INFANT BEHAVIOR AND DEVELOPMENT 9, 189-202 (1986)

Discrimination Between Solid Forms in Early Infancy*

MICHAEL COOK.ROSEMARY BIRCH,AND KATHLEEN GRIFFITHS Australian National University

This paper describes three experiments concerned with the ability of 3-month-old infants to discriminate between stotionary solid forms. Experiment 1 confirmed, using a habituation/recovery technique, the finding of Cook, Field, and Griffiths (1978) that the infants do not distinguish between a cube and a truncated pyramid (and a range of other solid forms). Experiment 2 showed that infants can dis- tinguish the square face of the cube from a trapezoidal face of the truncated pyramid when these shapes are presented in isolation. Experiment 3 showed that this discrimination is not made when the corresponding faces (square and trape- zoid) of the solid cube and trapezoid are colored distinctively. These results are discussed in terms of limitations in the infants’ capacity to resolve solid form.

hobituation discrimination form perception

Several studies have shown that young infants have shape constancy, in that they are capable of responding to solid objects, or non-frontoparallel plane objects, on the basis of their forms in three-dimensional space (Bower, 1966; Caron, Caron, & Carlson, 1978, 1979; Cook & Birch, 1984), and some infor- mation is available concerning the cues available to the infant in its three- dimensional space perception. Fox, Aslin, Shea, and Dumais (1980) showed the presence of stereopsis in the fourth month of infancy, and Owsley (1983) demonstrated three-dimensional form perception based upon motion perspec- tive cues in 4-month-old infants. On the other hand, there is no evidence of an ability to use static monocular depth cues within the first 6 months (Yonas, Cleaves, & Petterson, 1978).

Relatively little is known about how well infants perceive the forms of objects which lie outside the frontoparallel plane. This question will be the concern of the present study. Some information concerning the quality of in- fant form perception is provided by the published studies of infant shape con- stancy. The studies by Bower (1966), Caron et al. (1978, 1979), and Cook and Birch (1984) each show that infants of 3 months or less can see the forms of stationary, slanted plane objects sufficiently well to distinguish a rectangle (or square) from a trapezoid, and Owsley’s (1983) results imply that 4-month-old

l We would like to thank the Infant Welfare Sisters of the Australian Capital Territory Health Commission for their help in obtaining subjects, and the mothers and babies who participated. This work was supported by A.R.G.C. Grant No. A79115565.

Correspondence and requests for reprints should be sent to Michael Cook, Department of Psychology, Australian National University, P.O. Box 4. Canberra, A.C.T., Australia, 2601.

189

190 ’ COOK, BIRCH. AND GRIFFITHS

infants can distinguish a cube from a “wedge-like” solid. However, Cook and Birch (1984) found no evidence of discriminations between various non-rect- angular plane polygons in 3-month-old infants, and Cook, Field, and Griffiths (1978) found no evidence of discrimination between a stationary cube and several other convex polyhedral forms.

Doubts are raised about the negative findings of Cook et al. (1978), since they failed to show a discrimination between a cube and a truncated pyramid. This seems to contradict the well established finding that 3-month-olds can distinguish a rectangle from a trapezoid. If these shapes can be distinguished when presented in isolation, why could this discrimination not be made when the shapes were presented as faces of a solid object? It is, of course, possible that Cook et al’s results are simply a consequence of the use of an insensitive technique, and that other methods might demonstrate that the infants can dis- tinguish between a cube and a truncated pyramid. On the other hand, the dis- crepancy in the results may have theoretical significance for our understanding of the way in which the infant resolves the forms of solid objects. This paper will describe three experiments which examined this question.

