first occur can be determined. In a similar fashion, other more specific discriminations, such as individual recognition, cart be addressed. Further, once the age of onset of particular types of differentiation are determined, this procedure can be used to determine what visual cues underlie the discrimination. T h a t is, by systematically varying the types of visual cues that are changed during the dishabituation phase the investigator can determine which cues are important to the discrimination (i.e., which changes lead to dishabituation) and which cues are not involved in the discrimination (i.e., which changes do not lead to dishabituation). This procedure has been used recently in our laboratory to study perception of social
318
K.B. SWARTZ
facial cues (facial expression)-in h u m a n infants (Swartz & Keane, in preparation). A brief description of this study will provide an example of how the Cohen procedure can be applied to questions of social perception. T h e subjects were 24 three-month-old and 24 six-month-old h u m a n infants, balanced for sex in each age group and experimental condition. The procedure was an infant-controlled visual habituation procedure. The infant sat on a parent's lap facing a display screen. A blinking light, directly in front of the infant, signaled the onset of each trial. Once the infant looked at the light it went off and a slide was projected to the infant's right. Infants were allowed one unrestricted look at each slide. W h e n the infant looked away the slide was turned off and the light reappeared, beginning a new trial. Fixation times were recorded from a closed-circuit television monitor. The stimuli were photographs of four male college students portraying either a happy facial expression (smile) or sad expression (frown, downturned mouth). T h e habituation series consisted of pictures of three of the college students showing one of the facial expressions. T h e subjects were presented with these three faces, always with the same facial expression, repeated in the same order for four times, giving a series of 12 habituation trials. H a l f the subjects were habituated with h a p p y faces and half with sad faces, but the people pictured and the order of presentation were the same for all subjects. Following the 12 habituation trials, subjects in the Experimental group were presented with three test slides in balanced order: (1) a repetition of one of the habituation slides (nochange control), (2) a slide of one of the individuals pictured in the habituation trials but with the other facial expression (expression change), and (3) a slide of an unfamiliar individual showing the same facial expression demonstrated during the habituation trials (person change). Rather than being presented with changing test trials, the subjects in the Control group were presented with another repetition of the three habituation slides. T h e results showed, first, no effect of gender, age, or facial expression on habituation. All subjects showed a significant decrease in fixation time from the first three habituation trials to the last three trials of the habituation series, i.e., all subjects habituated to the series of slides. The results of major interest were the subjects' performance on the test trials (shown in Table 1). Subjects in the Experimental group showed a significant inTable 1
Mean fixation scores on habituation and test trials
Group
Type of trial
Control
Experimental
t
Mean
First three habituation trials Last three habituation trials "Test" trials
0.95 0-66 0.56
~" First three habituation trials Last three habituation trials No-change test Expression-change test Person-change test
0.81 0-52 0.46 0.54 0.58
* Transformed fixation times: X' = logl0(X + 1). crease in fixation time to both test trials in which there was a change, but no increase to the no-change test trial. As expected, the control subjects showed no increase during the fifth repetition of the habituation trials. The results of this study demonstrated that both the three- and six-month-old infants could recognize a change in individuals whom they had seen only a few times (i.e., they
PERCEPTUAL, COGNITIVE AND SOCIAL DEVELOPMENT
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dishabituated to the person-change test stimulus). This finding has implications for short-term m e m o r y in these infants, as well as suggesting the limits of familiarity and unfamiliarity in very young infants. T h a t these infants recognized a change in individuals w h o m they had seen presented for only four trials suggests that it requires very little experience for a face to become familiar to a three- or a six-month-old h u m a n infant. The developmental progression of facial recognition in h u m a n infants remains to be determined. More importantly, the results of the study demonstrated that the infants could recognize a change in facial expression even when they were presented with multiple exemplars of that expression. This ability to abstract a common element from multiple specific exemplars of a category and to recognize a change in that abstract feature even when not all the features change (i.e., the person pictured did not change even though the facial expression did) can at least be called an instance of discrimination of facial expressions, if not categorization. Whether this finding can be called an instance of concept acquisition as Cohen (1977) has defined it, is not clear. I f the infants have acquired a concept, then it is a social concept. This definitional issue does not detract from the finding that infants as young as three months old could discriminate a change in facial expression as well as a change in the individuals portraying the expressions. This multiple exemplar habituationdishabituation procedure could be used quite effectively to study other social stimuli than facial expression, including the social categories such as species, gender, and age discussed previously. Although this specific procedure has not been used with macaque subjects, H u m p h r e y (1972, 1974; H u m p h r e y & Keeble, 1976) has used a variant of the habituation-dishabituation paradigm with adult rhesus to study differentiation of social and non-social pictorial stimuli. Further, a visual fixation response has been used to study visual acuity in young macaque infants (Teller et al., 1978), and Gunderson (personal communication) has recently employed a visual fixation novelty preference paradigm (originally developed with h u m a n infant subjects; see Fantz et al., 1975) to study recognition memory in young infant macaques. I t should be possible, using this multiple exemplar visual habituation procedure in conjunction with more global experimental procedures such as the selfselection circus, to develop a theoretical understanding of the ontogeny of social perception in macaques, including specification of the visual cues contributing to early social differentiation. Systematic manipulation of the perceptual and cognitive demands of the experimental paradigm can also allow determination of the role of these basic processes in the development of social perception.
