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Why do children confuse mirror-image obliques?
JOURNAL
OF
EXPERIMENTAL
CHILD
Why Do Children MICHAEL
PSYCHOLOGY
Confuse
C. CORBALLIS McGill
24, 5166513
(1977)
Mirror-image AND MARSHA
Obliques? C.ZALIK
iJnit,rrL
Thirty-five children aged from 4.17 to 6.58 years were given a delayed-matching task in which they had to choose on each trial which of two lines was the same as a previously displayed standard line. Their choices were no better than random when the lines differed in degree of slope but not in left-right orientation and were only marginally more accurate when the lines were left-right mirror images. Performance improved significantly when the lines differed both in degree of slope from the vertical and in left-right orientation and improved still further if at least one of the lines was horizontal or vertical. The results suggest that young children have extreme difficulty encoding in memory either the degree of slope or the left-right orientation of an oblique line.
It is well established that children find it difficult to discriminate mirrorimage oblique lines. There is some question, however, as to whether the difficulty arises because the stimuli are oblique, or because they are mirror images. On the one hand, Appelle (1972) has reviewed a large body of evidence which shows that many species, including humans, perform more poorly on a variety of tasks when the critical stimuli are oblique lines or gratings than when they are horizontal or vertical. Appelle dubbed this the “oblique effect.” On the other hand, there is an equally impressive although partially overlapping accumulation of evidence, reviewed by Corballis and Beale (19761, that most species, including human children, suffer special difficulty in discriminating left-right mirror images. The difficulty includes, but is not restricted to, mirror-image obliques. Corballis and Beale (1970, 1976) argue that the problem is fundamentally one of telling left from right. One thing that is clear is that the difficulty of discriminating mirrorimage obliques is a problem of memory rather than of perception. For instance, children have no special difficulty on a simultaneous-matching task in which they must decide which of two mirror-images obliques is the same as a standard oblique, simultaneously displayed. However, 5year-olds perform at no better than chance on delayed matching, in which the choice mirror-image pair is presented 5 set after the standard (Bryant, This research was supported by a research grant from the National Canada. The author’s address is Department of Psychology. McGill Gregor Avenue, Montreal, Quebec, Canada H3A IBI.
Research University.
Council of I705 MC
MIRROR-IMAGECONFUSIONS
517
1969, 1973). Because the difficulty seems to occur only when the child has to remember the oblique orientation (see also Over & Over 1967a,b; Tee & Riesen, 1974), one might suppose that it has more to do with leftright confusion than with the oblique effect. Left-right confusion is almost certainly a problem of remembering rather than of perceiving which way round things are (Corballis, 1974; Corballis & Beale, 1976). whereas much of the evidence for the oblique effect comes from experiments on perception or detection of oblique lines (Appelle, 1972). Yet Bryant (1974) is adamant that the problem is related to obliqueness rather than to left-right confusion; he writes (p. 69) that it is “virtually certain that the mirrorimage analysis is quite wrong, at least as far as young children are concerned. Obliques are difficult because they are obliques.” Bryant’s reasoning is based on the results of two experiments (Bryant. 1969, 1973), in which he found that children were still unable to discriminate between obliques in a delayed-matching task when the choice stimuli were not exact mirror images. In both experiments, however. the stimuli were oriented to either side of the vertical but differed in degree of tilt away from the vertical. The stimuli therefore differed both with respect to left-right orientation and with respect to the degree of obliqueness. Bryant may have been correct in asserting that the difficulty was not one of mirror-image discrimination per se, but it may nevertheless be at least partly a problem of telling left from right. Children may be unable to encode in memory either the left-right orientation or the degree of obliqueness of an oblique line. The present experiment was designed to separate these components. Children were tested on a delayed-matching task, in which they were required to discriminate obliques which differed in degree of tilt but not in left-right orientation, obliques which differed in left-right orientation but not in degree of tilt (i.e., left-right mirror images), and obliques which differed both in degree of tilt and in left-right orientation. In addition, there were control conditions in which one or both of the choice stimuli was either a horizontal or a vertical line. METHODS
Thirty-five preschool children, 19 boys and 16 girls, participated in the experiment. Their ages ranged from 4.17 to 6.58 years with a mean of 5.59 years.
