Spatial coding and oblique discrimination by children

JOURNAL

OF EXPERIMENTAL

CHILD

PSYCHOLOGY

Spatial Coding and Oblique HOWARD

S. HOCK Florida

27, 96-104 (1979)

Discrimination AND THOMAS

Atlantic

by Children

HILTON

University

Children from five to eight years of age learned to discriminate between mirrorimage oblique lines more readily when cards bearing the obliques were presented vertically than when they were presented horizontally. The diierence in performance between the vertical and horizontal planes could not be accounted for by differences in either the external visual context or the availability of asymmetrical body information (e.g., left vs right hand). The superiority of the vertical plane was attributed to the congruence of objective, bodily, and retinal vertical axes for the vertical, but not the horizontal plane, It was concluded, on this basis, that at least part of children’s difficulty discriminating between mirror-image obliques is due to their difficulty establishing the internalized vertical axis necessary for the left-right spatial coding of the obliques.

In their classic study, Rude1 and Teuber (1963) investigated developmental changes in children’s ability to discriminate between mirror-image (MI) oblique lines. They found, within a session of 50 trials, that the discrimination was learned to criterion (pointing at the “correct” oblique nine times over a span of 10 trials) by only 11% of children between 3% and 5Y2 years of age. Children between 6Y2 and W2 years of age showed substantial improvement, but 39% still failed to reach criterion. Children’s difficulty discriminating between MI obliques is not due to their inability to establish a perceptual difference between them. Experiments by Over and Over (1967) and Bryant (1969, 1973) have shown that discrimination between MI obliques is difficult only when children are required to form a memory representation corresponding to the “correct” or standard oblique. Bryant has contended that for children under six, the formation of such memory representations involves coding the obliques in terms of external contextual information that differentiates between them. Consistent with this hypothesis, he found that oblique discrimination was improved by the introduction of multicolored oblique reference axes that provided either a match or mismatch with the standard oblique. A version of this paper was presented at the 1976 meeting of the Psychonomic Society. We are grateful to Ms. Barbara Bittner, Director, and the faculty of A. D. Henderson University School, Boca Raton, Florida, for their cooperation in conducting this study. We would also like to thank David Bjorklund for his careful reading of the manuscript. Thomas Hilton is currently at Texas Christian University. Reprint requests should be sent to the first author, Howard S. Hock, Department of Psychology, Florida Atlantic University, Boca Raton, FL 33431. 96 0022-0965/79/O 10096-09$02.00/O Copyright @ I979 hy Acadenuc Pre\\, Inc. All rights of repruduction tn any form reserved

OBLIQUE

HORIZONTAL

SURFACE

97

DISCRIMINATION

VERTICAL

SURFACE

FIG. 1. Illustration of the reIationship between objective, bodily, and retinal vertical axes for horizontal and vertical surfaces.

Although Corballis and Beale (1976) acknowledge that information in the visual context can influence MI discrimination, they emphasize the importance of asymmetrical body information (e.g., the left vs the right side) in MI discrimination. Young children’s difficulty with MI discrimination, they assert, is due to insufficient lateralization of bodily response systems (e.g., instability of hand preference). Consistent with this point of view, they cite evidence that training procedures which associate a different motor activity with each stimulus in a MI pair improves discrimination performance (Jeffrey, 1958, 1966; Clarke & Whitehurst, 1974; Femald, 1943). It is implicit in Corballis and Beale’s theory that bodily left and right must be defined in relation to an internalized vertical reference axis in order to be useful in spatially coding MI stimuli. This suggests that children’s difficulty with MI discrimination may not be due only to the absence of body asymmetry, as Corballis and Beale claim. That is, children’s difficulty with MI discrimination may be due, at least in part, to their difficulty establishing the internalized vertical axis necessary to utilize left-right body information. Evidence indicating the involvement of an internalized vertical axis in MI discrimination was provided in this study by comparing MI discrimination performance under two conditions: (1) when the stimulus cards bearing the oblique lines were presented in a vertical plane, and (2) when the stimulus cards bearing the oblique lines were presented in a horizontal plane. The nature of the contrast between the vertical and horizontal orientations of the stimulus cards is illustrated in Fig. 1. When viewed from above, the distal edge of a horizontal surface is retinally “higher” than the proximal edges1 A vertical reference axis based on retinally cued “top” and “bottom” could therefore be assigned to 1 Retinal “top” and “bottom” denotes the manner in which visual information onto the retina is coded, not the literal top and bottom of the retina.

