- Email: [email protected]
Infants' discrimination of internal and external pattern elements
JOURNAL
OF
EXPERIMENTAL
CHILD
PSYCHOLOGY
22,229-246
(1976)
Infants’ Discrimination of Internal and External Pattern Elements ALLAN E. MILEWSKI De Paul
University
Human infants’ discrimination of changes in internal and external elements of compound visual patterns was investigated in four experiments employing a familiarization-novelty paradigm in which visual reinforcing patterns were presented contingent upon rate of high-amplitude nonnutritive sucking. In Experiment 1, 4-month infants discriminated changes in the shape of internal, external and both internal and external figures. One-month infants discriminated external changes in both internal and external figures, but failed to show reliable response recovery when only internal figures were changed. Experiments 2 and 3 failed to explain the l-month results on the basis of poor resolution of internal figures by showing comparable discrimination of small and large singly-presented figures and by failing to find improved internal discrimination with large separation between internal and external figures. In Experiment 4, l-month infants showed response recovery to figure additions made adjacent to the initial figure, but not to internal additions. The results are interpreted in terms of attentiveness by young infants to external pattern elements and may indicate early processing of figure-ground information. The developmental differences observed suggest an increased breadth of attention to pattern elements.
Recent investigations of infant visual scanning behavior have reported that young infants, when presented with compound patterns containing internal and external elements, tend to concentrate their visual scans on portions of the external pattern more than on internal elements. For infants of 1 month and younger, external-contour scanning has been found to be predominant both with abstract, two-dimensional stimuli (Salapatek, Note 1,1975) and with facial stimuli (Bergman, Haith, & Mann, Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4). In contrast to the predominant external-contour scanning by l-month infants, 2-month infants in these investigations have shown broader scanning of both internal and external contours, occasionally with greater scanning of internal compared with external contours. The developmental changes that have been observed in infants’ scanning of compound patterns may have implications for some common empirical This research is based in part on a Ph.D. thesis submitted to Brown University and was supported by NIH Grant HD 03386 to E. R. Siqueland. The author thanks E. R. Siqueland for his help in all phases of the project. Requests for reprints should be sent to Allen E. Milewski, Department of Psychology, De Paul University, 2323 North Seminary Avenue, Chicago, Illinois 60614. 229 Copyright 0 1976 by Academic Press. Inc. All rights of reproduction in any form reserved.
230
ALLEN
E. MILEWSKI
findings suggesting early changes in visual attention and perception. For example, the literature on responsiveness to facial stimuli (see Gibson, 1969) suggests that infants younger than 2 or 3 months of age may not distinguish between faces that are equated for brightness and contour amount. Similarily, several studies have suggested that young infants may not discriminate between complex patterns differing in the arrangement of elements (see Bond, 1972). A large proportion of the information needed to discriminate between faces, and often between the abstract stimuli that have been used in arrangement-discrimination studies, is contained in internal portions of the patterns. Therefore, it is possible that the developmental changes observed in discrimination of faces and abstract patterns may be partly attributed to the young infant’s failure to visually scan information within the borders of complex patterns. However, there is a serious reservation in drawing conclusions about infant perception solely on the basis of findings from visual scanning studies. There is a lack of empirical evidence concerning the relationship between the measured locus of visual scanning in infants and the information that is actually processed about the visual stimulus. While several writers have emphasized the role of eye movements and directed visual regard in pattern perception and its development (Hebb, 1949; Hochberg, 1968; Noton & Stark, 1971; Piaget, 1969; Vurpillot, 1968), there is considerable evidence for pattern perception, by adults at least, under conditions which prohibit eye movements and/or macular investigation of the stimulus (Evans, 1965; Mooney, 1959; Zusne, 1970, pp. 307-309). The present investigation was designed to study infants’ responsiveness to internal and external pattern elements that is actually processed and stored by the infant. The high-amplitude operant sucking (HAS) technique was used in a stimulus familiarization-novelty paradigm (Siqueland & DeLucia, 1969). The HAS technique has previously provided important information regarding young infants’ auditory discrimination of speech stimuli (Eimas, Siqueland, Jusczyk, & Vigorito, 1971; Morse, 1972; Eimas, Note 5>, as well as visual discrimination of color and pattern (Milewski & Siqueland, 1975, Siqueland, Note 6, Note 7). Briefly, the HAS procedure involves the presentation of a visual or auditory reinforcing stimulus contingent upon the rate of nonnutritive high-amplitude sucking. Typically, as the infant learns that his HAS rate determines stimulus presentation, his rate initially increases above baseline and then decreases, presumably as a result of stimulus familiarization and a reduction in the reinforcing properties of the once-novel stimulus. If the familiar stimulus is subsequently replaced with a novel stimulus without change in the reinforcement contingencies and HAS rate increases relative to that of control subjects who continue to receive the familiar stimulus, discrimination of the stimulus change is inferred. The purpose of the present study was twofold: (i) to directly investigate
INFANTS’
DISCRIMINATION
OF PATTERNS
231
infants’ processing of compound patterns by comparing discrimination performance for changes in internal and external elements; and (ii) to investigate possible explanations for differences in younger infants’ discrimination of internal and external pattern changes. GENERAL
METHOD
The procedure and apparatus employed in the present investigation were identical to those described in detail by Milewski and Siqueland (1975). The apparatus provided visual reinforcement contingent upon the subject’s rate of nonnutritive high-amplitude sucking. Pressure variations due to the subject’s sucking on a sterilized, sealed nipple were converted to an analogue record by a gas pressure transducer. The analogue signal was digitized by a Schmitt trigger on the basis of a high-amplitude response criterion set for each subject during a prebaseline period. A PDP Lab 8/E mini-computer provided minute by minute storage of the digital sucking data. Visual stimulation was produced by increasing the voltage to a Kodak slide projector proportional to the rate of supracriterion sucking. Maximum stimulus illumination (60 V to the projector) was maintained by a momentary sucking rate of two or more sucks per second. The stimuli were white line figures on a black surround, projected onto a rear-projection screen which was 25 cm from the subjects. Angular subtense of the lines for all stimuli was approximately 55 min. The component shapes were a circle, a square, and an equilateral triangle. Infants were tested during their normal waking hours and were positioned in an infant seat facing the rear projection screen in a darkened, sound attenuated cubicle. While one experimenter monitored the recording and programming equipment in a separate room, another experimenter who was hidden from the subject’s view by a linen sheet held the nipple in the infant’s mouth approximately at midline. The procedure was a modification of the operant sucking methodology described by Siqueland and DeLucia (1969) and consisted of three segments: (i) During the first 1 or 2 min, the high-amplitude response criterion and baseline rate of responding were determined in the absence of visual reinforcement. The amplitude criterion was set independently for each subject at approximately the median of his baseline sucking amplitudes. Throughout the session, “high-amplitude sucks” weredefined as responses falling above the criterion amplitude; (ii) Immediately after the baseline rate was obtained, subjects received the initial visual reinforcing stimulus contingent on their rate of high-amplitude sucking. Following 3 min of acquisition training, assessment of familiarization effects began. A performance measure of stimulus familiarization was employed requiring each subject to hold, for two consecutive minutes, a decrement in sucking rate of 20% or more compared with the minute
232
ALLEN
E. MILEWSKI
immediately preceding the first minute of decrement. This second segment then had a minimum duration of 5 min, but varied across subjects as a function of the time required to meet the performance measure of familiarization. In order to ensure some minimal stimulus exposure immediately prior to the test segment, subjects were excluded from the study if they failed to meet a minimum response criterion of an average of 14 responses in the fast 2 min of familiarization. Subjects were also excluded if they failed to meet the performance measure of familiarization within 22 min; (iii) Following stimulus familiarization, a 5-min test segment began with no change in the stimulus feedback contingenices. During this segment experimental shift groups received a novel stimulus contingent upon sucking, while separate no-shift control groups continued to receive the initial visual stimulus. Experiment
1
Experiment 1 investigated the discrimination of internal and external pattern element changes developmentally. One- and four-month infants initially received compound visual stimuli. Independent groups received subsequent changes in either the internal, the external, or both the internal and external figures. Based on the developmental changes observed in the locus of infants’ visuaI scanning for compound figures (Salapatek, Note 1: Bergman et af ., Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4), it was expected that the l-month infants, but not the 4-month infants, would show greater response recovery to changes in the external compared with internal figures. Method Subjects. Subjects were 32 l-month and 32 4-month, full-term infants. The l-month subjects ranged from 27 days to 47 days @I = 35 days); the 4-month subjects ranged from 95 to 143 days (It4 = 124 days). Additional subjects were excluded from the study on the basis of behavior or experimental errors prior to the test segment. Thirty-nine l-month infants were excluded: 20 due to sleeping, 11 for excessive crying or rejection of the nipple, three due to experimenter error or equipment failure, and five for failure to meet the performance criteria specified in the General Method Section. Forty-eight 4-month infants were excluded: 13 for sleeping; 33 for crying; and two due to experimenter error. Stimuli. Each stimulus consisted of two different component figures, one centered within the other. The internal figures subtended an average horizontal visual angle of approximately 12”; the average visual angle of the external figures was approximately 32”. Within the large external and smaller internal groupings, figures were similar in area and contour amount.