The test stimuli consisted of circular white cards, 12.7 cm in diameter, each with a black line 7.6 cm long and 1.27 cm wide through its
518
CORBALLIS
AND
ZALIK
center. The line was oriented at either 0. 30, 60, or 90” from the vertical, either clockwise or counterclockwise. An additional set of cards with simple colored drawings in different orientations served as a training set. The cards could be inserted in a square placing tray, 71 by 71 cm. The tray consisted of two black cardboard squares taped back to back. The top square had three holes cut out of it. each the same size as the stimulus cards. The center of the top hole was 17.8 cm from the top of the tray and was centered horizontally. This hole was used for placement of the standard stimulus card. The choice cards were placed in two lower holes, with centers 20.3 cm apart and 15.2 cm below the center of the standard. The cards could be placed in the holes and held in fixed orientations by means of tabs, which were invisible to the subject.
The subjects were tested individually. Each child sat in a small chair at a table, facing the experimenter. The cards were in a prearranged order on the experimenter’s lap, out of sight of the subject. The experimenter explained to the child that she was going to place a “picture” in the top hole and then take it away and hide it. She was then going to place two pictures in the bottom holes, and the child was to point to the picture that was the same as the one the experimenter was hiding. A training card was then presented for 5 set and then removed. Immediately after removal of this standard card, the two choice cards were presented. One was identical to the standard, the other differed only in orientation. Subjects were given feedback to help them master the task, and when an error occurred the standard was put back to help the child correct his/her error. This training procedure was continued until each child achieved a training criterion offour consecutive correct choices. One child failed to reach this criterion and was rejected. After the training procedure, the experimenter explained that the cards would now not have pictures on them. but would have lines instead. Otherwise the “game” was the same. Each standard stimulus card was presented for 5 set followed by a 5set interval. and then the two choice cards were presented. One of the choice cards was identical to the standard. the other differed only in orientation of the line. During the test trials. the children were given no feedback. The children were randomly assigned to two groups. For Group A. the standards consisted of a vertical line. a line 30” to the left of vertical. and a line 60” to the right of vertical. For Group B, they consisted of a horizontal line, a line 60” to the left of vertical. and a line 30” to the right of vertical. There were 18 subjects in Group A and 17 in Group B. For the choice pairs. each standard was coupled with each of the other five line
MIRROR-IMAGE
CONFUSlONS
519
orientations twice, once with the standard on the left and once with the standard on the right. Thus, each subject completed a total of 30 test trials. 10 with each standard. The trials were randomly ordered. RESULTS For the purpose of analysis the trials were divided into five categories: those involving a choice between obliques with different slopes but with the same left-right orientation (O/OS), those involving a choice between mirror-image obliques (O/Om), those involving a choice between obliques oriented in opposite directions from the vertical but differing in degree of slope (OiOd), those involving a choice between an oblique and either a vertical or a horizontal line (OIVH). and those involving the choice between vertical and horizontal (V/H). Table I shows the mean percentage of errors in each category. Since each subject had only two V/H trials and only six of them made an error (one each), these scores were not included in the statistical comparisons. but it is clear that performance was well above chance. For the remaining categories, Group A was first compared with Group B by means of the Wilcoxon Rank Sum Test.’ Although Group B made a higher percentage of errors than Group A in all discrimination categories, the differences were nowhere significant and the groups were combined for all subsequent analyses.
A Friedman two-way analysis of variance by ranks among the four conditions involving obliques (O/OS, O/Om, OiOd, and O/VH) revealed a significant main effect xr2 = 395.2, df = 3. p < .OOl . Separate Wilcoxon signed-rank tests revealed that conditions O/OS and OiOm did not differ significantly from each other, but that all of the remaining comparisons were significant at the .Ol level or better. More specifically, the OiOd condition produced significantly fewer errors than either the O/OS or OiOm conditions, and the ONH condition produced significantly fewer errors than the OiOd condition.
Performance under the same four conditions was also compared with chance performance (i.e.. 50% correct). again by means of the Wilcoxon signed-rank test. Performance under the O/OS condition did not deviate significantly from chance. Performance under the OiOm was marginally better than chance (p < .05. one-tailed).
Performance
under the OiOd and
OiVH conditions were clearly above chance (p < .Ol. in both cases). Condition O/OS involved degree but not direction of slope. condition ’ Each child received only four trials under each of the oblique conditions (O/OS. OiOm. and OiOd), SOthe data were effectively scored on a T-point scale (O-3). To avoid the potential consequences of violating normality assumptions. we chose to use rank tests rather than analysis of variance to test hypotheses ahout the data. The los< in power in xo choosing is minimal (Siegel, 19%).