projected

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oblique lines presented on a horizontal surface. In three-dimensional space, however, the distal and proximal edges of a horizontal surface are equally high above the ground, and equally high in relation to the body of an observer in an upright posture. That is, the objectively and bodily defined vertical axes are not congruent with the retinally defined vertical axis. This is not the case for vertically oriented surfaces. For vertical surfaces, the retinally defined “top” is higher than the retinally defined “bottom” both objectively and in relation to the observer’s body. It was because of this congruence that oblique discrimination was expected to be better when the stimulus cards bearing the oblique lines were vertically oriented than when they were horizontally oriented. In the absence of differences in either the external visual context (Bryant) or the leftright position of the stimulus array relative to the subject’s body (Corballis & Beale), the superior performance hypothesized for the vertical plane would provide evidence for the involvement of an internalized vertical reference axis in the spatial coding of the MI oblique lines. EXPERIMENT

I

Method Stimuli. The stimuli used in this experiment were similar to Rude1 and Teuber’s (1963). They comprised two white Plexiglas cards (12.5 cm high, 12.5 cm wide, 0.3 cm thick), each bearing an oblique line (a strip of black tape 7.5 cm long, 0.5 cm wide) centered on the card. The oblique line was tilted 45’ to the left of vertical on one card, and 45” to the right of vertical on the other card. Procedure. The children sat on an adjustable stool that was adjacent to the table on which the stimulus cards were presented. The stool was positioned so that the center of the stimulus cards was about 30 cm from each child’s midsection. The height of the stool was adjusted so that each child’s line of sight with regard to the center of the stimulus cards formed an angle of approximately 45” with the surface of the stimulus cards

FIG. 2. Results tical and horizontal

for Experiment I comparing planes of presentation.

oblique

discrimination

performance

for ver-

OBLIQUE

DISCRIMINATION

99

(see Fig. 1). This adjustment, which was repeated at the start of each experimental session, matched the retinal projections of the oblique lines for the vertical and horizontal planes of presentation. Since the experiment required extensive discrimination training for the children, it was decided that minimizing their discomfort was more important than using a bite plate or chin rest to maintain their line of sight at precisely 45”. The line of sight, though slightly variable, remained close to 45’ for both the vertical and horizontal conditions.z The two stimulus cards were presented side-by-side in the vertical plane for one group of subjects, and side-by-side in the horizontal plane for a second group of subjects. For half the subjects in each condition (vertical or horizontal) one oblique was designated as “correct.” For the remaining subjects the other oblique was designated as “correct.” The “correct” oblique for each subject remained the same for the entire experiment, which involved daily sessions of 30 discrimination trials. Within each block of 30 trials the lateral position of the left and right oblique was varied according to a modified Gellerman series (Gellerman, 1933). A different series was generated for successive sessions, but within a given daily session the series was the same for every subject. At the start of each session the experimenter denoted the correct oblique, and on each succeeding stimulus presentation the subjects responded by pointing to what they thought was the correct oblique. Corrective feedback was provided after each trial. At the end of each session the children were given one M & M candy for each correct response. Pilot data indicated that the difference between the vertical and horizontal planes of presentation emerged only after many days of practice. For this reason, discrimination trials were conducted for a total of 18 daily sessions. Subjects. Twenty-four kindergarten students at the University School of Florida Atlantic University served as subjects for the entire 18 days of the experiment. The subjects in each condition were matched in sex, and their mean chronological age was 5.3 years in each condition. Four subjects, two from the vertical condition and two from the horizontal condition, were dropped from the experiment after exhibiting perfect performance for the first two days of the experiment. These subjects were apparently capable of oblique discrimination prior to their participation in the experiment. 2 Although the retinal projections of the obliques were matched for the vertical and horizontal planes, the retinal projections of the edges of the stimulus cards were different for the two planes of presentation. For both planes, the retinal projection of the pair of stimulus cards was trapazoidal. For the vertical plane, the retinally “higher” edge of the trapazoid was longer than the retinally “lower” edge. For the horizontal plane, however, the retinally “higher” edge of the trapazoid was shorter than the retinally “lower” edge. Nonetheless, the oblique lines were laterally symmetric and their lateral positions were counterbalanced for both planes. As a result, the projective spatial relations between the obliques and the edges of the stimulus cards were the same for the vertical and horizontal pIanes.