INFANTS’
DISCRIMINATION
233
OF PATTERNS
FIG. 1. Representative stimuli used in Experiment
1.
Design. The design included four experimental groups (each with N = 8) at each of the two ages. All groups were treated identically during the initial reinforcement and familiarization segments, and differed only in the locus of the shape change received during the test segment. Subjects received either a shift in the shape of the internal figure (Group I), the external figure (Group E), both the internal and external figures (Group I-E), or no shift in the initial visual reinforcing stimulus (Group NS). For six of eight subjects, each group was counterbalanced for the specific combination of component figures used in the internal and external positions with the restriction that the internal and external figures not be the same shape. For the remaining two subjects in each group, the initial stimulus was determined randomly. In Group I-E, stimulus changes were restricted such that the internal and external components were never simply reversed with respect to each other. Experimental design and stimulus examples are represented in Fig. 1. Results and Discussion Preshift performance. Since all experimental groups received identical treatment prior to the test segment, no group differences in performance were expected during familiarization. Separate analyses of preshift performance were done for l- and Cmonth subjects. Mixed design analyses of variance done on the minute by minute sucking rates for the 5 min preceding the test segment showed no significant group differences.
234
ALLEN
E. MILEWSKI
Moreover, for each age level, analyses of variance indicated no differences between experimental groups in baseline sucking rates, the amount of time required to meet the response decrement criterion, or the amount of response decrement at the end of familiarization. Finally, to assess response acquisition during the preshift segment, the data for each subject were vincentized by equally dividing into two blocks the period from the beginning of conditioning to the minute preceding the two minutes of 20% decrement. Two factor (Groups x Time blocks) mixed design analyses of variance were done on the mean number of criterion sucks over the two time blocks. Only the time factors were significant, indicating an overall increase in response levels over acquisition for l-month subjects, F(1,28) = 17.62,~ < .OOl, and 4-month subjects, F(1,28) = 33.92,~ < .OOl. In summary, for both age levels, no reliable differences in performance were found between experimental groups prior to the test segment. Test segment performance. In order to provide a sensitive measure of response change during the test segment relative to the familiarization segment, difference scores were computed for each subject by subtracting his average response rate during the last 2 min preceding the test segment from his rate for each test segment minute, providing five difference scores for each subject. Separate two factor (Experimental group x Postshift time) mixed design analyses of variance were performed on the difference scores for l- and 4-month subjects. One-month subjects. Analysis of l-month subjects’ difference scores revealed a significant main effect of groups,F(3,28) = 3.57,~ < .05. Since no other main effects or interactions were significant, results were collapsed across the five test segment minutes for subsequent analysis. Figure 2 shows the collapsed mean group difference scores for one-month subjects. A post-hoc Dunnett’s t test for comparison of each experimental group with the control (Winer, 1962) indicated that although Groups E and I-E differed from Group NS (p < .Ol andp < .05, respectively), Group I did not differ from the control. Four-month subjects. Analysis of the test segment difference scores of 4-month subjects showed a significant groups effect, F(3,28) = 10.60, p < .Ol, as well as a reliable time effect, F(4,112) = 5.12,~ < .Ol. Mean group difference scores for 4-month subjects are shown in Fig. 2. A Dunnett’s c test performed on the difference scores collapsed across time revealed that each of the stimulus shift groups (i.e., Groups I, E, and I-E) differed significantly from the NS control (p < .Ol). A trend analysis to determine the nature of the significant time effect showed an overall quadratic component, F(1.7) = 5.76. p < .05, with no significant groups x time interaction. The main interest of this study concerns performance differences between groups; therefore, since the effect was found for all groups, the cause of the curvilinear recovery function will not be considered further. These results indicate response recovery by 4-month infants to shape
INFANTS’
DISCRIMINATION
OF PATTERNS
235
FIG. 2. Mean group difference scores collapsed across the 5-min test segment, for l-month infants (solid) and for 4-month infants (cross-hatched) in Experiment 1.
changes in internal figures, external figures and in both internal and external figures. In contrast, l-month infants showed recovery to external shape changes and changes in both internal and external figures, but no evidence was found for recovery by l-month infants to changes in the shape of internal figures. Since for both age levels, no reliable group differences were found in performance prior to the test segment, group differences during the test segment suggest differential discrimination of stimulus changes and cannot be explained as an artifact of preshift performance. These results are consistent with the findings of visual scanning studies in which younger infants show more fixations to external compared with internal pattern elements (Salapatek, Note 1; Bergman et al., Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4). For convenience, both the evidence for discrimination of external but not internal figure changes, and the predominant visual scanning of external elements observed in l-month infants are subsequently referred to as the “externality effect.” At least two explanations for the externality effect are possible. The externality effect may have its basis in attentional processes wherein external and internal pattern elements differ with respect to salience for the young infant. An alternative class of explanations assumes differential availability of external and internal pattern information for the l-month infant, possibly related to the poor acuity in young infants (Fan% Ordy, & Udelf, 1962; Teller, Morse, Borton, & Regal, in press). Experiments 2 and 3 were designed to investigate whether the discrimination performance of l-month infants for internal and external elements can be explained in terms of differential availability of pattern information. Experiment
2
discrimination The results of Experiment 1, showing differential performance by l-month infants for internal and external figure changes, may have resulted from differences in the discriminability of the
236
ALLEN
E. MILEWSKI
independent component figures due to their relative sizes. It is possible, for example, that the small internal figures were not resolved by the l-month infants well enough to be discriminated. Although all figures were well within l-month grating acuity limits, there is no evidence pertaining to size thresholds in infants’ discrimination of geometric figures. Therefore, Experiment 2 was designed to test the possibility that differential discriminability of the individual component figures may account for the externality effect observed in l-month infants. This experiment compared response recovery to shape changes in either small or large figures presented singly in the visual field. Method Subjects. Twenty-four full-term l-month infants ranging from 27 to 40 days @I = 32 days) were randomly assigned to one of four experimental groups. Thirty-six additional subjects were tested, but were omitted from further analysis: 17 for sleeping, 14 for crying, and five for failing to meet the performance criteria described in the General Method Section. Stimuli. The stimuli were either large or small figures projected singly on a dark surround. The component shapes, stimulus sizes, and viewing conditions were identical to those used in Experiment 1. Design. The design provided for a comparison of four experimental groups (each with N = 6): two groups received a stimulus change during the test segment, and two groups were no-shift controls. One stimulus change group (Group L) received a single large figure contingent upon sucking during the familiarization segment followed by a different large figure in the test segment. Group S received a small single figure initially, followed by a different small figure. A large-stimulus no-shift group (Group LNS) and a small-stimulus no-shift group (Group SNS) continued to receive the same stimulus in the initial familiarization and test segments. Experimental groups were counterbalanced with respect to the specific component shapes presented during the familiarization and test segments of the experiment. Results and Discussion No evidence for systematic group differences on preshift performance was obtained by analyses of variance on the mean response rate during baseline and 5 min prior to the test segment, the amount of response decrement at the end of familiarization, the latency of response decrement or the amount of increase in response rates between vincentized preshift time blocks. The analysis of vincentized data did indicate a significant time effect suggesting an overall increase in response rate over acquisition, F(1,20) = 19.92,~ -=c.OOl. As in Experiment 1, difference scores were used to assess response
INFANTS’
2 0 s Ly
DISCRIMINATION
237
IO -
S-
s z if
0
2 f
-5 -
z
OF PATTERNS
SNS
LNS
-10 -
-15-
FIG. 3. Mean difference scores for experimental groups in Experiment 2, collapsed across the 5-min test segment.