References
Clifton, R. K. & Nelson, M. N. (1976). Developmental study of habituation in infants: The importance of paradigm, response system, and state. In (T.J. Tighe, R. N. Leaton, Eds) Habituation, pp. 159-205. Hillsdale, N. J.: Lawrence Erlbaum. Cohen, L. B. (1977). Concept acquisition in the human infant. Paper presented at Society for Research in Child Development Meeting, New Orleans. Cohen, L. B. & Caputo, N. F. (1978), Instructing infants to respond to perceptual categories. Paper presented at the Midwestern Psychological Association Convention, Chicago. Cohen, L. B. & Strauss, M. S. (1979). Concept acquisition in the human infant. Child Development 50, 419-424. Fantz, R. L., Fagan, J. F. I I I & Miranda, S. B. (1975). Early visual selectivity. In (L. B. Cohen & P. Salapatek, Eds) Infant perception : From sensation to cognition, vol. I, pp. 249-345. New York: Academic Press.
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Humphrey, N. K. (1972). "Interest" and "pleasure" : Two determinants of a monkey's visual preferences. Perception 1, 395-416. Humphrey, N. K. (1974). Species and individuals in the perceptual world of monkeys. Perception 3, 105114. Humphrey, N. K. & Keeble, G. R. (1976). How monkeys acquire a new way of seeing. Perception 5, 51-56. Jeffrey, W. E. & Cohen, L. B. (1971). Habituation in the h u m a n infant. In (H. W. Reese, Ed.) Advances in child development and behavior, vol. 1, pp. 63-97. New York: Academic Press. Rosenblum, L. A. (1971). Infant attachment in monkeys. In (H. R. Schaffer, Ed.) The origins of human social relations, pp. 85-113. New York: Academic Press. Rosenblum, L. A. & Alpert, S. (1974). J~lear of strangers and specificity of attachment in monkeys. In (M. Lewis & L. A. Rosenblum, Eds) The origins of fear, pp. 165-193. New York: Wiley. Rosenblum, L. A. & Alpert, S. (1977). Response to mother and stranger: A first step in socialization. In (S. Chevalier-Skolnikoff & F. Poirier, Eds) Primate bio-social development, pp. 463-477. New'York: Garland. Rosenblum, L. A. & Kaufman, I. C. (1968). Variations in infant development and response to maternal loss in monkeys. American Journal of Orthopsychiatry 38, 418-426. Sackett, G. P. (1970). Unlearned responses, differential rearing experiences and the development of soclal attachments by rhesus monkeys. I n (L. A. Rosenblum, Ed.) Primate behavior, vol. 1, pp. 112-140. New York: Academic Press. Swartz, K. B. & Keane, T. (in preparation) Visual habituation to facial expressions in three- and sixmonth-old h u m a n infants. Teller, D. Y., Regal, D. M., Videen, T. O. & Pulos, E. (1978). Development of visual acuity in infant monkeys (Macaca nemestrina) during the early postnatal weeks. Vision Research 18, 561-566.