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Results

Nine of 12 subjects in the vertical condition, as opposed to three of 12 in the horizontal condition, learned the discrimination to the level of perfect performance. The probability that this difference between the groups occurred by chance was less than .05 (Fisher’s test of exact probability). As can be seen in Fig. 2, the difference between the vertical and horizontal plane emerged only after nine days of discrimination trials. The subjects who learned the discrimination were evenly divided with respect to sex, and their mean age was about one month younger than the nonlearners. While the learners maintained perfect performance for 120 to 420 consecutive trials, the nonlearners performed at chance level for the entire experiment. Of further interest was the observation that four successful learners, all from the vertical condition, repeatedly traced their finger in a direction parallel to what they responded to as the correct oblique. This finding was consistent with previous evidence that the kinesthetic information introduced by tracing (e.g., Jeffrey, 1958, 1966) can facilitate the leftright coding of the obliques. Whether or not tracing is necessary in order to obtain a difference betwen the vertical and horizontal planes was examined in Experiment II. EXPERIMENT

II

Method Stimuli. The stimuli and viewing conditions were the same as in the previous experiment. The apparatus, however, was altered to include a large board as a vertical backdrop (extending 90 cm above the table top and 120 cm across). The backdrop served two functions. First, it was large enough to shield the subject from peripheral visual cues that were unrelated to the experiment; it intercepted a visual angle of approximately 113’ vertically and 126’ horizontally. Second, by placing the stimulus cards flush against this backdrop, the vertical and horizontal planes of presentation were matched with regard to potential parallax cues. Procedure. In order to investigate the effect of tracing on oblique discrimination, subjects were required to respond by either tracing their finger along what they thought was the correct oblique (the stimulus cards were secured so that they could not be dislodged by tracing) or pressing a button alongside what they thought was the correct oblique (two identical buttons were fastened onto the horizontal table top). The orthogonal combination of vertical plane vs horizontal plane and tracing vs no tracing resulted in four experimental conditions. Twelve children were assigned to each of the four conditions. The four groups of children were matched with respect to their mean performance in a preliminary oblique discrimination screening task. The latter comprised 25 oblique discrimination trials

OBLIQUE

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in the horizontal

plane. Subjects with 17 or fewer correct responses (76% of the total number of children tested) were continued in the experiment. Their average performance in the screening trials was at chance. Following the screening task, subjects remaining in the experiment participated in one of the four experimental conditions described above for 15 days, or until they reached a criterion of two consecutive days with 29/30 correct responses. The design of the experiment was otherwise the same as in Experiment I. For half the subjects in each group, the +45” was designated as “correct;” for the other half the -45’oblique was designated as correct. The lateral position of the left and right oblique was varied according to a modified Gellerman series, the series was the same for every subject within a given session, and a different series was generated for successive sessions. subjects. Five kindergarten, 22 first-grade, and 21 second-grade students at the University School participated in the entire experiment. Children from the three grades were assigned to the four experimental conditions in an approximately balanced fashion. As indicated above, 12 children were assigned to each condition. The four groups of children were matched in sex, mean performance in a preliminary screening task, and mean chronological age (mean = 7.3 years for each condition). Fifteen additional children who had more than 17 correct responses (out of 25 trials) during the preliminary screening test did not participate in the remainder of the experiment. Results

The results of Experiment II are presented in Fig. 3. A much larger proportion of the children in this experiment learned the MI discrimination than was the case in Experiment I, and those that learned it did so with much less practice than in Experiment I. This difference was not

OAYS

3. Results for Experiment II comparing oblique discrimination performance for vertical and horizontal planes of presentation. Subjects responded by either tracing along an oblique (trace) or pressing a button (no trace). FIG.

