- Email: [email protected]
Hemispheric differences in the discrimination of curvature
Neuropsychologia,
1976,
Vol. 14, pp.
195-202.
Pergamon
Press. Printed
in England.
HEMISPHERIC DIFFERENCES IN THE DISCRIMINATION OF CURVATURE K. LONGDEN, C. ELLIS and SUSAN D. IVERSEN Department
of Experimental
Psychology,
Downing Street, Cambridge,
England
(Received 2O_May 1975) Abstract-Pairs of curves were presented to left or right of the Fixation point and Ss were asked to indicate as quickly as possible whether the curves in a pair were the same or different. In Experiment I, reaction times (RT) for “different” and “same” responses were similar, although there was a tendency to a left field advantage this did not reach significance. In Experiment II, separate groups of subjects were used for the “different” and “same” judgements. In this case a significant LVF advantage was obtained with the “different” judgements. RT’s were also related to the difficulty of the discrimination.
INTRODUCTION OVER THE last 10 yr a great deal of information on localization of function between the hemispheres has been obtained by the study of human patients with discrete brain damage. MILNER [l] studied large series of patients who have undergone surgery of the temporal lobe for the relief of epilepsy. Psychological testing of these patients has provided the largest single body of evidence that the left and right hemispheres are specialized for different aspects of perceptual analysis and for the higher cognitive functions related to those kinds of information. It is also possible to investigate lateralization of function in normal subjects by comparing the speed of handling of certain kinds of information by the two hemispheres. In the visual mode, information can be preferentially passed to one or other hemisphere by presenting the stimuli to the right or left visual field. In the auditory mode the dominant crossed auditory projection to the cortex achieves a similar dissociation with the right ear projecting preferentially to the left hemisphere and the left ear to the right hemisphere. Perceptual differences between the two hemispheres are deduced on the basis of the accuracy or speed of discrimination under the two input conditions. Combining these two sources of information, (reviewed in [I] and [2]), it is known that the left hemisphere is faster at discriminating letters [3] and verbal features in sounds [4, 51 whereas the right hemisphere is superior with nonsense figures, faces, spatial discriminations, unfamiliar patterns of sound and shapes presented tactually. This pattern of results encourages the generalization that in right-handed subjects the left hemisphere is principally concerned with verbal function, whereas the right hemisphere is concerned with non-verbal processing. However, surprising results sometimes emerge which indicate that our definition of “verbalness” is imprecise in many senses. For example, UMILTA et al. [6] find that the left hemisphere is faster at discriminating rectangles orientated in the vertical, horizontal and oblique planes, (90”, 0” or 45”). However, the right hemisphere gains superiority when orientations of 30” and 45” from the vertical or 15”, 30”, 45”, and 60” from the vertical have to be compared. The authors suggest that horizontal, vertical and oblique are more easily verbally coded than 195
196
K. LONGLNN, C. ELI.IS and SUSAN D. IVERSEN
the intermediate oblique lines. In seeking further evidence of right hemisphere superiority in non-verbal perceptual processing one is clearly hampered in that the criterion of verbalness is not an easy one to apply. An alternative approach is to consider the evidence from electrophysiological recording experiments in subhuman species and psychophysical experiments in man, which indicate that neurons in the sensory cortices are tuned to analyze particular features of a sensory signal. From such evidence it is proposed that in the visual mode colour, line orientation, spatial frequency, depth, contrast and motion are coded by specialized detectors. We have been interested in curvature as another potential visual dimension and there is limited psychophysical and neurophysiological evidence to support this suggestion. Curvature shows classical after effect phenomena [7]. More recently, RIGGS [8] has described the MCCOLLOUGH [9] effect on coloured curved lines, suggesting that curvature and colour may be coded by the same neurons just as line orientation and colour are thought to be. Furthermore, in their unit recording experiment on the cat HUBEL and WIESEL [IO] make guarded reference to certain responses of hypercomplex cells which “can, in a sense, serve to measure curvature; the smaller the activating part of the field, the smaller the optimal radius of curvature would be”. Such cells receive their input from line orientation detectors and recent psychophysical work of BLAKEMOREand OVER [ll] strongly supports the view that curvature perception is mediated by such detectors rather than by specific curvature units. We have therefore, compared the speed of discrimination of pairs of curved lines presented to the right or left visual field of normal human subjects. As an additional feature of the experiment we also compared “same” and “different” judgements of the stimuli. EGETH and EPSTEIN 1121reported that the nature of the judgement seemed a more important determinant of hemisphere superiority than the basic verbal/non-verbal nature of the material. When asked to make a “different” judgement of pairs of letters, subjects showed a LVF advantage but a RVF when “same” judgements were made. However, this matter remains unresolved. ATKINSON and EGETH [13] found a LVF advantage for both “same” and “different” judgements when line orientation was judged.