EXPERIMENT 1

Cook et al. (1978) based their conclusion that the infants were unable to distin- guish between the cube and truncated pyramid on the finding that the rate of habituation of fixation for alternated presentations of the two objects did not differ from the rate of habituation for repeated presentations of a cube. How- ever, the results of Cook, Hine, and Williamson (1982) demonstrate that this particular technique is less sensitive than the habituation/recovery technique. Since this latter method was used by Caron et al. (1978, 1979) and Cook and Birch (1984) in demonstrating the rectangle-trapezoid discrimination, it might be concluded that Cook et al’s negative results do indeed merely reflect the use of an insensitive technique. Experiment 1 was therefore designed to reex- amine the ability of 3-month-old infants to discriminate between a cube and a truncated pyramid, using the more sensitive habituation/recovery technique. At the same time, the opportunity was taken to examine discrimination be- tween the cube and a sample of other solid forms.

The infants were tested for discrimination between the cube and a rhom- boidal prism, a triangular prism (wedge), a truncated pyramid, a cylinder, a sphere, and the L-form employed by Cook et al. (1978). The method used cor- responded closely to that of Cook and Birch (1984). Their results show that this procedure is reliably sensitive in demonstrating a rectangle-trapezoid dis- crimination.

Method

The apparatus and procedure were identical to those employed by Cook and Birch (1984). Each subject was habituated to repeated presentations of a cube and tested for recovery of fixation to two other stimulus forms. The habitua-

INFANT FORM DISCRIMINATION 191

tion stimulus was presented until a criterion level of fixation was reached. The subjects then received three test trials in which they were presented with the habituation stimulus and the two novel stimuli (the order of the three test trials being counterbalanced across subjects). Fixation time for the cube test trial provided the reference for assessment of recovery in the novel-form test trials. Following the three test trials, a control trial was given in which the subject was presented with a highly salient object-a red ball covered with black thumb tacks and mounted in front of a jagged white collar. Subjects were rejected if total fixation time to this control object was less than 10 s.

A varying-orientation procedure was employed in which the orientation of the cube was changed between presentations during the habituation se- quence. This procedure corresponded to that employed by Caron et al. (1978, 1979) and Cook and Birch (1984), and was analogous to that employed by Cook et al. (1978). The use of a varying-orientation procedure by Caron et al. and by Cook and Birch was motivated primarily by the specific requirement of the paradigm that the subjects be desensitized to changes in orientation. The motivation for its use by Cook et al. (1978) and for its use in the present experi- ment was somewhat different, namely, to desensitize the subjects to change in general. If a fixed-orientation procedure were employed, it would be possible for the infant to distinguish between the cube and a novel form merely by iden- tifying that some change had occurred in the proximal stimulus. Under varying- orientation conditions, it would be necessary for the infant to identify those aspects of stimulation which were invariant as the orientation of the stimulus object was changed, and to distinguish these from the changes due to the sub- stitution of the novel form. This ability is a prerequisite for an ability to per- ceive spatial form.

Three groups of infants were used. The novel stimuli presented were: Group l-rhomboidal prism and cylinder; Group 2-wedge and truncated pyramid; Group 3-sphere and L-form. Fixation to the various stimulus ob- jects was examined in pretests carried out on three other groups of 12 infants. In these tests, the test trials were replicated without prior habituation to the cube. In no case did fixation time for a “novel” form differ from that for the cube.

Subjects. Each group was comprised of 6 boys and 6 girls. The means (and standard deviations) of the ages in days were: Group 1, M= 88.5 (SD=7.3); Group 2, M= 90.7 (SD = 7.2); Group 3, M= 89.0 (SD = 7.5). These means do not differ significantly. An additional 4 infants were rejected: 2 for crying, 1 for failing to fixate the stimulus for more than 4 s on each of the first two habituation trials, and 1 as a result of experimenter error.

Apparatus. The subjects were seated so that the stimuli lay in their sagital and visual horizontal plane at a distance of 30 cm. The stimuli were mounted against circular blue collars, 28 cm in front of a vertical blue screen. The ob- server viewed from a darkened chamber behind this screen through a large

192 * COOK, BIRCH, AND GRIFFITHS

aperture which provided a clear binocular view of the infant and the rear of the stimulus collar. The collar concealed the stimulus identity from the observer. The aperture was concealed from the infant by blue cheesecloth stretched over the screen. During habituation, stimulus orientation was changed by the ob- server through an access door in the rear screen. The test stimuli were intro- duced by a device controlled by the observer from within the viewing chamber. They were arranged by a second experimenter and were unknown to the ob- server. Between trials the stimuli were hidden by a curtain.