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surprising given that the children in Experiment II were, on the average, two years older than the children in Experiment I. In Experiment II, only two subjects, one from the horizontal-trace and the other from the horizontal no-trace condition, failed to reach criterion within the 15 discrimination sessions allowed for acquisition. For the 46 subjects who reached criterion, the mean number of days preceding criterion was 1.4 days for the subjects in the vertical condition and 3.3 days for the subjects in the horizontal condition. An analysis of variance, with “days preceding criterion” serving as the dependent measure, indicated that the effect of plane of presentation (vertical vs horizontal) was significant [F(l,38) = 16.83, p < .005]. The effect of tracing [F(l,38) < I.01 and the interaction between tracing and plane of presentation [F( 1,38) < 1 .O] were not significant. Likewise, the effect of sex, and all interactions involving the sex variable were not signmcant [R’(l,38) < l.0].3 On the day following the achievement of criterion performance, subjects were transferred to the condition opposite their training condition (e.g., from vertical-trace to horizontal-no trace). In all transfer conditions discrimination performance was virtually perfect (mean correct = 98%). DISCUSSION

The results for both Experiment I and II indicated a significant difference in MI discrimination between the vertical and horizontal planes of presentation. Since the stimuli, viewing conditions, and procedure were the same for both the vertical and horizontal planes, the difference in discrimination performance between them could not be attributed to subjects redefining the task (e.g., by responding to the configurational properties of the pair of MI obliques) so that the left-right discrimination became a simpler updown discrimination (Corballis & Beale, 1970). Furthermore, the visual context provided by the edges of the stimulus cards was the same for the vertical and horizontal planes, and the possibility that subjects would utilize peripheral visual cues in the experimental room was minimized in Experiment II by the use of a large vertical backdrop. Therefore, the difference in discrimination performance between the vertical and horizontal planes could not be attributed to differences in the availability of external contextual information for coding the MI obliques (Bryant, 1969, 1973). Finally, the left-right position of the obliques relative to the subject’s body was the same for the vertical and horizontal planes, and s Since two subjects failed to reach criterion, the analysis of variance followed the leastsquares-procedure for unequal cell frequencies (Winer, 1971). A logarithmic transformation was performed on the data in order to obtain homogeneity of variance. Although the experiment was not designed with grade (K, 1, 2) as an independent variable, an examination of the data indicated that the difference in discrimination performance between the vertical and horizontal planes was consistent at each grade level.