EXPERIMENT
1
Method Subjects nrzd design. Ten right handed undergraduates served as volunteers. For all the Ss the task was a one response go-no-go matching task. Pilot testing indicated that the task was fairly difficult, therefore, a 94 o/oaccuracy criterion was adopted specifying that no more than 2 errors of omission (including reaction times above 2 set) or commission (false alarms) should be made in any test batch of 32 stimuli. Stimuli. Thirty-two cards were prepared for use in a tachistoscope. Each card had a pair of curves drawn on it, one directly above the other; to the right or left of the fixation point (Fig. la). Four curves and a straight line were used as stimuli. The height of the curves and the arc length varied but the chord length was a constant 1 in. The 4 curves selected subtended angles of 45”, 90”, 135” and 180 at their centres (Fig. lb). The stimuli appeared as black curves vertically mounted and separated by 2.25” on a white background. This distance was compounded of 1.5” from the bottom of the lower curve to the midpoint between the curves and 0.75” from the lowest point of the upper curve to the mid-point. This arrangement gave the best symmetry, as judged by visual inspection, within the physical parameters of the cards. Projection distance and viewing distance were such that each curve’s chord length subtended 3”. The horizontal distance between the fixation point and the closest edge of the curve w& also 3” (since curves were drawn from their mid-points 4.5” from the fixation point). The 32 cards represented a balanced series of the angles used, position (left or right) and vertica!ity (higher or lower).
HEMISPHERICDIFFERENCES IN THE DISCRIMINATIONOF CURVATURE
PO‘
4 5
197
”
FIG. 1 (a). Upper. Illustration of the layout of the curved lines. This would be a “same” judgement on the 90” curve. (b) Lower. The four curved lines which together with a straight line were used for the “same”, “different” curvature judgements. The chord lengths were constant and the arc lengths and the radii varied.
Procedure. On each trial one card (i.e. one pair of curves) was presented tachistoscopically for 150 msec. Six test batches of the 32 cards were made up, using each of three randomized series twice. Ss responded to either “same” or “different” on alternative test batches of 32 stimuli. The ordering of the test batches was randomized between Ss. Ss were instructed to respond by depressing a key with a finger of their right hand. Under conditions of instruction to respond when the curves were the same, Ss depressed the key when they judged the curves were the same, but refrained from responding when the curves were judged to be different. The opposite requirements operated when the instructions were to respond for different. A ready signal from the experimenter about 2-3 set before each trial reminded Ss to fixate a dot at the centre of the screen. On each trial the reaction time was measured with an electronic timer from the onset of the stimulus to the initiation of the key press. There were 32 practice stimuli, followed by the 6 test batches.
Table 1. Median reaction times in msec “5ame”
iudserrents
'DifferentHiuduements
RWF
LVF
(R-L)
RVF
LVF
(S-L)
KL
270
566
+?J
532
570
-38
CrnL
I!?2
jot;
l
575
523
l56
AT
h"?
558
.'i
755
652
.103
JJ
55d
534
l24
529
529
0
RP
55Y
566
-7
578
559
r19
JriN
U‘4S
432
*lb
421
433
-12
CT
562
551
lll
565
533
-19
rlH
509
hY4
l15
436
471
-35
DL
671
'192
+29
S9S
539
*lo
MR
515
408
.27
539
507
l32
%
K. LONODEN,C. ELLIS and SUSAND. IVERSEN
198
Results
The median reaction times and the differences between them (compared with RVF latencies-LVF latencies) are presented in Table 1. An analysis of variance of the median RTs yielded the F values shown in Table 2. No influence of field upon RT could be demonstrated. The raw data were examined in some detail and it was found that particular cards, particular angles or particular batches of stimuli had not had any significant effect upon the results.