Stimuli. Figure 1 shows the cube and the various novel objects in the orientations used in the test trials (as seen by the subject). The objects were mounted on horizontal rods (which were hidden by the stimuli) so that they pre- sented bilaterally symmetrical views to the infant. The cube had a IO-cm side and subtended approximately 22’ at the subject. The dimensions of the other ob- jects were selected so that all stimuli had approximately the same apparent size to an adult observer. The stimuli had a coarse surface texture and were painted flourescent orange. Illumination was placed so as to provide good contrast be- tween the stimulus surfaces. Surface luminance lay in the range 6-35 cd/m*.

The habituation procedure entailed presentation of cubes in various ori- entations. A single cube was used for this purpose. It was identical in size, color, and texture to that used in the test trials, and it was attached rigidly to its mounting rod to present the view illustrated in Figure 1. Changes in orienta- tion were effected between trials by rotating the horizontal mounting rod in its mounting bush. These changes, which corresponded to those used by Cook et al. (1978), rotated the cube about the line of sight. While these orientation changes did not alter the shape of the retinal projection substantially, they rotated the image on the retina and changed the distribution of illumination. They had the effect of altering the cube’s orientation in space (tipping it for- wards, backwards, laying it on its side, etc.), with consequent changes in the binocular and motion parallax cues to the object’s form.

A total of eight cube orientations were used during habituation, generated by rotating the cube progressively in 45 ’ steps from the orientation illustrated in Figure 1. These were arranged in a fixed sequence of contrasting orientations which were presented cyclically to the infant until criterion was reached. The initial orientation used was selected randomly for each infant.

It should be noted that in contrast to the studies by Caron et al. (1979) and Cook and Birch (1984), the novel test stimuli employed in this study pro- vided quite novel retinal projections. Because the demonstration of shape con- stancy was not an objective here, it was not necessary to control proximal novelty. Since the test stimuli employed in this experiment were both proximally and distally novel, they might have been expected to provide rather greater opportunities for discrimination than those used in the Caron et al. and Cook and Birch studies.

Procedure. The trials lasted 20 s from the first fixation and the inter-trial interval was 5 s. The observer closed a microswitch during fixations. Trial

INFANT FORM DISCRIMINATION 193

initiation and termination, and the reaching of criterion were signaled to the observer acoustically by computer. Following the procedure employed by Caron et al. (1979), the habituation series consisted at least 6 and not more than 12 trials. Within these limits, the sequence was terminated if: (a) each of the last two trials yielded fixation times less than 50% of the longest fixation time in the first two trials, or (b) the total fixation time over the last three trials was less than 5 s.

Results

Figure 1 shows mean total fixation times for the cube during the habituation trials, averaging over the three groups. The curve gives the mean time for the larger of the two first trials and the last five trials ((c-4), . . . ,c). In addition, the figure shows the mean total fixation time obtained in the test trial for the cube and the two novel stimuli for these three groups. The mean number of trials to criterion was 6.8 (SD = 1.2).

1,2 (c-4) (c-2) c gg HABITUATION 0 !? :

03 TRIALS

op. 1 op2 op.3

cube cylinder rh. prism wedge tr. PY~. sphere L-form

Flgure 1. Results of Experiment 1: Mean fixation times in habituation and test trials for the three experimental groups.

194 . COOK, BIRCH, AND GRIFFITHS

A Groups x Tests ANOVA was performed on the fixation times for the test trials. (In this analysis, the two novel-form tests for each group were ordered as shown in Figure 1. This order is, of course, arbitrary.) In view of the skewed distributions of times, log measures were employed in the analysis. The analysis yielded no significant main effect for Groups, F(2,33) = 1.14, p = -33, but did yield a significant main effect for Tests, F(2,66) = 5.99, p c .005, and a signifi- cant Groups x Tests interaction, F(4,66) = 3.35, p< .02. The simple effect of Tests was insignificant for both Group 1, F(2,22) = 1.16, p = .33, and Group 2, F(2,22) = .26, p = .77, but there was a significant effect of Tests for Group 3, F(2,22) = 5.35, p< ,015. For this group, fixation time for the L-form was sig- nificantly greater than for the cube, t(22) =4.60, p< .OOl. The difference in fixation time for the sphere and cube approached, but did not reach signifi- cance, 1(22) = 1.88, p = .087.