change during the test segment. A two factor (Groups x Postshift time) mixed design analysis of variance performed on the difference scores indicated a significant groups effect, F(3,20) = 5.80, p < .Ol. No other main effects or interactions were significant. Figure 3 shows the group difference scores averaged across the five test segment minutes. A conservative Tukey’s “hsd” test (Winer, 1%2) revealed the following results: (i) Group L differed from Group LNS (p < .Ol); (ii) Group S and Group SNS were different (p < .Ol); (iii) no evidence of reliable differences was found between Groups L and S or between Groups LNS and SNS. These results indicate comparable discrimination performance by l-month infants of shape changes in small and large singly presented figures. Since the small and large stimulus sizes were identical to the sizes of internal and external figures of the compound patterns in Experiment 1, it can be inferred that the evidence for an externality effect in l-month infants did not result from differences in discriminability due to the size of the independent component figures. Instead, the results of Experiments 1 and 2 together suggest that the externality effect is dependent upon the presentation of figures in compound displays. Experiment
3
There is a second possible explanation for the externality effect which is consistent with the suggestion that the effect arises from the use of compound displays and which assumes differential availability of information from internal and external figures. This explanation assumes degradation of internal pattern information as the result of spatial interaction between contours of internal and external figures. According to this explanation, while the interaction effects are mutual between internal
238
ALLEN
E. MILEWSKI
and external figures, there may be relatively more degradation of internal pattern information due to interference from both the surrounding figure and nearby internal contours. It is not possible to speculate about a specific spatial interaction mechanism with respect to the externality effect in infants, but lateral interference effects have been reported in diverse areas of the adult and animal perceptual literature (e.g., Bridger, 1971: Ratliff. 1965). Experiment 3 was designed to investigate the role of spatial interaction of contours in young infants’ discrimination of internal figure changes. It was reasoned that increasing the separation between internal and external components by the use of large external figures should reduce the amount of interaction between figures. Hence, if spatial interaction of contours accounts for the externality effect, the use of large external figures should result in improved discrimination of internal changes by l-month infants. Method Subjects. Subjects were eight l-month infants ranging from 28-34 days (M = 31 days). An additional 16 subjects were rejected: eight for sleeping: seven for crying; and one for failure to meet the performance criterion described in the General Method Section. Stimufianddesign. Following the initial familiarization with a compound pattern, all subjects received a shift in the shape of the internal figure (Group I’). The stimuli and stimulus shifts were identical to those used in the internal shift group of Experiment 1 with the exception that the external figures in this experiment were larger, subtending an average horizontal visual angle of 44”. A separate no-shift control group was not run for this experiment. Instead, the data from the one-month Group NS of Experiment I were compared with the data of Group I’. Results and Discussion Analyses of preshift performance which were identical to those used in the previous experiments failed to find any differences between Group I’ and Group NS prior to the introduction of stimulus novelty conditions. An analysis of variance comparing response rate across vincentized acquisition blocks failed to reach significance but showed trends similar to those of Experiments 1 and 2, F(1,14) = 4.09, p = .06. Figure 4 shows the mean difference scores for Groups I’ and NS averaged across the five test segment minutes. A two factor (Experimental groups x Postshift time) analysis of variance failed to find a significant main effect either of groups or of postshift time. In summary, Experiment 3 failed to find evidence for discrimination of changes in the shape of figures centered within large external figures. These results replicate the failure in Experiment 1 to find discrimination of
INFANTS’
DISCRIMINATION
OF PATTERNS
239
FIG. 4. Mean, collapsed test segment difference scores for each group in Experiment 3.
internal figures by I-month infants. Moreover, if it is assumed that the increased size of the external figures in this experiment was sufficient to significantly reduce or eliminate the effects of spatial interaction, the lack of improved internal figure discrimination in l-month infants is not consistent with an interpretation of the externality effect postulating degradation of internal figure information due to an interaction between internal and external contours. Experiment
4
Experiments 2 and 3 failed to provide evidence in support of two explanations of the externality effect in young infants which assume differential availability of internal and external pattern information. The results of these experiments suggest that the findings of Experiment 1 were not due to an inability of the young infants to resolve the internal figures either because of their size or because of lateral interference between contours. An alternative class of explanations assumes equal availability of internal and external information but proposes that the externality effect has its basis in attentional mechanisms. There are at least two ways in which an attentional hypothesis can explain the externality effect. One explanation stresses the relative size of the internal and external figures, while another explanation emphasizes the relative location of component figures in the pattern. Obviously, in both the present discrimination study and the visual scanning studies reporting externality effects, the external
240
ALLEN
E. MILEWSKI
figures were larger than the internal figures. Therefore, it is possible to explain the results of these studies by assuming an attentional preference in young infants for larger as compared with smaller elements in compound displays. In contrast to an explanation based on attentional preferences as a function of element size, it is possible that the externality effect is mediated by a tendency in young infants to attend to external pattern elements more than to internal elements. Experiment 4 investigated whether inferior discrimination of small figures in compound patterns occurs only when the small figure is centered within a larger figure, or whether inferior discrimination will also occur when the small figure is in a position adjacent to the larger figure. Hypotheses assuming that a size preference underlies the externality effect in l-month infants would predict poor discrimination for changes in small figures positioned either within or adjacent to the larger figure. However, results indicating discrimination of adjacent small figures but not internal small figures would implicate the relative locations of figures in a pattern as the basis of the externality effect. Rather than using changes in the shape of the small figures as the novelty condition, Experiment 4 employed novelty conditions involving the addition of a small figure either in the internal or adjacent position. Previous pilot work with a small number of subjects failed to find discrimination when a single large familiarized figure was changed to a compound by the addition of an internal component. Experiment 4 tested whether the externality effect can be found even with the large amounts of change in brightness and contour associated with the addition of an internal figure, and compared discrimination of internal additions with identical additions in an adjacent position. Method
Subjects. Sixteen full-term infants ranging from 27 to 44 days (M = 36 days) were randomly assigned to one of two groups (each with N = 8). In addition, 17 infants were rejected for sleeping, seven for crying, and one for failing to meet the minimum response criterion. Stimuli and design. Subjects initially received a single large figure as the familiarization stimulus. The stimulus change conditions during the test segment consisted of the addition of a small figure either centered within the initial large figure (Group IA) or in a position adjacent to the large figure (Group AA). For Group AA, the center of the small figure was positioned parallel to and 29” from the center of the large figure; the adjacent figures were positioned on either side of the center of the projection screen. The large figure was on the right for half the subjects and on the left for the other half. For Group AA, the initial familiarization stimulus was positioned off-center so that no change in the position of the large figure occurred
INFANTS’
DISCRIMINATION
OF
PAlTERNS
241
between familiarization and test segments. The data from Groups IA and AA were compared with control data obtained by combining the data from the six subjects of Group LNS (from Experiment 2) with two additional no-shift subjects. For six of eight subjects, each group was counterbalanced for the specific component shapes used in the familiarization and test segments. The stimulus figures for the remaining two subjects in each group were determined randomly. Results and Discussion With the exception of one measure, analysis of preshift performance failed to find any differences between experimental groups. A two factor (Experimental groups x Time blocks) analysis of variance performed on the vincentized acquisition data showed a reliable effect of time blocks, F(2,21) = 35.73, p < .OOl. In contrast to the previous experiments, this analysis also revealed a significant groups x time interaction F(2,21) = 4.48,~ < -05 reflecting a smaller amount of increase in response rates between vincentized blocks for Group AA compared with either Groups IA or LNS. The mean difference scores for each experimental group, averaged across the five test segment minutes are shown in Fig. 5. A two factor (Experimental groups x Postshift time) mixed-design analysis of variance performed on these scores revealed a significant effect of group, F(2,21) = 5.15,~ < .05; no other main effect of interaction was reliable. A post-hoc Dunnett’s t test showed that only Group AA differed significantly from the LNS control group (p < .05). Groups IA and LNS did not differ from each other. The results of this experiment replicate, under conditions involving a large amount of stimulus change in the internal position, the failure of Experiment 1 to find significant response recovery to internal changes, while recovery was found for adjacent figure additions. These results are qualified somewhat by the reliable preshift interaction found between experimental groups and vincentized acquisition blocks. This single preshift interaction is somewhat surprising since groups were treated identically prior to the test segment and since similar preshift conditions in Experiments 1,2, and 3 failed to result in a similar interaction. However, Milewski and Siqueland (1975) failed to find a significant correlation between individual differences in vincentized acquisition data and subsequent test segment performance; hence, it appears unlikely that this interaction qualifies the test segment results in any important way. The demonstration of response recovery to adjacent additions but not to internal additions tentatively argues against an explanation of the externality effect based on attentional preferences as a function of figure size. Instead, these results appear to suggest differential responsiveness to
242
ALLEN
E. MILEWSKI
FIG. 5. Mean, collapsed difference scores for experimental groups in Experiment 4.
information in the internal compound patterns.