Table 2. Analysis of variance SOllrCP
df
EL5
S (subjects)
9
13,308.Z
C (condition-some/ differrnt)
1
354.0
CSS
3
27b5.0
1
1575.0
9
425.7
C*k
1
7.2
c s I<.* s
9
F (field
YXb
Total
- RVF/LW)
l?
Sic;E.
0.129
NS
3.700
I\Ts
0.012
NS
584.8
39
Discussion
Despite the failure to find a significant field effect, 9/10 subjects showed a LVF advantage for the “same” judgement. It was, therefore, decided to repeat the experiment using the identical stimuli but different testing procedures.
EXPERIMENT
II
Method Subjects and design. Nineteen right handed undergraduates who had not performed in Experiment I served as subjects. The general design testing conditions and stimuli were the same as Experiment I with the following modifications: 1. It was considered possible that the requirements to oscillate between “same” and “different” judgements in Experiment I could have interfered with stable performance. Groups were not informed of the other testing condition to exclude the possibility that the experiment “primed” responses to one of the conditions preferentially. Separate groups of subjects were tested on the “same” and “different” discriminations. 2. The rejection criterion was made more stringent; Ss who had difficulty reaching or maintaining the accuracy criterion of 94% were rejected; so were Ss who had reaction times consistently longer than 650 msec. 3. Individual long reaction times (l-2 set) to a trial were not accepted. Cards producing such long RTs were reintroduced at the end of the test batch as long as the total number of errors plus long RTs did not exceed a 90% criterion (i.e. 3 cards in a test batch of 32). 4. A shorter testing time was introduced, to avoid any variability produced by fatigue. This condition was satisfied since only 3 test batches needed to be used per subject in Experiment II as Ss were now tested in only one condition (“same” or “different”).
HEMISPHERIC
DIFFERENCES
IN
THE
DISCRIMINATION
OF
CURVATURE
199
5. Two response keys, one in each hand, were used in an effort to exclude any bias due to output processing. Ss were instructed to press the keys simultaneously and the faster response cut off the digital counter used to measure RTs.
Results The median reaction times for individual subjects are presented in Table 3. There is a LVF advantage of 9.2 msec for “same” and 14.7 msec for “different” judgements overall. The group performing “same” judgements did not show a significant field effect. But, those performing “different” judgements showed a significant LVF advantage (t = 3.6, df = 9, P < O-01). Table 3. The Median reaction times for individual subjects Median
Reaction
%.Wlle~ iudwments
Times "DifferentH
RWF
LWf
(R-L)
M.4
547
533
t14
KS
537
509
TH
484
JB
524
RR
iudqements
RVF
LVF
(R-L)
DC
555
528
+30
+20
Jb4
489
484
+5
525
-41
AN
510
500
+lO
543
-19
JE
522
500
+22
506
475
+31
PM
499
461
l30
SN
619
592
+27
TB
514
432
+22
BP
596
564
l32
JC
596
590
+6
LP
521
532
-11
RS
540
526
+I4
JP
525
503
+22
EJ
602
607
-5
AB
576
571
+5
Further analysis on a Friedman two-way analysis of variance revealed that on the “different” judgements the difficulty of the discrimination signScantly influenced RTs, (P = O-042 for X2r = 6 when K = 4, N = 2). This was not the case for “same” judgements. However, RTs for judging difference of curvature did not decrease as the difference in arc lengths or heights of the curve pairs increased, as one might expect on a simple matchmismatch hypothesis. Thus some other parameter of curvature is likely to be used in detecting differences. Discussion A left visual field superiority in reaction time was found with respect to “different” judgements. There was no significant effect of field of presentation on “same” judgements, although 6 out of 9 subjects again produced faster reaction times for curves presented in the left visual field. These results are somewhat surprising in view of the results of the first experiment where, although there were no significant differences, “same” judgements produced more consistent left visual field advantages. It is possible that this is due to the operation of unwanted range effects inherent in the within-subject design of the first experiment. Thus subjects
200
K. LONGDEN. C. ELLIS
and
SUSAN D.