520
COKBALLIS
ANU ZALIK
TABLE I MEAN
PHKENTAGE
OF ERRORS (CHANCY. = 50%) FOR EACH OF DISCRIMINATION. IN EACH GROUP
CATEGORY
Category OIOS
OiOm
O/Od
OIVH
V/H
Group B
43.06 51.47
38.89 47.06
x!.21 30.88
8.71 16.21
5.56 11.76
Total
47.14
42.86
26.43
12.35
8.57
Group
A
OiOm involved direction but not degree, while condition OiOd involved both cues. It is of interest therefore to inquire whether performance on O/Od can be predicted from performance on O/OS and OiOm alone. Using a standard correction for guessing. we computed from the mean proportion of errors that the probability that the subjects made use of degree of slope as a cue in O/OS was .0573, while the probability that they made use of left-right orientation in O/Om was .1428. One would predict therefore that the probability of effectively using either degree or direction, or both. in O/Od would be [I - (1 - .0573) ( I - .1428)], which is .1918. Assuming that those who could use neither cue were merely guessing, this represents an error rate of 40.50%. which is clearly above the error rate of 26.43% actually obtained. In short, performance on O/Od is better than one would predict from performance on O/OS and OiOm. DISCUSSION The simplest interpretation of our results is that young children have extreme difficulty encoding either the degree of slope or the left-right orientation of an oblique line. Thus discrimination proved virtually impossible when the choice lines differed in degree of slope but not in leftright orientation (condition O/OS), and just about as difficult when they differed in left-right orientation but not in degree of slope (condition O/Om). Contrary to Bryant’s (1969, 1973) results. discrimination improved significantly when the choice lines differed both in the degree and in the direction of slope (condition OiOd). It is not immediately apparent why Bryant obtained a different result, particularly since he presented more extreme differences in slope than we did. One possibility is that his subjects may have been more likely than ours were to adopt a strategy of matching the lines to horizontal and vertical lines in the immediate environment and were thus less sensitive to variations among different oblique lines. We shall discuss this “match-mismatch” strategy more fully below. We saw that performance in the presence of both discriminative cues was actually better than one would expect on the basis of performance in
MIRROR-IMAGE
CONFUSIONS
521
the presence of either cue alone. A possible explanation for this is that the relative difficulty among the three conditions may have been influenced to some extent by variations in the angle between the two choice lines. Thus the lines differing only in degree of slope (condition O/OS) differed from each other by only 30”. while those differing in both degree and direction (condition OiOd) were always 90” apart. We might note that the subjects had no difficulty discriminating lines 30” apart when one of them was either horizontal or vertical (mean percent errors = 9.2), although evidence on the judgment of stimulus orientation (see Appelle, 1971, for review) might lead one to suspect that angular disparity would be more critical when both lines are oblique. We doubt that angular disparity played a significant role in the difference between O/Om and OiOd. Under the mirror-image condition (OiOm) the angle between the lines, if extended to meet, was either 60” or IZO”, and performance was essentially equal in the two cases. One could say, however, that the mirror-image lines at 60”from the vertical differ by only 60”, not 120”. Further research is therefore needed to clarify the role of angular separation in discrimination of the various oblique pairs. Bryant (1969, 1974) has argued that discrimination between lines in the delayed matching task depends on a “match-mismatch” code: Children encode the orientation of a line with reference to some parallel line (or lines) in the immediate stimulus environment. For horizontal and vertical lines, appropriate reference lines are usually to be found in the borders of the card displaying the stimulus lines, as well as in other rectangular features of the environment, but appropriate oblique lines are seldom to be found. However, Bryant showed that young children cl~tz discriminate obliques if matching obliques are made available. either by surrounding the standard and the test displays with diamond-shaped borders (Bryant. 1969) or simply by including a line which matches the standard along with both the standard and the test pair (Bryant. 1974). It is perhaps unlikely that a match-mismatch strategy played a very significant role in our experiment. For one thing. we attempted to minimize the role of borders by using circular cards. although it is still possible that the edges of the placing tray itself or of the table served as matching reference lines for the horizontal and vertical stimuli. There is no ready explanation in terms of matching lines for the difference in difficulty among the three conditions involving obliques (i.e., O/OS. OiOm, and OiOd). Moreover, Fellows and Brooks (1973) failed to replicate Bryant’s (1969) results with diamond-shaped borders, reporting that children were unable to discriminate mirror-image obliques but easily able to discriminate horizontal from vertical regardless of whether the borders were rectangular or diamond-shaped. Perhaps the crucial difference was that Bryant gave his subjects special instructions designed to draw their attention to the borders. whereas Fellows and Brooks did not.