Discussion

The results of this experiment indicate that there was a recovery of fixation to the L-form, but there was no evidence of recovery to the rhomboidal prism, the wedge, the truncated pyramid, or the cylinder. The result for the sphere should be regarded as indeterminate. These results, therefore, confirm the finding of Cook et al. (1978) that 3-month-olds can distinguish an L-form from a cube. If failure to recover is taken to indicate absence of discrimina- tion, the results imply a failure to distinguish the cube from either the rhom- boidal prism, the wedge, the truncated pyramid, the cylinder, or (possibly) the sphere. The results are therefore consistent with the conclusions of Cook et al. (1978) that the resolution of solid form by 3-month-olds is poor, in spite of the use of the more sensitive habituation/recovery technique. The fact that the particular solid form which provided the basis for these negative conclusions was a cube is particularly interesting, since the results imply that the sensitivity to orthogonality demonstrated by Cook and Birch (1984) for 3-month-olds does not generalize from plane to solid forms.

Of course, negative evidence is weak, and it might be argued that these results merely reflect low statistical power or inadequacy in experimental pro- cedure. However, the sensitivity of the technique is demonstrated by the fact that, when appropriate stimuli were used, the same method was capable of yielding evidence of discrimination, both in Experiment 2 of this paper and in the study by Cook and Birch (1984).

In this experiment the subjects showed no evidence that they discriminated between the cube and the truncated pyramid. This finding raises again the question of why they should have failed to make this discrimination, given their ability to distinguish between the faces of these objects when they are presented in isolation as a square and trapezoid (Bower, 1966; Caron et al., 1978, 1979; Cook & Birch, 1984). Since the surfaces of the solids present multi- ple examples of these shapes, the solids might have been expected, if anything, to be more discriminable than the plane forms.

INFANT FORM DISCRIMINATION 195

EXPERIMENT 2

If 3-month-old infants were to find discrimination between solid forms more difficult than discrimination between analogous plane forms, this would carry interesting implications concerning the nature of infant form perception. How- ever, there are some alternative explanations of the relatively poor performance obtained with the solids in this experiment. One possibility is that the particu- lar rectangle and trapezoid which constituted the critical faces of the solids in this experiment simply happened to be less discriminable than the particular examples of these forms employed in the other studies. A second, and more in- teresting, explanation lies in the differences between the stimulus orientations used in the various experiments. The orientations used in the present experi- ment for both cube and truncated pyramid were such that their faces bore a complex relationship to the primary visual axes of the infant. This contrasts with the orientations used in the studies of the rectangle-trapezoid discrimina- tion. In each of these, the rectangle edges were orthogonal to the median and horizontal planes, while the base of the trapezoid was either horizontal (Caron et al., 1979; Cook & Birch, 1984) or vertical (Bower, 1966). It is possible that the ability to make the rectangle-trapezoid discrimination is specific to situa- tions where the forms are aligned with the primary visual axes. Experiment 2 tested all of these possibilities.

Method

A single group of infants was habituated to a square, which was presented in various orientations, and tested for recovery to a triangle and a trapezoid. These stimuli were precisely the same shape, size, and color, and they were mounted in the same orientations and positions relative to the stimulus collars, as the object faces marked with stars (*) in Figure 1. In effect, the subjects were presented with the cube, wedge, and truncated pyramid of Experiment 1, with all but a single face removed. Recovery to the novel forms under these conditions would eliminate the possibility that the failures to discriminate be- tween the shapes of the faces of the corresponding solids in Experiment 1 were due to the particular orientations (or any other attributes) of these faces.

The apparatus and experimental procedure were identical to those em- ployed for Group 2 of Experiment 1, with the exception of the substitution of the plane forms for the corresponding solids.