IVERSEN
performed batches of “press for same” alternatively with batches of “press for different” on the go-no-go paradigm, and there was a total of 6 batches in all. Subjects seemed to find no difficulty in switching from one strategy to another and examination of the results showed no consistent trends with respect to which condition came first. However, POULTON [14] has shown that even when each subject receives a number of experimental conditions in random order, unwanted range effects can revise the true rank order of the conditions, and hence give rise to erroneous results. In the second experiment, not only did each subject perform only one of the conditions but he was also ignorant of the existence of the alternative condition and received no practice in it. It is possible that subjects in the first experiment developed a set for the “press for same” condition despite equal preliminary training on both conditions. Indeed, many people seemed to find the “press for same” condition easier though this was not always borne out with respect to speed in the experimental results. Thus whereas previous experiments have found that “same” judgements are performed consistently faster this was not the case with several of our subjects. A second difference between the two experiments was in the mode of responding. In the first experiment, subjects held a single button between finger and thumb of the right hand. Because of contralateral control of distal musculature this biases against any right hemisphere advantage in perceptual processing. In the second experiment subjects held two buttons, one in either hand, and were instructed to respond with both. It was felt that this would equalize any laterality differences in the speed of producing a response enabling us to ascribe any laterality differences found to differences in perceptual processing. Without, however, analysis of hand-field combinations it is not possible to distinguish between dominance and asymmetry and the laterality of differences due to interhemispheric crossing times. Furthermore, it is possible that testing in the first experiment was too long for a reaction time study. Despite several breaks after batches of stimuli, each subject was presented with 192 cards plus the training required to reach an accurate level of performance. In the second experiment each subject received only half that number and did not have the added problem of dealing with two conditions. It was essential that subjects were accurate but to avoid any speed-accuracy trade ofl’, subjects were also encouraged to react as quickly as possible within those limits. Analysis of the errors indicated that there had been no substantial trade-off since there were no more errors in one half of the visual field than the other. There was, however, a wide variation in mean reaction time between subjects irrespective of condition. This may be indicative of the operation of different mechanisms in different subjects. Indeed, in the second experiment we rejected any subject who consistently reacted with a speed of over 650 msec, since this was well outside the “normal” range. The analysis of variance of the first experiment indicated that inter-subject variance was large enough to obscure any individual’s laterality differences, and possibly any “population” laterality effect with respect to a single mechanism of processing. Population trends in cerebral asymmetry appear to be very fragile, and it may be better to investigate individuals intensively and test the consistency of their results rather than trying to form “across-population” conclusions. There is some evidence that laterality differences become less marked with practice, and if this were so the conclusions based upon limited testing of many subjects may well be misleading. Since little is known of the mechanisms of perception of curvature with respect to “same” and “different” judgements, it is difficult to make firm prior hypotheses as to
201
HEMISPHERIC DIFFERENCES IN THE DISCRIMINATION OF CURVATURE
expected laterality effects. Previous experiments have suggested that “same” judgements mediated preferentially by the left hemisphere, may be carried out by “global gestalt” processes while “different” judgements are more analytic in nature and depend on the right hemisphere. In our experiments there was a tendency to left field advantage for both kinds of judgemerits. But the fact that the “different” judgement in Experiment II gave the only significant effect would support a special role for the right hemisphere in such judgements.
~4cknowledgenzerrts-We thank Prof. 0. L. ZANGWILL helpful comments on the manuscript.
for support of our work and Dr. G. BERLUCCHIfor
REFERENCES 1. MILNER,B. Interhemispheric Bull. 27,272-271,
differences in the localization of psychological
processes in man. Br. rned
1971.