portion
compared
with other portions
of
GENERAL DISCUSSION
The results of the present investigation are consistent with previous HAS studies which have shown that human infants between 1 and 4 months are differentially responsive to novel and familiar visual pattern reinforcers and show recovery of operant sucking to presentations of novel patterns. The present results extend this conclusion by showing differential novelty effects in l-month infants as a function of the locus of change in compound patterns. While l-month infants in Experiment 1 showed discrimination of external figure changes, Experiments 1, 3, and 4 provide no evidence for discrimination of internal changes. This result is consistent with those indicating predominant visual scanning of external contours by young infants (Salapatek, Note 1; Bergman& al., Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4), and provides indirect evidence for a relationship between the locus of scanning and visual discrimination performance in one-month infants. In contrast to the differential discrimination of internal and external elements by l-month infants, 4-month infants in Experiment 1 showe+d comparable discrimination of all pattern changes. This result is consistent with the visual scanning data in suggesting a developmental increase in breadth of attention, but fails to support the previous report of greater attentiveness to internal elements by older infants (Salapatek, Note 1). It is possible that the locus of visual scanning may not be as important for older
INFANTS’
DISCRIMINATION
OF
PATTERNS
243
infants’ discrimination as it is for young infants, or that the visual scanning patterns of the 4-month infants used in the present study may not be comparable to those of the 2-month infants used in the visual scanning studies. Possibly, the interpretation of results from Experiment 1 suggesting developmental differences in responsiveness to internal and external figure changes should be qualified. An overall analysis of test segment performance including l- and 4-month data revealed no evidence for the groups x age interaction that would provide strong support for the conclusion of developmental differences. However, since post-hoc tests which were run on the combined data indicated a reliable difference between Groups I and NS for, the 4-month subjects but not the l-month subjects, the failure to find a significant groups x age interaction in recovery scores may have been a function of the large variability both within groups and ages. Since 4-month subjects in Experiment 1 showed reliable recovery in both Groups I and E, evidence for additivity in Group I-E might be expected; however, Group I-E failed to show greater recovery compared with either Group I or E. The HAS procedure may be insensitive as a measure of cue additivity, possibly due to ceiling effects in the level of criterion responding during the test segment, Previous HAS studies have failed to find greater recovery to changes in both color and form of visual patterns compared with changes in either cue alone (Milewski & Siqueland, 1975; Siqueland, Note 7). Two classes of explanations for the externality effects observed in l-month infants were investigated in the present study. Experiments 2 and 3 failed to provide evidence in support of explanations based on differential availability of internal and external information either due to figure size or because of lateral interference effects. These experiments provided: (i) evidence for comparable discrimination of singly-presented small and large figures and (ii) no evidence for improved internal discrimination when the spatial separation between internal and external elements was increased. Since only two amounts of separation were used in this study, it should be noted that the present results do not provide strong proof against a lateral interference explanation. Further study comparing internal discrimination under several amounts of separation is needed. The second class of explanations assumes that internal information is available to the young infant, but that differential attention to pattern elements mediates the externality effect. The results of Experiment 4, while qualified by a preshift interaction between groups and acquisition rate, suggest discrimination of adjacent additions but no reliable recovery to internal additions. This finding does not support an explanation based on differential attention due to the relative size of elements, but tentatively suggests that discrimination varies as a function of the relative position of
244
ALLEN
E. MILEWSKI
pattern elements. Given the present results, the evidence for externality effects observed in young infants’ discrimination and visual scanning may be most easily explained in terms of differential attention to internal and external portions of patterns. While the present evidence provides no direct proof of an attentional explanation, compared with the alternative hypotheses discussed above, an attentional interpretation confronts few problems. One proposal which might explain the greater attention to external elements assumes that the young infant maintains fixations on and processes only the first contour contacted (Haith. In press). However, Salapatek (1975) has noted that the majority of young infants in the visual scanning studies show some scanning of internal elements and in some cases move to an external contour from an initial internal fixation. Another explanation of the externality effect (Salapatek, 1975) has suggested that the predominance of external scanning by l-month infants may reflect maintenance of the young infants’ line of sight at those portions of the pattern which are most relevant to the segregation of figure from ground. In this context, Salapatek discusses a physiological distinction between a midbrain-mediated “secondary” visual system which segregates figures from ground on the basis of gross sensory differences, and a cortically-mediated “primary” visual system which processes more detailed information from previously segregated figures (Held, 1972; Humphrey, 1974; Schneider, 1969; Trevarthen, 1968). Bronson (1975) has recently argued that the young infant is cortically immature and that his perceptual abilities may be explained by assuming control by the “secondary” visual system. Finally, several writers have suggested that primitive organizational information regarding figure-ground segregation is a particularly salient feature of the young infant’s visual environment (Gibson, 1969; Hebb, 1949). Hence, an interpretation of the externality effect found in the present investigation which assumes early processing of figure-ground information is consistent with current notions of visual physiology as well as with major theories of perceptual development. REFERENCES Bond, E. K. Perception 225-245. Bridger, B. Metacontrast Bronson. G. The postnatal Eimas, P. D., Siqueland, Science,
1971, 171,
of form by the human infant. Psychological
Bulletin.
1972, 77,
and lateral inhibition. Psychological ReGcn,. 1971, 78, 528-539. growth of visual capacity. Child Development. 1974.45,873-890. E. R., Jusczyk. P., & Vigorito. J. Speech perception in infants. 303-306.
Evans, C. R. Some studies of pattern perception using stabilized retinal images. British Journal of Psychology, 1965. 56, 121-133. Fantz, R. L.. Ordy. J. M., & Udelf. M. S. Maturation of pattern vision in infants during the first six months. Journal of Compurntive and Ph~~siologkwl Psychology. 1962. 55, 907-917.
INFANTS’
DISCRIMINATION
OF PATTERNS
245
Gibson, E. J. Principles of perceptual learning and development. New York: AppletonCentury-Crofts, 1969. Haith, M. M. Visual competence in early infancy. In R. Held, H. Leibowitz, & H. L. Teuber (Eds.), Handbook of Sensory Physiology (Vol. 8) New York: Springer-Verlag. in p-ess. Hebb, D. 0. Organization ofbehavior. New York: Wiley. 1949. Held, R. Two modes of processing spatially distributed visual stimulation. In The Neurosciences: Second study program. New York: Rockefeller University Press, 1972. Hochberg, J. In the mind’s eye. In R. R. Haber (Ed.), Contemporary theory and research in visual perception. New York: Holt, Rinehart & Winston, 1%8. Humphrey, N. K. Vision in a monkey without striate cortex: A case study. Perception, 1974. 3, 241-255. Milewski, A. E., & Siqueland, E. R. Discrimination of color and pattern novelty in one-month human infants. Journal of Experimental Child Psychology, 1975, 19, 122-136. Mooney, C. M. Recognition of novel visual configurations with and without eye movements. Journal of Experimental Psychology, 1958, 56, 133- 138. Morse, P. The discrimination of speech and nonspeech stimuli early in infancy. Journal of Experimental child Psychology, 1972, 3, 477-492. Noton, D., & Stark, L. Scan paths in saccadic eye movements while viewing and recognizing patterns. Vision Research, 1971, 11, 929-942. Piaget, J. The mechanisms of perception. New York: Basic Books, 1969. Ratliff, F. Much Bands: Quantitative studies on neural networks in the retina. San Francisco: Holden-Day, 1%5. Salapatek, P. Pattern perception in early infancy. In L. B. Cohen & P. Salapatek (Eds.), Infant perception: From sensation to cognition. New York: Academic Press, 1975. Schneider, G. E. Two visual systems. Science, 1969, 163, 895-902. Siqueland, E. R., & DeLucia, D. Visual reinforcement of nonnutritive sucking in human infants. Science, 1969, 165, ll44- 1146. Teller, D. Y., Morse, R., Borton, R.. & Regal, D. Visual acuity for vertical and diagonal gratings in human infants. Vision Research, 1974, 14, 1433- 1439. Trevarthen, C. B. Two mechanisms of vision in primates. Psycologische Forschung, 1968.31, 299-337. Vurpillot. E. The development of scanning strategies and their relation to visual differentiation. Journal of Experimental Child Psychology, 1%8,6, 622-650. Winer, B. J. Statistical principles in experimental design. New York: McGraw-Hill, 1962. Zusne, L. Visual perception ofform. New York: Academic Press, 1970.
REFERENCE NOTES Salapatek, P. The visual investigation of geometric pattern by the one and two month old infan?. Paper presented at meetings of the American Association for the Advancement of Science, Boston, Massachusetts, 1969. Bergman, T., Haith, M. M., & Mann, L. Development of eye contact andfacial scanning in infanfs. Paper presented at meetings of the Society for Research in Child Development, Minneapolis, Minnesota, 1971. Donnee, L. H. Infants’ developmental scanning patterns to face and nonface stimuli under various auditory conditions. Paper presented at meetings of the Society for Research in Child Development, Philadelphia, Pennsylvania, 1973. Maurer, D., & Salapatek, P. Developmental changes in the scanning of faces by infants. Paper presented at meetings of the Society for Research in Child Development, Denver, Colorado, 1975.
246
ALLEN
E. MILEWSKI
Eimas, P. D. Linguistic processing of speech by young irzfanrs. Paper presented at the conference on “Language intervention with the mentally retarded,” June 1973. Siqueland, E. R. Visual reinforcement and exploratory behavior in infants. Paper presented at meetings of the Society for Research in Child Development, Worchester, 1968. Siqueland, E. R. The development of instrumental exploratory behavior during thefirst year of human life. Paper presented at meetings of the Society for Research in Child Development, Santa Monica, California. 1969. RECEIVED:
February 13. 1975;
REVISED:
April 12, 1976.