Subjects. The subjects were 12 boys and 12 girls. Their mean age in days was 92.8 (SD=7.6). This does not differ significantly from the mean ages of the three groups in Experiment 1. Three infants were rejected for crying.

Stimuli. The stimuli were cut from thin metal plate. Although they were mounted obliquely to the line of sight, no appreciable edges were visible. Stim- ulus luminance lay in the range 6-35 cd/m2. Pretests carried out on a separate group of infants showed no differences in fixation time to these stimuli in the absence of a preceding habituation sequence.

196 . COOK, BIRCH, AND GRIFFITHS

Results

Figure 2 shows mean total-fixation times for the habituation sequence and for each of the test trials. The mean number of trials to criterion was 6.88 (SD = 1.42).

An ANOVA performed on the log-fixation times for the three test trials yielded a significant treatment effect, F(2,46) = 8.33, p< .OOl and mean log- fixation time for each novel stimulus was significantly greater than for the square (triangle, t(46) =2.99, p< .005; trapezoid, t(46) = 3.90, pc .OOl).

For purposes of comparison between experiments, the fixation time for each infant on each novel-form test trial was converted to a recovery score, given by the ratio: (T(nove1 form) - T(square))/(T(novel form) + T(square)). The means (and standard deviations) of these scores were: triangle, M= .26 (SD = .30); trapezoid, M= .20 (SD = .33). The fixation times for Group 2 of Experiment 1 were also converted to recovery scores. The means (and standard deviations) of the scores were: wedge, M= 0.08 (SD = .3 1); truncated pyramid,

1,2 (c-4) . (c-2) . c

HABITUATION TRIALS

Flgure 2. Results of Experiment 2: Mean fixation times in habituation and test trials.

INFANT FORM DISCRIMINATION 197

M= 0.03 (SD = 0.3 1). In each case, the mean recovery score for the plane form in Experiment 2 was significantly greater than that for the corresponding solid in Experiment 1 (truncated pyramid/trapezoid, t(34) = 2.92, p< .Ol; wedge/ triangle, t(34) = 3.17, p< .Ol).

Discussion

The recoveries of fixation manifested in the test trials show that the infants were able to distinguish both the triangle and the trapezoid from the square that had been seen during the habituation sequence. This contrasts with the infants’ apparent failure in Experiment 1 to distinguish between the solid counterparts of these objects. The positive results obtained in Experiment 2 confirm that the difficulties with solids manifested in the previous experiment were genuine. The recoveries of fixation which occurred in this experiment demonstrate the sensitivity of the technique employed in Experiment 1, and the comparison of performance in the two experiments provides positive statis- tical evidence that discrimination between the plane forms was superior to dis- crimination between their solid counterparts.

The results of Experiment 2 demonstrate that the infants’ failure to dis- tinguish the square faces of the cube from the trapezoidal faces of the truncated pyramid in Experiment 1 (and the corresponding failure in the cube-wedge discrimination), were not due to the particular sizes or shapes of those sur- faces, nor to the particular orientations in which they were presented. This finding shows that the failure to respond to the distinctive faces of the solids in the earlier experiment was specifically due to the presence of the additional faces of these objects. The finding is also of significance in showing that the rectangle-trapezoid discrimination demonstrated in earlier studies does not de- pend upon an alignment of the stimuli with the primary visual axes.

EXPERIMENT 3

A possible explanation for the improvement in discrimination effected by the removal of the additional faces of the solids in Experiment 2, lies in the conse- quent increase in the chromatic contrast at the edges of the stimuli. In Experi- ment 2, each edge of the stimulus was seen against the blue background screen. On the other hand, the corresponding object face in Experiment 1 was con- nected to two like-colored faces. It is possible that this reduction in chromatic contrast at two of the surface boundaries was sufficient to prevent the infant from processing its form. Experiment 3 was carried out to examine this possi- bility.