2. BLAKEMORE, C., IVERSEN,S. D. and ZANGWILL,0. L. Brain Functions.
A. Rev. Psychol. 23,413-456, 1972. 3. UMILTA,C., FROST, N. and HUMAN, R. Interhemispheric effects on choice reaction times to one- twoand three-letter displays. J. exp. Psychol. 93, 198-204, 1972. 4. STUDDERT-KENNEDY,M. and SHANKWEILER,D. Hemispheric specialization for speech perception. J. acoust. Sot. Am. 48, 579-594, 1970. 5. DARWIN, C. J. Ear differences in the recall of fricatives and vowels. Q. JI. exp. Psyclzol. 23,46-62, 1971. 6. UMILTA,C., RIZZOLATTI,G., MARZI, C. A., ZAMBONI,G., FRANZINI,C., CAMARDA,R. and BERLUCCIII, G. Hemispheric differences in the discrimination of line orientation. Neuropsychologia 12,165-174,1974. 7. COLTHEART,M. Visual feature-analyzers and after-effects of tilt and curvature. Psycho/. Rev. 78,114-121, 1971. 8. RIGGS, L. A. Curvature as a feature of pattern vision. Science 181, 1070-1072, 1973. 9. MCCOLLOUGH,C. Colour adaptation of edge detectors in the human visual system. Science 149,11151116, 1965. IO.HUBEL, D. H. and WIESEL,T. N. Receptive fields and functional architecture in two non-striate visual areas (18 and 19) of the cat. J. Neurophysiol. 28,229-289, 1965. Il. BLAKEMORE,C. and OVER, R. Curvature detectors in human vision. Perception 3, 3-7, 1974. 12. EGETH, H. and EPSTEIN,J. Differential specialization of the cerebral hemispheres for the perception of sameness and difference. Percept. Psychophys. 12, 218-220, 1972. 13. ATKINSON,J. and EGETH, H. Right hemisphere superiority in visual orientation matching. Can. J. Psychol.
27,152-158,
1973.
I4 POULTON,E. C. The new psychophysics:
six models for magnitude estimation.
Psychol. Bull. 69, 1-19,
1968.
ResumtS
:
paires
de courbes
On a pr&sentd
b droite
en demandant
que possible
si les courbes
rentes"..:ans
l'experience
ses
"differentes"
une
tendance
niveau separes
de
et
3 un avantage
de sujets
pour
ce cas, un avantage jugements
de la paire 1,
les
"identiques"
signification.
t@ de la discrimination.
temps
Les
gauche
similaires mais
blen
gauche
RT 6taier.t aussi
oc "diffeles repon-
cyu'il exista
qui n'attcignait
et
des
rapidement
(RT) pour
2, on a utilisf
"differents"
du champ
de fixation aussi
"identiquer"
de r@action
l'cxp,&rience
les jugements
du point
d'indiquer
Etaient
dtaient
du champ
Dans
siqnificatif
'differents".
et 2 qauche
aux sujets
pas
"identiaues".
Ftait
obtenu
en relation
Ic
des c[roupcs
avec
Dans les
a la difficul-
202
K. LONGDEN,C.ELLISand SUSAND. IVERSEN Deutschsprachige Zusammenfassung: Rechts und links vom Fixierpunkt w&den Kurvenpaare dargeboten. Versuchspersonen wurden gebeten, so schnell wie moglich anzugeben, welche Kurven eines Paares identisch oder diverent waren. In einem ersten Experiment waren die Reaktionszeiten (RI')fiir lfdifferentell und "identische" Antworten ahnlich, allerdings war eine gewisse Linksfeldiiberlegenheitzu erkennen, sie war aber nicht signifikant. In einem zweiten Experiment wurden getrennte Gruppen fiir die Beurteilung "different" und "identisch'lverwandt. In diesem Fall wurde eine signifikante tfberlegenheitfiir die linke Gesichtsfeldhalfte bei der Beurteilung lldifferentlf nachgewiesen. Die Reaktionszeiten entsprachen demnach dem Schwierigkeitsgrad der Unterscheidung.
1976,
Vol. 14, pp.
195-202.
Pergamon
Press. Printed
in England.
HEMISPHERIC DIFFERENCES IN THE DISCRIMINATION OF CURVATURE K. LONGDEN, C. ELLIS and SUSAN D. IVERSEN Department
of Experimental
Psychology,
Downing Street, Cambridge,
England
(Received 2O_May 1975) Abstract-Pairs of curves were presented to left or right of the Fixation point and Ss were asked to indicate as quickly as possible whether the curves in a pair were the same or different. In Experiment I, reaction times (RT) for “different” and “same” responses were similar, although there was a tendency to a left field advantage this did not reach significance. In Experiment II, separate groups of subjects were used for the “different” and “same” judgements. In this case a significant LVF advantage was obtained with the “different” judgements. RT’s were also related to the difficulty of the discrimination.