522
CORBALLIS
AND
ZALIK
With respect to the role of left-right coding, it is also important to note that Bryant’s experiments with matching lines beg the critical question. In his earlier experiment with the diamond-shaped border, the two top edges of the diamond were colored differently. one red. one blue, so that the child simply had to code the standard according to whether it was parallel to a red or a blue line. The left-right distinction was thus transformed into an easily encoded color discrimination. Similarly. in the later experiment, the presence of a single matching oblique line provided a consistent left-right cue. Thus neither of Bryant’s experimental paradigms tested the children’s ability to tell left from right (cf. Corballis & Beale, 1970, 1976), which could at least partly explain why the children readily discriminated the mirror-image lines under these conditions. (Fellows and Brooks, 1973. also included different colored borders but in their experiment this did not improve discrimination, probably again because the experimenters did not draw attention to the relevance of the borders). We do not wish to claim that Bryant’s “match-mismatch” hypothesis is wrong, but simply that it has limited generality. No doubt constant environmental cues do serve as mnemonic aids when they are available and when attention is drawn to them. However. the problem of left-right discrimination is really only of interest when no such cues are available: Corballis and Beale (1970. 1976) have pointed out that it is precisely because left and right are fundamentally egocentric concepts that it is difficult to tell them apart, and to provide a constant external left-right cue is to beg the question of whether or not the left-right distinction can be internally coded in memory. Bryant’s experiments therefore do not even address themselves to the question of whether left-right confusion contributes to the inability of children to discriminate mirror-image obliques. The present experiment suggests that left-right confusion may indeed play a role, especially in situations where appropriate matching stimuli are unavailable or unnoticed. It would perhaps be surprising if this were not so. since children have as much difficulty discriminating left-right mirror-image U-shapes, which contain no obliques at all. as they do in discriminating mirror-image obliques. and the developmental trends in the ability to learn the two discriminations are essentially parallel (Rude1 Rc Teuber, 1963: Serpell, 1971). For all it:, apparent simplicity, the oblique line poses problems which are just that. oblique. REFERENCES
MIRROR-IMAGE
CONFUSIONS
Bryant,
523
P. E. Perception and understanding irr >loung children. New York: Basic Books, 1974. Corballis, M. C. The left-right problem in psychology. Canadian Psychologisf. 1974. 15, 16-33. Corballis, M. C., & Beale. 1. L. Bilateral symmetry and behavior. Psychologicul Re\~ie~r. 1970, 77, 451-464. Corballis, M. C.. & Beale. I. L. The psychology oj‘ /e.fi anti righr. Hillsdale, N. J.: Lawrence Erlbaum Associates, 1976. Fellows, B. J., & Brooks. B. An investigation of the role of matching and mismatching frameworks upon the discrimination of differently oriented line stimuli in young children. Journal of Child Psychology & Psych&y, 1973, 14, 293-299. Over, R.. & Over. J. Detection and recognition of mirror-image obliques by young children. Journal qf Comparutire & Physiologicd Psychology, 1967a, 64, 467-470. Over, R.. & Over, J. Kinesthetic judgments of direction of line in children. Quarter!\ Journal qf’E.uperirnental Psychology, 1967b. 19, 337-340. Rudel. R. G., & Teuber, H.-L. Discrimination of direction of line in children. J<~tnlcr/ I?/ Comparutive & Physiological Psychology, 1963, 56, 892-898. Serpell. R. Discrimination of orientation by Zambian children. Jortrnd of Compurarive & Ph.vsiologicu/ Psychology. 1971, 75, 312-316. Siegel, S. Nonparametric sfatisrics. New York: McGraw-Hill, 1956. Tee. K. S.. & Riesen, A. H. Visual left-right confusions in animal and man. In G. Newton & A. H. Riesen (Eds.). Advances in P.s,vchobiology. New York: Wiley, 1974. Vol. 3. RECEIVED:
September
17. 1976: REVISED:
December
3. 1976.