OBLIQUE

DISCRIMINATION

103

differential motor responses (i.e., tracing) did not affect discrimination performance for either plane in Experiment II. It could be concluded, therefore, that the vertical-horizontal difference in discrimination performance was not the result of differences in the availability of asymmetrical body information for left-right spatial coding of the MI obliques (Corballis & Beale, 1976). The only apparent difference between the vertical and horizontal planes of presentation involved the congruence of alternative vertical reference axes. For the horizontal plane, objective and bodily vertical are congruent, but neither are congruent with retinal vertical. For the vertical plane, however, objective, bodily and retinal vertical are ail congruent. This congruence in establishing an internalized vertical axis seems the likely basis for the superior discrimination performance obtained for the vertical plane, The occurrence of spontaneous tracing in the vertical plane for the relatively young children in Experiment I (mean age of 5.3 years) was consistent with evidence that differential body responses would facilitate left-right coding for young children (e.g., Jeffrey, 1958, 1966). The absence of any tracing effect for the older children in Experiment II (mean age of 7.3 years) was consistent with Corballis and Beale’s contention that the development of body asymmetry (e.g., stable hand preference) makes overt body responses unnecessary for the left-right coding of MI stimuli. Regardless of the need for overt body responses, the superior discrimination performance obtained for the vertical compared with the horizontal plane indicated that establishing an internalized vertical reference axis is necessary in order to utilize left-right body information for spatially coding MI obliques. Differences in performance between vertical and horizontal planes of presentation are not exclusive to MI discrimination. Benson and Yonas (1973) have found that three-year-olds could accurately discriminate between mounds and craters when photographs of these discriminanda were vertically oriented, but not when they were horizontally oriented. In a study involving incongruence of spatial information within the vertical plane, Yonas and Hagen (1973) had three- and seven-year-olds make size judgments for stimuli presented in vertically oriented photographs. They found that the presence of motion parallax cues, which indicated that the stimuli in the photographs were objectively equidistant from the observer, reduced the children’s use of pictorial depth information in the photographs. Returning, in conclusion, to a study involving the discrimination of MI obliques, Olson and Boswell (Note 1) have obtained results that converge with those of the present study. They found that increasing the amount of congruent spatial information biased toward diagonahty increased discrimination accuracy for MI obliques and decreased discrimination accuracy for vertical and horizontal lines. They attributed these results to subjects establishing an internalized oblique reference system for coding the discriminanda.

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In general, the results of the present research, together with the research of Olson and Boswell (Note 1), Benson and Yonas (1973), and Yonas and Hagen (1973), suggest that in tasks requiring the spatial coding of visual information, children’s performance depends on the degree of congruence between alternative spatial reference axes. REFERENCES Benson, C. & Yonas, A. Development of sensitivity to static pictorial depth information. Perception & Psychophysics, 1973, 13, 361-366. Bryant, P. E. Perception and memory of the orientation of visually presented lines by children. Nature, 1%9, 224, 1331-1332. Bryant, P. E. Discrimination of mirror images by young children. Journal of Comparative and Physiological Psychology, 1973, 82, 415-425. Clarke, J. E., & Whitehurst, G. J. Asymmetrical stimulus control and the mirror-image problem. Journal of Experimental Child Psychology, 1974, 17, 147- 166. Corballis, M. C., &i Beale, I. L. Bilateral symmetry and behavior. Psychological Review, 1970, 77, 451-461. Corballis, M. C., & Beale, I. L. The psychology of left and right. Hillsdale, NJ: Erlbaum Associates, 1976. Femaid, G. M. Remedial techniques in basic school subjects. New York: McGraw-Hill, 1943. Gellerman, L. W. Chance orders of alternating stimuli in visual discrimination experiments. Journal of Genetic Psychology, 1933, 42, 206-208. Jeffrey, W. E. Variables in early discrimination learning: I. motor responses in the training of left-right discrimination. Child Development, 1958, 29, 269-275. Jeffrey, W. E. Discrimination of oblique lines by children. Journal of Comparative & Physiological Psychology, 1966, 62, 154- 156. Over, R., & Over, J. Detection and recognition of mirror-image obliques by young children. Journal of Comparative and Physiological Psychology, 1%7,64, 467-470. Rudel, R. G., & Teuber, H. L. Discrimination of the direction of line by young children. Journal of Comparative and Physiological Psychology, 1%3, 56, 892-898. Wirier, B. J. Statistical principles in experimental design. New York: McGraw-Hill, 1971. Yonas, A., & Hagen, M. Effects of static and motion parallax depth information on perception of size in children and adults. Journal of Experimental Child Psychology, 1973, 15, 254-265.

REFERENCE 1. Olson, R. K., & Boswell, S. L. Children’s tion

is based

on orientation

of a retinally

NOTE

dif$culty with independent

right-left diagonal discriminaCartesian reference system.

Paper presented at a meeting of the Psychonomic Society, Denver, Colorado, RECEIVED: October 11, 1977;

REVISED:

December 29, 1977;

R.&lmry

21, 1978

1975.