Method

A single group of infants was habituated to a cube and tested for recovery of fixation to a wedge and a truncated pyramid. The procedure and apparatus were identical to those employed for Group 2 of Experiment 1, with the excep- tion that the faces not marked with stars (*) in Figure 1 were colored white in-

198 ’ COOK, BIRCH, AND GRIFFITHS

stead of orange. In effect, the stimuli in this experiment were those employed in Experiment 2, with the addition of two more, chromatically distinct, faces.

Subjects. The subjects were 12 boys and 12 girls, with a mean age of 91 .O days (SD= 5.0). This mean does not differ significantly from the mean ages of the groups in the other experiments. An additional 9 infants were rejected: 5 for crying, 2 for failure to fixate for more than 4 s on the first two habituation trials, and 1 for failing to reach criterion on the control object.

Stimuli. The stimulus forms were the same as the cube, wedge, and trun- cated pyramid of Experiment 1, the hue and luminances of the critical stimulus faces being identical to those for Experiment 2. The luminances of the white faces lay in the range lo-46 cd/m2. Pretests carried out on a separate group of infants established that fixation times for the three stimuli did not differ in the absence of a preceding habituation sequence.

Results

Figure 3 shows mean total fixation times in the habituation trials and for each of the test trials. The mean number of trials to criterion was 7.1 (SD = 1.5). Performance in the test trials was examined by means of an ANOVA carried out on the log fixation times. This yielded no significant effect for Tests, F(2,46) = .263.

For purposes of comparison with Experiment 2, the performance of each subject on each novel-form test trial was converted to a recovery score. The means (and standard deviations) of these scores were: wedge, M= - .009 (SD= .243); and truncated pyramid, M= .017 (SD = .184). In each case, the mean recovery score for the plane form in Experiment 2 was significantly greater than that for the corresponding solid in Experiment 3 (truncated pyra- mid/trapezoid, t(46) =4.74, p< .Ol; wedge/triangle, 1(46) = 6,83, p-c .Ol).

Discussion

In Experiment 1 the infants showed no evidence of distinguishing the cube from either the wedge or the truncated pyramid, even though the infants in Ex- periment 2 were able to distinguish between the distinctive faces of these ob- jects when they were presented in isolation. It was concluded that the ability to distinguish between the shapes of these faces was masked in Experiment 1 by the presence of the two extra faces of the solids. Experiment 3 examined the possibility that the improvement in performance observed in Experiment 2 was simply due to increased chromatic contrast at the edges of the stimuli. How- ever, the results of this experiment do not support that suggestion, since the solids still presented problems to the infants when the distinctive faces were a different color from the adjoining faces.

GENERAL DISCUSSION

The results of Experiments 1 and 3 confirm the findings of Cook et al. (1978) that 3-month-old infants fail to discriminate between a cube and a range of

INFANT FORM DISCRIMINATION 199

HABITUATION TRIALS

Figure 3. Results of Experiment 3: Mean fixation times in habituation and test trials.

convex forms, when these are stationary and viewed binocularly. However, the results of Owsley (1983) raise the question as to whether the resolution of the spatial forms of solid objects in these experiments might have been improved if the stimulus objects had been presented in motion. Owsley habituated 4-month- old infants to a “wedge-like” convex polyhedron, which was seen monocularly, and tested for recovery of fixation to a binocularly seen cube. The infants evi- denced discrimination between the wedge and the cube when the wedge was rotated during the habituation trials, but not when it was stationary (the stimuli were always stationary in the recovery trials). Although the wedge in Owsley’s study was seen monocularly (in contrast to the binocular viewing conditions of the present study), her results imply a relatively good resolution of the form of this object when it was seen in motion.

200 COOK, BIRCH, AND GRIFFITHS

Several comments may be made regarding the implications of Owsley’s results for the present study. Her study implies that the perception of the form of a solid object is enhanced by moving it. While such a finding is not surprising in view of the enriched optical information provided by the moving object (Ruff, 1980), it does not contradict the conclusion that the resolution of the forms of stationary objects is poor. Nor is it necessarily the case that the use of moving stimuli has greater ecological validity than the static conditions used in the present study. Stimulus rotations of the magnitude employed in Owsley’s study are only encountered by the infant when it or the object are in motion, and the conditions of present study model the not inconsiderable part of the infant’s visual experience which entails the perception of stationary objects by the stationary child. Both the present results and Owsley’s own results suggest that resolution of solid form under these conditions is limited.