OF
EXPERIMENTAL
CHILD
Why Do Children MICHAEL
PSYCHOLOGY
Confuse
C. CORBALLIS McGill
24, 5166513
(1977)
Mirror-image AND MARSHA
Obliques? C.ZALIK
iJnit,rrL
Thirty-five children aged from 4.17 to 6.58 years were given a delayed-matching task in which they had to choose on each trial which of two lines was the same as a previously displayed standard line. Their choices were no better than random when the lines differed in degree of slope but not in left-right orientation and were only marginally more accurate when the lines were left-right mirror images. Performance improved significantly when the lines differed both in degree of slope from the vertical and in left-right orientation and improved still further if at least one of the lines was horizontal or vertical. The results suggest that young children have extreme difficulty encoding in memory either the degree of slope or the left-right orientation of an oblique line.
It is well established that children find it difficult to discriminate mirrorimage oblique lines. There is some question, however, as to whether the difficulty arises because the stimuli are oblique, or because they are mirror images. On the one hand, Appelle (1972) has reviewed a large body of evidence which shows that many species, including humans, perform more poorly on a variety of tasks when the critical stimuli are oblique lines or gratings than when they are horizontal or vertical. Appelle dubbed this the “oblique effect.” On the other hand, there is an equally impressive although partially overlapping accumulation of evidence, reviewed by Corballis and Beale (19761, that most species, including human children, suffer special difficulty in discriminating left-right mirror images. The difficulty includes, but is not restricted to, mirror-image obliques. Corballis and Beale (1970, 1976) argue that the problem is fundamentally one of telling left from right. One thing that is clear is that the difficulty of discriminating mirrorimage obliques is a problem of memory rather than of perception. For instance, children have no special difficulty on a simultaneous-matching task in which they must decide which of two mirror-images obliques is the same as a standard oblique, simultaneously displayed. However, 5year-olds perform at no better than chance on delayed matching, in which the choice mirror-image pair is presented 5 set after the standard (Bryant, This research was supported by a research grant from the National Canada. The author’s address is Department of Psychology. McGill Gregor Avenue, Montreal, Quebec, Canada H3A IBI.
Research University.
Council of I705 MC
MIRROR-IMAGECONFUSIONS
517
1969, 1973). Because the difficulty seems to occur only when the child has to remember the oblique orientation (see also Over & Over 1967a,b; Tee & Riesen, 1974), one might suppose that it has more to do with leftright confusion than with the oblique effect. Left-right confusion is almost certainly a problem of remembering rather than of perceiving which way round things are (Corballis, 1974; Corballis & Beale, 1976). whereas much of the evidence for the oblique effect comes from experiments on perception or detection of oblique lines (Appelle, 1972). Yet Bryant (1974) is adamant that the problem is related to obliqueness rather than to left-right confusion; he writes (p. 69) that it is “virtually certain that the mirrorimage analysis is quite wrong, at least as far as young children are concerned. Obliques are difficult because they are obliques.” Bryant’s reasoning is based on the results of two experiments (Bryant. 1969, 1973), in which he found that children were still unable to discriminate between obliques in a delayed-matching task when the choice stimuli were not exact mirror images. In both experiments, however. the stimuli were oriented to either side of the vertical but differed in degree of tilt away from the vertical. The stimuli therefore differed both with respect to left-right orientation and with respect to the degree of obliqueness. Bryant may have been correct in asserting that the difficulty was not one of mirror-image discrimination per se, but it may nevertheless be at least partly a problem of telling left from right. Children may be unable to encode in memory either the left-right orientation or the degree of obliqueness of an oblique line. The present experiment was designed to separate these components. Children were tested on a delayed-matching task, in which they were required to discriminate obliques which differed in degree of tilt but not in left-right orientation, obliques which differed in left-right orientation but not in degree of tilt (i.e., left-right mirror images), and obliques which differed both in degree of tilt and in left-right orientation. In addition, there were control conditions in which one or both of the choice stimuli was either a horizontal or a vertical line. METHODS
Thirty-five preschool children, 19 boys and 16 girls, participated in the experiment. Their ages ranged from 4.17 to 6.58 years with a mean of 5.59 years.