OF
EXPERIMENTAL
CHILD
PSYCHOLOGY
22,229-246
(1976)
Infants’ Discrimination of Internal and External Pattern Elements ALLAN E. MILEWSKI De Paul
University
Human infants’ discrimination of changes in internal and external elements of compound visual patterns was investigated in four experiments employing a familiarization-novelty paradigm in which visual reinforcing patterns were presented contingent upon rate of high-amplitude nonnutritive sucking. In Experiment 1, 4-month infants discriminated changes in the shape of internal, external and both internal and external figures. One-month infants discriminated external changes in both internal and external figures, but failed to show reliable response recovery when only internal figures were changed. Experiments 2 and 3 failed to explain the l-month results on the basis of poor resolution of internal figures by showing comparable discrimination of small and large singly-presented figures and by failing to find improved internal discrimination with large separation between internal and external figures. In Experiment 4, l-month infants showed response recovery to figure additions made adjacent to the initial figure, but not to internal additions. The results are interpreted in terms of attentiveness by young infants to external pattern elements and may indicate early processing of figure-ground information. The developmental differences observed suggest an increased breadth of attention to pattern elements.
Recent investigations of infant visual scanning behavior have reported that young infants, when presented with compound patterns containing internal and external elements, tend to concentrate their visual scans on portions of the external pattern more than on internal elements. For infants of 1 month and younger, external-contour scanning has been found to be predominant both with abstract, two-dimensional stimuli (Salapatek, Note 1,1975) and with facial stimuli (Bergman, Haith, & Mann, Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4). In contrast to the predominant external-contour scanning by l-month infants, 2-month infants in these investigations have shown broader scanning of both internal and external contours, occasionally with greater scanning of internal compared with external contours. The developmental changes that have been observed in infants’ scanning of compound patterns may have implications for some common empirical This research is based in part on a Ph.D. thesis submitted to Brown University and was supported by NIH Grant HD 03386 to E. R. Siqueland. The author thanks E. R. Siqueland for his help in all phases of the project. Requests for reprints should be sent to Allen E. Milewski, Department of Psychology, De Paul University, 2323 North Seminary Avenue, Chicago, Illinois 60614. 229 Copyright 0 1976 by Academic Press. Inc. All rights of reproduction in any form reserved.
230
ALLEN
E. MILEWSKI
findings suggesting early changes in visual attention and perception. For example, the literature on responsiveness to facial stimuli (see Gibson, 1969) suggests that infants younger than 2 or 3 months of age may not distinguish between faces that are equated for brightness and contour amount. Similarily, several studies have suggested that young infants may not discriminate between complex patterns differing in the arrangement of elements (see Bond, 1972). A large proportion of the information needed to discriminate between faces, and often between the abstract stimuli that have been used in arrangement-discrimination studies, is contained in internal portions of the patterns. Therefore, it is possible that the developmental changes observed in discrimination of faces and abstract patterns may be partly attributed to the young infant’s failure to visually scan information within the borders of complex patterns. However, there is a serious reservation in drawing conclusions about infant perception solely on the basis of findings from visual scanning studies. There is a lack of empirical evidence concerning the relationship between the measured locus of visual scanning in infants and the information that is actually processed about the visual stimulus. While several writers have emphasized the role of eye movements and directed visual regard in pattern perception and its development (Hebb, 1949; Hochberg, 1968; Noton & Stark, 1971; Piaget, 1969; Vurpillot, 1968), there is considerable evidence for pattern perception, by adults at least, under conditions which prohibit eye movements and/or macular investigation of the stimulus (Evans, 1965; Mooney, 1959; Zusne, 1970, pp. 307-309). The present investigation was designed to study infants’ responsiveness to internal and external pattern elements that is actually processed and stored by the infant. The high-amplitude operant sucking (HAS) technique was used in a stimulus familiarization-novelty paradigm (Siqueland & DeLucia, 1969). The HAS technique has previously provided important information regarding young infants’ auditory discrimination of speech stimuli (Eimas, Siqueland, Jusczyk, & Vigorito, 1971; Morse, 1972; Eimas, Note 5>, as well as visual discrimination of color and pattern (Milewski & Siqueland, 1975, Siqueland, Note 6, Note 7). Briefly, the HAS procedure involves the presentation of a visual or auditory reinforcing stimulus contingent upon the rate of nonnutritive high-amplitude sucking. Typically, as the infant learns that his HAS rate determines stimulus presentation, his rate initially increases above baseline and then decreases, presumably as a result of stimulus familiarization and a reduction in the reinforcing properties of the once-novel stimulus. If the familiar stimulus is subsequently replaced with a novel stimulus without change in the reinforcement contingencies and HAS rate increases relative to that of control subjects who continue to receive the familiar stimulus, discrimination of the stimulus change is inferred. The purpose of the present study was twofold: (i) to directly investigate
INFANTS’
DISCRIMINATION
OF PATTERNS
231
infants’ processing of compound patterns by comparing discrimination performance for changes in internal and external elements; and (ii) to investigate possible explanations for differences in younger infants’ discrimination of internal and external pattern changes. GENERAL
METHOD
The procedure and apparatus employed in the present investigation were identical to those described in detail by Milewski and Siqueland (1975). The apparatus provided visual reinforcement contingent upon the subject’s rate of nonnutritive high-amplitude sucking. Pressure variations due to the subject’s sucking on a sterilized, sealed nipple were converted to an analogue record by a gas pressure transducer. The analogue signal was digitized by a Schmitt trigger on the basis of a high-amplitude response criterion set for each subject during a prebaseline period. A PDP Lab 8/E mini-computer provided minute by minute storage of the digital sucking data. Visual stimulation was produced by increasing the voltage to a Kodak slide projector proportional to the rate of supracriterion sucking. Maximum stimulus illumination (60 V to the projector) was maintained by a momentary sucking rate of two or more sucks per second. The stimuli were white line figures on a black surround, projected onto a rear-projection screen which was 25 cm from the subjects. Angular subtense of the lines for all stimuli was approximately 55 min. The component shapes were a circle, a square, and an equilateral triangle. Infants were tested during their normal waking hours and were positioned in an infant seat facing the rear projection screen in a darkened, sound attenuated cubicle. While one experimenter monitored the recording and programming equipment in a separate room, another experimenter who was hidden from the subject’s view by a linen sheet held the nipple in the infant’s mouth approximately at midline. The procedure was a modification of the operant sucking methodology described by Siqueland and DeLucia (1969) and consisted of three segments: (i) During the first 1 or 2 min, the high-amplitude response criterion and baseline rate of responding were determined in the absence of visual reinforcement. The amplitude criterion was set independently for each subject at approximately the median of his baseline sucking amplitudes. Throughout the session, “high-amplitude sucks” weredefined as responses falling above the criterion amplitude; (ii) Immediately after the baseline rate was obtained, subjects received the initial visual reinforcing stimulus contingent on their rate of high-amplitude sucking. Following 3 min of acquisition training, assessment of familiarization effects began. A performance measure of stimulus familiarization was employed requiring each subject to hold, for two consecutive minutes, a decrement in sucking rate of 20% or more compared with the minute
232
ALLEN
E. MILEWSKI
immediately preceding the first minute of decrement. This second segment then had a minimum duration of 5 min, but varied across subjects as a function of the time required to meet the performance measure of familiarization. In order to ensure some minimal stimulus exposure immediately prior to the test segment, subjects were excluded from the study if they failed to meet a minimum response criterion of an average of 14 responses in the fast 2 min of familiarization. Subjects were also excluded if they failed to meet the performance measure of familiarization within 22 min; (iii) Following stimulus familiarization, a 5-min test segment began with no change in the stimulus feedback contingenices. During this segment experimental shift groups received a novel stimulus contingent upon sucking, while separate no-shift control groups continued to receive the initial visual stimulus. Experiment
1
Experiment 1 investigated the discrimination of internal and external pattern element changes developmentally. One- and four-month infants initially received compound visual stimuli. Independent groups received subsequent changes in either the internal, the external, or both the internal and external figures. Based on the developmental changes observed in the locus of infants’ visuaI scanning for compound figures (Salapatek, Note 1: Bergman et af ., Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4), it was expected that the l-month infants, but not the 4-month infants, would show greater response recovery to changes in the external compared with internal figures. Method Subjects. Subjects were 32 l-month and 32 4-month, full-term infants. The l-month subjects ranged from 27 days to 47 days @I = 35 days); the 4-month subjects ranged from 95 to 143 days (It4 = 124 days). Additional subjects were excluded from the study on the basis of behavior or experimental errors prior to the test segment. Thirty-nine l-month infants were excluded: 20 due to sleeping, 11 for excessive crying or rejection of the nipple, three due to experimenter error or equipment failure, and five for failure to meet the performance criteria specified in the General Method Section. Forty-eight 4-month infants were excluded: 13 for sleeping; 33 for crying; and two due to experimenter error. Stimuli. Each stimulus consisted of two different component figures, one centered within the other. The internal figures subtended an average horizontal visual angle of approximately 12”; the average visual angle of the external figures was approximately 32”. Within the large external and smaller internal groupings, figures were similar in area and contour amount.