It may also be noted that in the present study the subjects were able to distinguish the square from the triangle and the trapezoid in Experiment 2. This demonstrates that the problems with the solids in Experiment 1 and 3 were not simply due to the fact that the stimuli were stationary.

One aspect of Owsley’s results is puzzling in the light of the present find- ings. Her infants recovered to the stationary binocular cube when it was pre- sented after habituation to the rotating wedge. This implies not only that the infants could see the form of the moving wedge, but, also, that they were able to see the stationary, binocular cube sufficiently well to distinguish it from the wedge, a conclusion which contradicts the results of this study and those of Cook et al. (1978). The reasons for this apparent contradiction are unclear. Several possibilities suggest themselves. For example, the inconsistency in the results might be attributed to the difference in age between the subjects in the two studies, or the difference in complexity between the two-faced view of the cube used in the Owsley study and the three-faced view used here. It is possible that the particular object selected by Owsley was more discriminable from a cube than the objects used in this study, although this seems unlikely in view of the negative result obtained here for objects as different as the cube and the cy- linder. Another possibility is that the rotation of the wedge in Owsley’s study enhanced processing by increasing the infant’s attention to the two stimuli (al- though there is no evidence for this in her fixation times). A more interesting alternative is that the dynamic presentation of the wedge during the habitu- ation procedure altered the nature of the infant’s perceptual representation of this object, so that it was now distinguishable from the static cube.

The central result of the present experiments is the demonstration that otherwise discriminable plane forms were not distinguished when these were presented as faces of solid objects. The apparent difficulties in discrimination caused by the additional faces of the solids in Experiments 1 and 3 could be ex- plained in several ways. One possibility is that the effect was due to the infants’ distribution of attention-that they were capable of perceiving the shapes of the individual faces of the solids, but did not do so because they were directing

INFANT FORM DISCRIMINATION 201

attention to the forms of the solids as wholes. If this account were correct, it might be possible to improve discrimination between the solids by procedures which directed attention appropriately. The fact that performance with the solids remained poor in Experiment 3, in spite of the enhanced distinctiveness of the critical faces, might be interpreted as providing some evidence contrary to this suggestion.

An alternative explanation is that infants may have been unable to’per- ceive the square, triangle, and trapezoid when these constituted parts of the surfaces of the corresponding solids. This could be a consequence of a limited processing capacity-the requirements for processing the relatively complex solid forms precluding the processing of individual faces-or perhaps it could reflect the way in which the infants organize the stimuli perceptually. For ex- ample, the presence of the square, triangle, and truncated pyramid in the cor- responding solids would not be evident if the latter were perceived in terms of the space enclosed by their surfaces, rather than their surface forms, or if the surfaces were perceived as continuous instead of being segmented into discrete “faces.”

The implication of these results is that the infants were not seeing, or, at least, not responding to, differences between the individual faces of the solid objects. However, this need not have prevented the infants from discriminating between the objects. Each of the test objects used in Experiments 1 and 3 could have been distinguished from the cube on the basis of many attributes other than the shapes of individual faces. The negative findings obtained in Experi- ments 1 and 3 imply the infants were not always able to make use of these alter- native cues. Bearing this in mind, the pattern of discrimination displayed in Experiments 1 and 3 may be seen as conveying information about the infants’ perception of the solid stimulus objects. The infants did not distinguish the cube from the various convex polyhedra. In addition, the cube was not dis- tinguished from at least one of the two curved convex forms, and the evidence for the second of these (the sphere) was inconclusive. On the other hand, they did distinguish the cube from the L-form. These results suggest that the infants may simply have been distinguishing between convex and concave forms. In turn, this classification might reflect their level of comprehension of the forms of the solid objects. However, definitive conclusions about this question can- not be drawn on the basis of the small sample of forms examined in this study.

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27 June 1985; Revised 27 November 1985 l