The test stimuli consisted of circular white cards, 12.7 cm in diameter, each with a black line 7.6 cm long and 1.27 cm wide through its
518
CORBALLIS
AND
ZALIK
center. The line was oriented at either 0. 30, 60, or 90” from the vertical, either clockwise or counterclockwise. An additional set of cards with simple colored drawings in different orientations served as a training set. The cards could be inserted in a square placing tray, 71 by 71 cm. The tray consisted of two black cardboard squares taped back to back. The top square had three holes cut out of it. each the same size as the stimulus cards. The center of the top hole was 17.8 cm from the top of the tray and was centered horizontally. This hole was used for placement of the standard stimulus card. The choice cards were placed in two lower holes, with centers 20.3 cm apart and 15.2 cm below the center of the standard. The cards could be placed in the holes and held in fixed orientations by means of tabs, which were invisible to the subject.
The subjects were tested individually. Each child sat in a small chair at a table, facing the experimenter. The cards were in a prearranged order on the experimenter’s lap, out of sight of the subject. The experimenter explained to the child that she was going to place a “picture” in the top hole and then take it away and hide it. She was then going to place two pictures in the bottom holes, and the child was to point to the picture that was the same as the one the experimenter was hiding. A training card was then presented for 5 set and then removed. Immediately after removal of this standard card, the two choice cards were presented. One was identical to the standard, the other differed only in orientation. Subjects were given feedback to help them master the task, and when an error occurred the standard was put back to help the child correct his/her error. This training procedure was continued until each child achieved a training criterion offour consecutive correct choices. One child failed to reach this criterion and was rejected. After the training procedure, the experimenter explained that the cards would now not have pictures on them. but would have lines instead. Otherwise the “game” was the same. Each standard stimulus card was presented for 5 set followed by a 5set interval. and then the two choice cards were presented. One of the choice cards was identical to the standard. the other differed only in orientation of the line. During the test trials. the children were given no feedback. The children were randomly assigned to two groups. For Group A. the standards consisted of a vertical line. a line 30” to the left of vertical. and a line 60” to the right of vertical. For Group B, they consisted of a horizontal line, a line 60” to the left of vertical. and a line 30” to the right of vertical. There were 18 subjects in Group A and 17 in Group B. For the choice pairs. each standard was coupled with each of the other five line
MIRROR-IMAGE
CONFUSlONS
519
orientations twice, once with the standard on the left and once with the standard on the right. Thus, each subject completed a total of 30 test trials. 10 with each standard. The trials were randomly ordered. RESULTS For the purpose of analysis the trials were divided into five categories: those involving a choice between obliques with different slopes but with the same left-right orientation (O/OS), those involving a choice between mirror-image obliques (O/Om), those involving a choice between obliques oriented in opposite directions from the vertical but differing in degree of slope (OiOd), those involving a choice between an oblique and either a vertical or a horizontal line (OIVH). and those involving the choice between vertical and horizontal (V/H). Table I shows the mean percentage of errors in each category. Since each subject had only two V/H trials and only six of them made an error (one each), these scores were not included in the statistical comparisons. but it is clear that performance was well above chance. For the remaining categories, Group A was first compared with Group B by means of the Wilcoxon Rank Sum Test.’ Although Group B made a higher percentage of errors than Group A in all discrimination categories, the differences were nowhere significant and the groups were combined for all subsequent analyses.
A Friedman two-way analysis of variance by ranks among the four conditions involving obliques (O/OS, O/Om, OiOd, and O/VH) revealed a significant main effect xr2 = 395.2, df = 3. p < .OOl . Separate Wilcoxon signed-rank tests revealed that conditions O/OS and OiOm did not differ significantly from each other, but that all of the remaining comparisons were significant at the .Ol level or better. More specifically, the OiOd condition produced significantly fewer errors than either the O/OS or OiOm conditions, and the ONH condition produced significantly fewer errors than the OiOd condition.
Performance under the same four conditions was also compared with chance performance (i.e.. 50% correct). again by means of the Wilcoxon signed-rank test. Performance under the O/OS condition did not deviate significantly from chance. Performance under the OiOm was marginally better than chance (p < .05. one-tailed).
Performance
under the OiOd and
OiVH conditions were clearly above chance (p < .Ol. in both cases). Condition O/OS involved degree but not direction of slope. condition ’ Each child received only four trials under each of the oblique conditions (O/OS. OiOm. and OiOd), SOthe data were effectively scored on a T-point scale (O-3). To avoid the potential consequences of violating normality assumptions. we chose to use rank tests rather than analysis of variance to test hypotheses ahout the data. The los< in power in xo choosing is minimal (Siegel, 19%).