INFANTS’
DISCRIMINATION
233
OF PATTERNS
FIG. 1. Representative stimuli used in Experiment
1.
Design. The design included four experimental groups (each with N = 8) at each of the two ages. All groups were treated identically during the initial reinforcement and familiarization segments, and differed only in the locus of the shape change received during the test segment. Subjects received either a shift in the shape of the internal figure (Group I), the external figure (Group E), both the internal and external figures (Group I-E), or no shift in the initial visual reinforcing stimulus (Group NS). For six of eight subjects, each group was counterbalanced for the specific combination of component figures used in the internal and external positions with the restriction that the internal and external figures not be the same shape. For the remaining two subjects in each group, the initial stimulus was determined randomly. In Group I-E, stimulus changes were restricted such that the internal and external components were never simply reversed with respect to each other. Experimental design and stimulus examples are represented in Fig. 1. Results and Discussion Preshift performance. Since all experimental groups received identical treatment prior to the test segment, no group differences in performance were expected during familiarization. Separate analyses of preshift performance were done for l- and Cmonth subjects. Mixed design analyses of variance done on the minute by minute sucking rates for the 5 min preceding the test segment showed no significant group differences.
234
ALLEN
E. MILEWSKI
Moreover, for each age level, analyses of variance indicated no differences between experimental groups in baseline sucking rates, the amount of time required to meet the response decrement criterion, or the amount of response decrement at the end of familiarization. Finally, to assess response acquisition during the preshift segment, the data for each subject were vincentized by equally dividing into two blocks the period from the beginning of conditioning to the minute preceding the two minutes of 20% decrement. Two factor (Groups x Time blocks) mixed design analyses of variance were done on the mean number of criterion sucks over the two time blocks. Only the time factors were significant, indicating an overall increase in response levels over acquisition for l-month subjects, F(1,28) = 17.62,~ < .OOl, and 4-month subjects, F(1,28) = 33.92,~ < .OOl. In summary, for both age levels, no reliable differences in performance were found between experimental groups prior to the test segment. Test segment performance. In order to provide a sensitive measure of response change during the test segment relative to the familiarization segment, difference scores were computed for each subject by subtracting his average response rate during the last 2 min preceding the test segment from his rate for each test segment minute, providing five difference scores for each subject. Separate two factor (Experimental group x Postshift time) mixed design analyses of variance were performed on the difference scores for l- and 4-month subjects. One-month subjects. Analysis of l-month subjects’ difference scores revealed a significant main effect of groups,F(3,28) = 3.57,~ < .05. Since no other main effects or interactions were significant, results were collapsed across the five test segment minutes for subsequent analysis. Figure 2 shows the collapsed mean group difference scores for one-month subjects. A post-hoc Dunnett’s t test for comparison of each experimental group with the control (Winer, 1962) indicated that although Groups E and I-E differed from Group NS (p < .Ol andp < .05, respectively), Group I did not differ from the control. Four-month subjects. Analysis of the test segment difference scores of 4-month subjects showed a significant groups effect, F(3,28) = 10.60, p < .Ol, as well as a reliable time effect, F(4,112) = 5.12,~ < .Ol. Mean group difference scores for 4-month subjects are shown in Fig. 2. A Dunnett’s c test performed on the difference scores collapsed across time revealed that each of the stimulus shift groups (i.e., Groups I, E, and I-E) differed significantly from the NS control (p < .Ol). A trend analysis to determine the nature of the significant time effect showed an overall quadratic component, F(1.7) = 5.76. p < .05, with no significant groups x time interaction. The main interest of this study concerns performance differences between groups; therefore, since the effect was found for all groups, the cause of the curvilinear recovery function will not be considered further. These results indicate response recovery by 4-month infants to shape
INFANTS’
DISCRIMINATION
OF PATTERNS
235
FIG. 2. Mean group difference scores collapsed across the 5-min test segment, for l-month infants (solid) and for 4-month infants (cross-hatched) in Experiment 1.
changes in internal figures, external figures and in both internal and external figures. In contrast, l-month infants showed recovery to external shape changes and changes in both internal and external figures, but no evidence was found for recovery by l-month infants to changes in the shape of internal figures. Since for both age levels, no reliable group differences were found in performance prior to the test segment, group differences during the test segment suggest differential discrimination of stimulus changes and cannot be explained as an artifact of preshift performance. These results are consistent with the findings of visual scanning studies in which younger infants show more fixations to external compared with internal pattern elements (Salapatek, Note 1; Bergman et al., Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4). For convenience, both the evidence for discrimination of external but not internal figure changes, and the predominant visual scanning of external elements observed in l-month infants are subsequently referred to as the “externality effect.” At least two explanations for the externality effect are possible. The externality effect may have its basis in attentional processes wherein external and internal pattern elements differ with respect to salience for the young infant. An alternative class of explanations assumes differential availability of external and internal pattern information for the l-month infant, possibly related to the poor acuity in young infants (Fan% Ordy, & Udelf, 1962; Teller, Morse, Borton, & Regal, in press). Experiments 2 and 3 were designed to investigate whether the discrimination performance of l-month infants for internal and external elements can be explained in terms of differential availability of pattern information. Experiment
2
discrimination The results of Experiment 1, showing differential performance by l-month infants for internal and external figure changes, may have resulted from differences in the discriminability of the
236
ALLEN
E. MILEWSKI
independent component figures due to their relative sizes. It is possible, for example, that the small internal figures were not resolved by the l-month infants well enough to be discriminated. Although all figures were well within l-month grating acuity limits, there is no evidence pertaining to size thresholds in infants’ discrimination of geometric figures. Therefore, Experiment 2 was designed to test the possibility that differential discriminability of the individual component figures may account for the externality effect observed in l-month infants. This experiment compared response recovery to shape changes in either small or large figures presented singly in the visual field. Method Subjects. Twenty-four full-term l-month infants ranging from 27 to 40 days @I = 32 days) were randomly assigned to one of four experimental groups. Thirty-six additional subjects were tested, but were omitted from further analysis: 17 for sleeping, 14 for crying, and five for failing to meet the performance criteria described in the General Method Section. Stimuli. The stimuli were either large or small figures projected singly on a dark surround. The component shapes, stimulus sizes, and viewing conditions were identical to those used in Experiment 1. Design. The design provided for a comparison of four experimental groups (each with N = 6): two groups received a stimulus change during the test segment, and two groups were no-shift controls. One stimulus change group (Group L) received a single large figure contingent upon sucking during the familiarization segment followed by a different large figure in the test segment. Group S received a small single figure initially, followed by a different small figure. A large-stimulus no-shift group (Group LNS) and a small-stimulus no-shift group (Group SNS) continued to receive the same stimulus in the initial familiarization and test segments. Experimental groups were counterbalanced with respect to the specific component shapes presented during the familiarization and test segments of the experiment. Results and Discussion No evidence for systematic group differences on preshift performance was obtained by analyses of variance on the mean response rate during baseline and 5 min prior to the test segment, the amount of response decrement at the end of familiarization, the latency of response decrement or the amount of increase in response rates between vincentized preshift time blocks. The analysis of vincentized data did indicate a significant time effect suggesting an overall increase in response rate over acquisition, F(1,20) = 19.92,~ -=c.OOl. As in Experiment 1, difference scores were used to assess response
INFANTS’
2 0 s Ly
DISCRIMINATION
237
IO -
S-
s z if
0
2 f
-5 -
z
OF PATTERNS
SNS
LNS
-10 -
-15-
FIG. 3. Mean difference scores for experimental groups in Experiment 2, collapsed across the 5-min test segment.