INTRODUCTION OVER THE last 10 yr a great deal of information on localization of function between the hemispheres has been obtained by the study of human patients with discrete brain damage. MILNER [l] studied large series of patients who have undergone surgery of the temporal lobe for the relief of epilepsy. Psychological testing of these patients has provided the largest single body of evidence that the left and right hemispheres are specialized for different aspects of perceptual analysis and for the higher cognitive functions related to those kinds of information. It is also possible to investigate lateralization of function in normal subjects by comparing the speed of handling of certain kinds of information by the two hemispheres. In the visual mode, information can be preferentially passed to one or other hemisphere by presenting the stimuli to the right or left visual field. In the auditory mode the dominant crossed auditory projection to the cortex achieves a similar dissociation with the right ear projecting preferentially to the left hemisphere and the left ear to the right hemisphere. Perceptual differences between the two hemispheres are deduced on the basis of the accuracy or speed of discrimination under the two input conditions. Combining these two sources of information, (reviewed in [I] and [2]), it is known that the left hemisphere is faster at discriminating letters [3] and verbal features in sounds [4, 51 whereas the right hemisphere is superior with nonsense figures, faces, spatial discriminations, unfamiliar patterns of sound and shapes presented tactually. This pattern of results encourages the generalization that in right-handed subjects the left hemisphere is principally concerned with verbal function, whereas the right hemisphere is concerned with non-verbal processing. However, surprising results sometimes emerge which indicate that our definition of “verbalness” is imprecise in many senses. For example, UMILTA et al. [6] find that the left hemisphere is faster at discriminating rectangles orientated in the vertical, horizontal and oblique planes, (90”, 0” or 45”). However, the right hemisphere gains superiority when orientations of 30” and 45” from the vertical or 15”, 30”, 45”, and 60” from the vertical have to be compared. The authors suggest that horizontal, vertical and oblique are more easily verbally coded than 195
196
K. LONGLNN, C. ELI.IS and SUSAN D. IVERSEN
the intermediate oblique lines. In seeking further evidence of right hemisphere superiority in non-verbal perceptual processing one is clearly hampered in that the criterion of verbalness is not an easy one to apply. An alternative approach is to consider the evidence from electrophysiological recording experiments in subhuman species and psychophysical experiments in man, which indicate that neurons in the sensory cortices are tuned to analyze particular features of a sensory signal. From such evidence it is proposed that in the visual mode colour, line orientation, spatial frequency, depth, contrast and motion are coded by specialized detectors. We have been interested in curvature as another potential visual dimension and there is limited psychophysical and neurophysiological evidence to support this suggestion. Curvature shows classical after effect phenomena [7]. More recently, RIGGS [8] has described the MCCOLLOUGH [9] effect on coloured curved lines, suggesting that curvature and colour may be coded by the same neurons just as line orientation and colour are thought to be. Furthermore, in their unit recording experiment on the cat HUBEL and WIESEL [IO] make guarded reference to certain responses of hypercomplex cells which “can, in a sense, serve to measure curvature; the smaller the activating part of the field, the smaller the optimal radius of curvature would be”. Such cells receive their input from line orientation detectors and recent psychophysical work of BLAKEMOREand OVER [ll] strongly supports the view that curvature perception is mediated by such detectors rather than by specific curvature units. We have therefore, compared the speed of discrimination of pairs of curved lines presented to the right or left visual field of normal human subjects. As an additional feature of the experiment we also compared “same” and “different” judgements of the stimuli. EGETH and EPSTEIN 1121reported that the nature of the judgement seemed a more important determinant of hemisphere superiority than the basic verbal/non-verbal nature of the material. When asked to make a “different” judgement of pairs of letters, subjects showed a LVF advantage but a RVF when “same” judgements were made. However, this matter remains unresolved. ATKINSON and EGETH [13] found a LVF advantage for both “same” and “different” judgements when line orientation was judged.