520
COKBALLIS
ANU ZALIK
TABLE I MEAN
PHKENTAGE
OF ERRORS (CHANCY. = 50%) FOR EACH OF DISCRIMINATION. IN EACH GROUP
CATEGORY
Category OIOS
OiOm
O/Od
OIVH
V/H
Group B
43.06 51.47
38.89 47.06
x!.21 30.88
8.71 16.21
5.56 11.76
Total
47.14
42.86
26.43
12.35
8.57
Group
A
OiOm involved direction but not degree, while condition OiOd involved both cues. It is of interest therefore to inquire whether performance on O/Od can be predicted from performance on O/OS and OiOm alone. Using a standard correction for guessing. we computed from the mean proportion of errors that the probability that the subjects made use of degree of slope as a cue in O/OS was .0573, while the probability that they made use of left-right orientation in O/Om was .1428. One would predict therefore that the probability of effectively using either degree or direction, or both. in O/Od would be [I - (1 - .0573) ( I - .1428)], which is .1918. Assuming that those who could use neither cue were merely guessing, this represents an error rate of 40.50%. which is clearly above the error rate of 26.43% actually obtained. In short, performance on O/Od is better than one would predict from performance on O/OS and OiOm. DISCUSSION The simplest interpretation of our results is that young children have extreme difficulty encoding either the degree of slope or the left-right orientation of an oblique line. Thus discrimination proved virtually impossible when the choice lines differed in degree of slope but not in leftright orientation (condition O/OS), and just about as difficult when they differed in left-right orientation but not in degree of slope (condition O/Om). Contrary to Bryant’s (1969, 1973) results. discrimination improved significantly when the choice lines differed both in the degree and in the direction of slope (condition OiOd). It is not immediately apparent why Bryant obtained a different result, particularly since he presented more extreme differences in slope than we did. One possibility is that his subjects may have been more likely than ours were to adopt a strategy of matching the lines to horizontal and vertical lines in the immediate environment and were thus less sensitive to variations among different oblique lines. We shall discuss this “match-mismatch” strategy more fully below. We saw that performance in the presence of both discriminative cues was actually better than one would expect on the basis of performance in
MIRROR-IMAGE
CONFUSIONS
521
the presence of either cue alone. A possible explanation for this is that the relative difficulty among the three conditions may have been influenced to some extent by variations in the angle between the two choice lines. Thus the lines differing only in degree of slope (condition O/OS) differed from each other by only 30”. while those differing in both degree and direction (condition OiOd) were always 90” apart. We might note that the subjects had no difficulty discriminating lines 30” apart when one of them was either horizontal or vertical (mean percent errors = 9.2), although evidence on the judgment of stimulus orientation (see Appelle, 1971, for review) might lead one to suspect that angular disparity would be more critical when both lines are oblique. We doubt that angular disparity played a significant role in the difference between O/Om and OiOd. Under the mirror-image condition (OiOm) the angle between the lines, if extended to meet, was either 60” or IZO”, and performance was essentially equal in the two cases. One could say, however, that the mirror-image lines at 60”from the vertical differ by only 60”, not 120”. Further research is therefore needed to clarify the role of angular separation in discrimination of the various oblique pairs. Bryant (1969, 1974) has argued that discrimination between lines in the delayed matching task depends on a “match-mismatch” code: Children encode the orientation of a line with reference to some parallel line (or lines) in the immediate stimulus environment. For horizontal and vertical lines, appropriate reference lines are usually to be found in the borders of the card displaying the stimulus lines, as well as in other rectangular features of the environment, but appropriate oblique lines are seldom to be found. However, Bryant showed that young children cl~tz discriminate obliques if matching obliques are made available. either by surrounding the standard and the test displays with diamond-shaped borders (Bryant. 1969) or simply by including a line which matches the standard along with both the standard and the test pair (Bryant. 1974). It is perhaps unlikely that a match-mismatch strategy played a very significant role in our experiment. For one thing. we attempted to minimize the role of borders by using circular cards. although it is still possible that the edges of the placing tray itself or of the table served as matching reference lines for the horizontal and vertical stimuli. There is no ready explanation in terms of matching lines for the difference in difficulty among the three conditions involving obliques (i.e., O/OS. OiOm, and OiOd). Moreover, Fellows and Brooks (1973) failed to replicate Bryant’s (1969) results with diamond-shaped borders, reporting that children were unable to discriminate mirror-image obliques but easily able to discriminate horizontal from vertical regardless of whether the borders were rectangular or diamond-shaped. Perhaps the crucial difference was that Bryant gave his subjects special instructions designed to draw their attention to the borders. whereas Fellows and Brooks did not.