change during the test segment. A two factor (Groups x Postshift time) mixed design analysis of variance performed on the difference scores indicated a significant groups effect, F(3,20) = 5.80, p < .Ol. No other main effects or interactions were significant. Figure 3 shows the group difference scores averaged across the five test segment minutes. A conservative Tukey’s “hsd” test (Winer, 1%2) revealed the following results: (i) Group L differed from Group LNS (p < .Ol); (ii) Group S and Group SNS were different (p < .Ol); (iii) no evidence of reliable differences was found between Groups L and S or between Groups LNS and SNS. These results indicate comparable discrimination performance by l-month infants of shape changes in small and large singly presented figures. Since the small and large stimulus sizes were identical to the sizes of internal and external figures of the compound patterns in Experiment 1, it can be inferred that the evidence for an externality effect in l-month infants did not result from differences in discriminability due to the size of the independent component figures. Instead, the results of Experiments 1 and 2 together suggest that the externality effect is dependent upon the presentation of figures in compound displays. Experiment
3
There is a second possible explanation for the externality effect which is consistent with the suggestion that the effect arises from the use of compound displays and which assumes differential availability of information from internal and external figures. This explanation assumes degradation of internal pattern information as the result of spatial interaction between contours of internal and external figures. According to this explanation, while the interaction effects are mutual between internal
238
ALLEN
E. MILEWSKI
and external figures, there may be relatively more degradation of internal pattern information due to interference from both the surrounding figure and nearby internal contours. It is not possible to speculate about a specific spatial interaction mechanism with respect to the externality effect in infants, but lateral interference effects have been reported in diverse areas of the adult and animal perceptual literature (e.g., Bridger, 1971: Ratliff. 1965). Experiment 3 was designed to investigate the role of spatial interaction of contours in young infants’ discrimination of internal figure changes. It was reasoned that increasing the separation between internal and external components by the use of large external figures should reduce the amount of interaction between figures. Hence, if spatial interaction of contours accounts for the externality effect, the use of large external figures should result in improved discrimination of internal changes by l-month infants. Method Subjects. Subjects were eight l-month infants ranging from 28-34 days (M = 31 days). An additional 16 subjects were rejected: eight for sleeping: seven for crying; and one for failure to meet the performance criterion described in the General Method Section. Stimufianddesign. Following the initial familiarization with a compound pattern, all subjects received a shift in the shape of the internal figure (Group I’). The stimuli and stimulus shifts were identical to those used in the internal shift group of Experiment 1 with the exception that the external figures in this experiment were larger, subtending an average horizontal visual angle of 44”. A separate no-shift control group was not run for this experiment. Instead, the data from the one-month Group NS of Experiment I were compared with the data of Group I’. Results and Discussion Analyses of preshift performance which were identical to those used in the previous experiments failed to find any differences between Group I’ and Group NS prior to the introduction of stimulus novelty conditions. An analysis of variance comparing response rate across vincentized acquisition blocks failed to reach significance but showed trends similar to those of Experiments 1 and 2, F(1,14) = 4.09, p = .06. Figure 4 shows the mean difference scores for Groups I’ and NS averaged across the five test segment minutes. A two factor (Experimental groups x Postshift time) analysis of variance failed to find a significant main effect either of groups or of postshift time. In summary, Experiment 3 failed to find evidence for discrimination of changes in the shape of figures centered within large external figures. These results replicate the failure in Experiment 1 to find discrimination of
INFANTS’
DISCRIMINATION
OF PATTERNS
239
FIG. 4. Mean, collapsed test segment difference scores for each group in Experiment 3.
internal figures by I-month infants. Moreover, if it is assumed that the increased size of the external figures in this experiment was sufficient to significantly reduce or eliminate the effects of spatial interaction, the lack of improved internal figure discrimination in l-month infants is not consistent with an interpretation of the externality effect postulating degradation of internal figure information due to an interaction between internal and external contours. Experiment
4
Experiments 2 and 3 failed to provide evidence in support of two explanations of the externality effect in young infants which assume differential availability of internal and external pattern information. The results of these experiments suggest that the findings of Experiment 1 were not due to an inability of the young infants to resolve the internal figures either because of their size or because of lateral interference between contours. An alternative class of explanations assumes equal availability of internal and external information but proposes that the externality effect has its basis in attentional mechanisms. There are at least two ways in which an attentional hypothesis can explain the externality effect. One explanation stresses the relative size of the internal and external figures, while another explanation emphasizes the relative location of component figures in the pattern. Obviously, in both the present discrimination study and the visual scanning studies reporting externality effects, the external
240
ALLEN
E. MILEWSKI
figures were larger than the internal figures. Therefore, it is possible to explain the results of these studies by assuming an attentional preference in young infants for larger as compared with smaller elements in compound displays. In contrast to an explanation based on attentional preferences as a function of element size, it is possible that the externality effect is mediated by a tendency in young infants to attend to external pattern elements more than to internal elements. Experiment 4 investigated whether inferior discrimination of small figures in compound patterns occurs only when the small figure is centered within a larger figure, or whether inferior discrimination will also occur when the small figure is in a position adjacent to the larger figure. Hypotheses assuming that a size preference underlies the externality effect in l-month infants would predict poor discrimination for changes in small figures positioned either within or adjacent to the larger figure. However, results indicating discrimination of adjacent small figures but not internal small figures would implicate the relative locations of figures in a pattern as the basis of the externality effect. Rather than using changes in the shape of the small figures as the novelty condition, Experiment 4 employed novelty conditions involving the addition of a small figure either in the internal or adjacent position. Previous pilot work with a small number of subjects failed to find discrimination when a single large familiarized figure was changed to a compound by the addition of an internal component. Experiment 4 tested whether the externality effect can be found even with the large amounts of change in brightness and contour associated with the addition of an internal figure, and compared discrimination of internal additions with identical additions in an adjacent position. Method
Subjects. Sixteen full-term infants ranging from 27 to 44 days (M = 36 days) were randomly assigned to one of two groups (each with N = 8). In addition, 17 infants were rejected for sleeping, seven for crying, and one for failing to meet the minimum response criterion. Stimuli and design. Subjects initially received a single large figure as the familiarization stimulus. The stimulus change conditions during the test segment consisted of the addition of a small figure either centered within the initial large figure (Group IA) or in a position adjacent to the large figure (Group AA). For Group AA, the center of the small figure was positioned parallel to and 29” from the center of the large figure; the adjacent figures were positioned on either side of the center of the projection screen. The large figure was on the right for half the subjects and on the left for the other half. For Group AA, the initial familiarization stimulus was positioned off-center so that no change in the position of the large figure occurred
INFANTS’
DISCRIMINATION
OF
PAlTERNS
241
between familiarization and test segments. The data from Groups IA and AA were compared with control data obtained by combining the data from the six subjects of Group LNS (from Experiment 2) with two additional no-shift subjects. For six of eight subjects, each group was counterbalanced for the specific component shapes used in the familiarization and test segments. The stimulus figures for the remaining two subjects in each group were determined randomly. Results and Discussion With the exception of one measure, analysis of preshift performance failed to find any differences between experimental groups. A two factor (Experimental groups x Time blocks) analysis of variance performed on the vincentized acquisition data showed a reliable effect of time blocks, F(2,21) = 35.73, p < .OOl. In contrast to the previous experiments, this analysis also revealed a significant groups x time interaction F(2,21) = 4.48,~ < -05 reflecting a smaller amount of increase in response rates between vincentized blocks for Group AA compared with either Groups IA or LNS. The mean difference scores for each experimental group, averaged across the five test segment minutes are shown in Fig. 5. A two factor (Experimental groups x Postshift time) mixed-design analysis of variance performed on these scores revealed a significant effect of group, F(2,21) = 5.15,~ < .05; no other main effect of interaction was reliable. A post-hoc Dunnett’s t test showed that only Group AA differed significantly from the LNS control group (p < .05). Groups IA and LNS did not differ from each other. The results of this experiment replicate, under conditions involving a large amount of stimulus change in the internal position, the failure of Experiment 1 to find significant response recovery to internal changes, while recovery was found for adjacent figure additions. These results are qualified somewhat by the reliable preshift interaction found between experimental groups and vincentized acquisition blocks. This single preshift interaction is somewhat surprising since groups were treated identically prior to the test segment and since similar preshift conditions in Experiments 1,2, and 3 failed to result in a similar interaction. However, Milewski and Siqueland (1975) failed to find a significant correlation between individual differences in vincentized acquisition data and subsequent test segment performance; hence, it appears unlikely that this interaction qualifies the test segment results in any important way. The demonstration of response recovery to adjacent additions but not to internal additions tentatively argues against an explanation of the externality effect based on attentional preferences as a function of figure size. Instead, these results appear to suggest differential responsiveness to
242
ALLEN
E. MILEWSKI
FIG. 5. Mean, collapsed difference scores for experimental groups in Experiment 4.
information in the internal compound patterns.