522
CORBALLIS
AND
ZALIK
With respect to the role of left-right coding, it is also important to note that Bryant’s experiments with matching lines beg the critical question. In his earlier experiment with the diamond-shaped border, the two top edges of the diamond were colored differently. one red. one blue, so that the child simply had to code the standard according to whether it was parallel to a red or a blue line. The left-right distinction was thus transformed into an easily encoded color discrimination. Similarly. in the later experiment, the presence of a single matching oblique line provided a consistent left-right cue. Thus neither of Bryant’s experimental paradigms tested the children’s ability to tell left from right (cf. Corballis & Beale, 1970, 1976), which could at least partly explain why the children readily discriminated the mirror-image lines under these conditions. (Fellows and Brooks, 1973. also included different colored borders but in their experiment this did not improve discrimination, probably again because the experimenters did not draw attention to the relevance of the borders). We do not wish to claim that Bryant’s “match-mismatch” hypothesis is wrong, but simply that it has limited generality. No doubt constant environmental cues do serve as mnemonic aids when they are available and when attention is drawn to them. However. the problem of left-right discrimination is really only of interest when no such cues are available: Corballis and Beale (1970. 1976) have pointed out that it is precisely because left and right are fundamentally egocentric concepts that it is difficult to tell them apart, and to provide a constant external left-right cue is to beg the question of whether or not the left-right distinction can be internally coded in memory. Bryant’s experiments therefore do not even address themselves to the question of whether left-right confusion contributes to the inability of children to discriminate mirror-image obliques. The present experiment suggests that left-right confusion may indeed play a role, especially in situations where appropriate matching stimuli are unavailable or unnoticed. It would perhaps be surprising if this were not so. since children have as much difficulty discriminating left-right mirror-image U-shapes, which contain no obliques at all. as they do in discriminating mirror-image obliques. and the developmental trends in the ability to learn the two discriminations are essentially parallel (Rude1 Rc Teuber, 1963: Serpell, 1971). For all it:, apparent simplicity, the oblique line poses problems which are just that. oblique. REFERENCES
MIRROR-IMAGE
CONFUSIONS
Bryant,
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P. E. Perception and understanding irr >loung children. New York: Basic Books, 1974. Corballis, M. C. The left-right problem in psychology. Canadian Psychologisf. 1974. 15, 16-33. Corballis, M. C., & Beale. 1. L. Bilateral symmetry and behavior. Psychologicul Re\~ie~r. 1970, 77, 451-464. Corballis, M. C.. & Beale. I. L. The psychology oj‘ /e.fi anti righr. Hillsdale, N. J.: Lawrence Erlbaum Associates, 1976. Fellows, B. J., & Brooks. B. An investigation of the role of matching and mismatching frameworks upon the discrimination of differently oriented line stimuli in young children. Journal of Child Psychology & Psych&y, 1973, 14, 293-299. Over, R.. & Over. J. Detection and recognition of mirror-image obliques by young children. Journal qf Comparutire & Physiologicd Psychology, 1967a, 64, 467-470. Over, R.. & Over, J. Kinesthetic judgments of direction of line in children. Quarter!\ Journal qf’E.uperirnental Psychology, 1967b. 19, 337-340. Rudel. R. G., & Teuber, H.-L. Discrimination of direction of line in children. J<~tnlcr/ I?/ Comparutive & Physiological Psychology, 1963, 56, 892-898. Serpell. R. Discrimination of orientation by Zambian children. Jortrnd of Compurarive & Ph.vsiologicu/ Psychology. 1971, 75, 312-316. Siegel, S. Nonparametric sfatisrics. New York: McGraw-Hill, 1956. Tee. K. S.. & Riesen, A. H. Visual left-right confusions in animal and man. In G. Newton & A. H. Riesen (Eds.). Advances in P.s,vchobiology. New York: Wiley, 1974. Vol. 3. RECEIVED:
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3. 1976.