portion
compared
with other portions
of
GENERAL DISCUSSION
The results of the present investigation are consistent with previous HAS studies which have shown that human infants between 1 and 4 months are differentially responsive to novel and familiar visual pattern reinforcers and show recovery of operant sucking to presentations of novel patterns. The present results extend this conclusion by showing differential novelty effects in l-month infants as a function of the locus of change in compound patterns. While l-month infants in Experiment 1 showed discrimination of external figure changes, Experiments 1, 3, and 4 provide no evidence for discrimination of internal changes. This result is consistent with those indicating predominant visual scanning of external contours by young infants (Salapatek, Note 1; Bergman& al., Note 2; Donnee, Note 3; Maurer & Salapatek, Note 4), and provides indirect evidence for a relationship between the locus of scanning and visual discrimination performance in one-month infants. In contrast to the differential discrimination of internal and external elements by l-month infants, 4-month infants in Experiment 1 showe+d comparable discrimination of all pattern changes. This result is consistent with the visual scanning data in suggesting a developmental increase in breadth of attention, but fails to support the previous report of greater attentiveness to internal elements by older infants (Salapatek, Note 1). It is possible that the locus of visual scanning may not be as important for older
INFANTS’
DISCRIMINATION
OF
PATTERNS
243
infants’ discrimination as it is for young infants, or that the visual scanning patterns of the 4-month infants used in the present study may not be comparable to those of the 2-month infants used in the visual scanning studies. Possibly, the interpretation of results from Experiment 1 suggesting developmental differences in responsiveness to internal and external figure changes should be qualified. An overall analysis of test segment performance including l- and 4-month data revealed no evidence for the groups x age interaction that would provide strong support for the conclusion of developmental differences. However, since post-hoc tests which were run on the combined data indicated a reliable difference between Groups I and NS for, the 4-month subjects but not the l-month subjects, the failure to find a significant groups x age interaction in recovery scores may have been a function of the large variability both within groups and ages. Since 4-month subjects in Experiment 1 showed reliable recovery in both Groups I and E, evidence for additivity in Group I-E might be expected; however, Group I-E failed to show greater recovery compared with either Group I or E. The HAS procedure may be insensitive as a measure of cue additivity, possibly due to ceiling effects in the level of criterion responding during the test segment, Previous HAS studies have failed to find greater recovery to changes in both color and form of visual patterns compared with changes in either cue alone (Milewski & Siqueland, 1975; Siqueland, Note 7). Two classes of explanations for the externality effects observed in l-month infants were investigated in the present study. Experiments 2 and 3 failed to provide evidence in support of explanations based on differential availability of internal and external information either due to figure size or because of lateral interference effects. These experiments provided: (i) evidence for comparable discrimination of singly-presented small and large figures and (ii) no evidence for improved internal discrimination when the spatial separation between internal and external elements was increased. Since only two amounts of separation were used in this study, it should be noted that the present results do not provide strong proof against a lateral interference explanation. Further study comparing internal discrimination under several amounts of separation is needed. The second class of explanations assumes that internal information is available to the young infant, but that differential attention to pattern elements mediates the externality effect. The results of Experiment 4, while qualified by a preshift interaction between groups and acquisition rate, suggest discrimination of adjacent additions but no reliable recovery to internal additions. This finding does not support an explanation based on differential attention due to the relative size of elements, but tentatively suggests that discrimination varies as a function of the relative position of
244
ALLEN
E. MILEWSKI
pattern elements. Given the present results, the evidence for externality effects observed in young infants’ discrimination and visual scanning may be most easily explained in terms of differential attention to internal and external portions of patterns. While the present evidence provides no direct proof of an attentional explanation, compared with the alternative hypotheses discussed above, an attentional interpretation confronts few problems. One proposal which might explain the greater attention to external elements assumes that the young infant maintains fixations on and processes only the first contour contacted (Haith. In press). However, Salapatek (1975) has noted that the majority of young infants in the visual scanning studies show some scanning of internal elements and in some cases move to an external contour from an initial internal fixation. Another explanation of the externality effect (Salapatek, 1975) has suggested that the predominance of external scanning by l-month infants may reflect maintenance of the young infants’ line of sight at those portions of the pattern which are most relevant to the segregation of figure from ground. In this context, Salapatek discusses a physiological distinction between a midbrain-mediated “secondary” visual system which segregates figures from ground on the basis of gross sensory differences, and a cortically-mediated “primary” visual system which processes more detailed information from previously segregated figures (Held, 1972; Humphrey, 1974; Schneider, 1969; Trevarthen, 1968). Bronson (1975) has recently argued that the young infant is cortically immature and that his perceptual abilities may be explained by assuming control by the “secondary” visual system. Finally, several writers have suggested that primitive organizational information regarding figure-ground segregation is a particularly salient feature of the young infant’s visual environment (Gibson, 1969; Hebb, 1949). Hence, an interpretation of the externality effect found in the present investigation which assumes early processing of figure-ground information is consistent with current notions of visual physiology as well as with major theories of perceptual development. REFERENCES Bond, E. K. Perception 225-245. Bridger, B. Metacontrast Bronson. G. The postnatal Eimas, P. D., Siqueland, Science,
1971, 171,
of form by the human infant. Psychological
Bulletin.
1972, 77,
and lateral inhibition. Psychological ReGcn,. 1971, 78, 528-539. growth of visual capacity. Child Development. 1974.45,873-890. E. R., Jusczyk. P., & Vigorito. J. Speech perception in infants. 303-306.
Evans, C. R. Some studies of pattern perception using stabilized retinal images. British Journal of Psychology, 1965. 56, 121-133. Fantz, R. L.. Ordy. J. M., & Udelf. M. S. Maturation of pattern vision in infants during the first six months. Journal of Compurntive and Ph~~siologkwl Psychology. 1962. 55, 907-917.
INFANTS’
DISCRIMINATION
OF PATTERNS
245
Gibson, E. J. Principles of perceptual learning and development. New York: AppletonCentury-Crofts, 1969. Haith, M. M. Visual competence in early infancy. In R. Held, H. Leibowitz, & H. L. Teuber (Eds.), Handbook of Sensory Physiology (Vol. 8) New York: Springer-Verlag. in p-ess. Hebb, D. 0. Organization ofbehavior. New York: Wiley. 1949. Held, R. Two modes of processing spatially distributed visual stimulation. In The Neurosciences: Second study program. New York: Rockefeller University Press, 1972. Hochberg, J. In the mind’s eye. In R. R. Haber (Ed.), Contemporary theory and research in visual perception. New York: Holt, Rinehart & Winston, 1%8. Humphrey, N. K. Vision in a monkey without striate cortex: A case study. Perception, 1974. 3, 241-255. Milewski, A. E., & Siqueland, E. R. Discrimination of color and pattern novelty in one-month human infants. Journal of Experimental Child Psychology, 1975, 19, 122-136. Mooney, C. M. Recognition of novel visual configurations with and without eye movements. Journal of Experimental Psychology, 1958, 56, 133- 138. Morse, P. The discrimination of speech and nonspeech stimuli early in infancy. Journal of Experimental child Psychology, 1972, 3, 477-492. Noton, D., & Stark, L. Scan paths in saccadic eye movements while viewing and recognizing patterns. Vision Research, 1971, 11, 929-942. Piaget, J. The mechanisms of perception. New York: Basic Books, 1969. Ratliff, F. Much Bands: Quantitative studies on neural networks in the retina. San Francisco: Holden-Day, 1%5. Salapatek, P. Pattern perception in early infancy. In L. B. Cohen & P. Salapatek (Eds.), Infant perception: From sensation to cognition. New York: Academic Press, 1975. Schneider, G. E. Two visual systems. Science, 1969, 163, 895-902. Siqueland, E. R., & DeLucia, D. Visual reinforcement of nonnutritive sucking in human infants. Science, 1969, 165, ll44- 1146. Teller, D. Y., Morse, R., Borton, R.. & Regal, D. Visual acuity for vertical and diagonal gratings in human infants. Vision Research, 1974, 14, 1433- 1439. Trevarthen, C. B. Two mechanisms of vision in primates. Psycologische Forschung, 1968.31, 299-337. Vurpillot. E. The development of scanning strategies and their relation to visual differentiation. Journal of Experimental Child Psychology, 1%8,6, 622-650. Winer, B. J. Statistical principles in experimental design. New York: McGraw-Hill, 1962. Zusne, L. Visual perception ofform. New York: Academic Press, 1970.
REFERENCE NOTES Salapatek, P. The visual investigation of geometric pattern by the one and two month old infan?. Paper presented at meetings of the American Association for the Advancement of Science, Boston, Massachusetts, 1969. Bergman, T., Haith, M. M., & Mann, L. Development of eye contact andfacial scanning in infanfs. Paper presented at meetings of the Society for Research in Child Development, Minneapolis, Minnesota, 1971. Donnee, L. H. Infants’ developmental scanning patterns to face and nonface stimuli under various auditory conditions. Paper presented at meetings of the Society for Research in Child Development, Philadelphia, Pennsylvania, 1973. Maurer, D., & Salapatek, P. Developmental changes in the scanning of faces by infants. Paper presented at meetings of the Society for Research in Child Development, Denver, Colorado, 1975.
246
ALLEN
E. MILEWSKI
Eimas, P. D. Linguistic processing of speech by young irzfanrs. Paper presented at the conference on “Language intervention with the mentally retarded,” June 1973. Siqueland, E. R. Visual reinforcement and exploratory behavior in infants. Paper presented at meetings of the Society for Research in Child Development, Worchester, 1968. Siqueland, E. R. The development of instrumental exploratory behavior during thefirst year of human life. Paper presented at meetings of the Society for Research in Child Development, Santa Monica, California. 1969. RECEIVED:
February 13. 1975;
REVISED:
April 12, 1976.