EXPERIMENT
1
Method Subjects nrzd design. Ten right handed undergraduates served as volunteers. For all the Ss the task was a one response go-no-go matching task. Pilot testing indicated that the task was fairly difficult, therefore, a 94 o/oaccuracy criterion was adopted specifying that no more than 2 errors of omission (including reaction times above 2 set) or commission (false alarms) should be made in any test batch of 32 stimuli. Stimuli. Thirty-two cards were prepared for use in a tachistoscope. Each card had a pair of curves drawn on it, one directly above the other; to the right or left of the fixation point (Fig. la). Four curves and a straight line were used as stimuli. The height of the curves and the arc length varied but the chord length was a constant 1 in. The 4 curves selected subtended angles of 45”, 90”, 135” and 180 at their centres (Fig. lb). The stimuli appeared as black curves vertically mounted and separated by 2.25” on a white background. This distance was compounded of 1.5” from the bottom of the lower curve to the midpoint between the curves and 0.75” from the lowest point of the upper curve to the mid-point. This arrangement gave the best symmetry, as judged by visual inspection, within the physical parameters of the cards. Projection distance and viewing distance were such that each curve’s chord length subtended 3”. The horizontal distance between the fixation point and the closest edge of the curve w& also 3” (since curves were drawn from their mid-points 4.5” from the fixation point). The 32 cards represented a balanced series of the angles used, position (left or right) and vertica!ity (higher or lower).
HEMISPHERICDIFFERENCES IN THE DISCRIMINATIONOF CURVATURE
PO‘
4 5
197
”
FIG. 1 (a). Upper. Illustration of the layout of the curved lines. This would be a “same” judgement on the 90” curve. (b) Lower. The four curved lines which together with a straight line were used for the “same”, “different” curvature judgements. The chord lengths were constant and the arc lengths and the radii varied.
Procedure. On each trial one card (i.e. one pair of curves) was presented tachistoscopically for 150 msec. Six test batches of the 32 cards were made up, using each of three randomized series twice. Ss responded to either “same” or “different” on alternative test batches of 32 stimuli. The ordering of the test batches was randomized between Ss. Ss were instructed to respond by depressing a key with a finger of their right hand. Under conditions of instruction to respond when the curves were the same, Ss depressed the key when they judged the curves were the same, but refrained from responding when the curves were judged to be different. The opposite requirements operated when the instructions were to respond for different. A ready signal from the experimenter about 2-3 set before each trial reminded Ss to fixate a dot at the centre of the screen. On each trial the reaction time was measured with an electronic timer from the onset of the stimulus to the initiation of the key press. There were 32 practice stimuli, followed by the 6 test batches.
Table 1. Median reaction times in msec “5ame”
iudserrents
'DifferentHiuduements
RWF
LVF
(R-L)
RVF
LVF
(S-L)
KL
270
566
+?J
532
570
-38
CrnL
I!?2
jot;
l
575
523
l56
AT
h"?
558
.'i
755
652
.103
JJ
55d
534
l24
529
529
0
RP
55Y
566
-7
578
559
r19
JriN
U‘4S
432
*lb
421
433
-12
CT
562
551
lll
565
533
-19
rlH
509
hY4
l15
436
471
-35
DL
671
'192
+29
S9S
539
*lo
MR
515
408
.27
539
507
l32
%
K. LONODEN,C. ELLIS and SUSAND. IVERSEN
198
Results
The median reaction times and the differences between them (compared with RVF latencies-LVF latencies) are presented in Table 1. An analysis of variance of the median RTs yielded the F values shown in Table 2. No influence of field upon RT could be demonstrated. The raw data were examined in some detail and it was found that particular cards, particular angles or particular batches of stimuli had not had any significant effect upon the results.
Table 2. Analysis of variance SOllrCP
df
EL5
S (subjects)
9
13,308.Z
C (condition-some/ differrnt)
1
354.0
CSS
3
27b5.0
1
1575.0
9
425.7
C*k
1
7.2
c s I<.* s
9
F (field
YXb
Total
- RVF/LW)
l?
Sic;E.
0.129
NS
3.700
I\Ts
0.012
NS
584.8
39
Discussion
Despite the failure to find a significant field effect, 9/10 subjects showed a LVF advantage for the “same” judgement. It was, therefore, decided to repeat the experiment using the identical stimuli but different testing procedures.
EXPERIMENT
II
Method Subjects and design. Nineteen right handed undergraduates who had not performed in Experiment I served as subjects. The general design testing conditions and stimuli were the same as Experiment I with the following modifications: 1. It was considered possible that the requirements to oscillate between “same” and “different” judgements in Experiment I could have interfered with stable performance. Groups were not informed of the other testing condition to exclude the possibility that the experiment “primed” responses to one of the conditions preferentially. Separate groups of subjects were tested on the “same” and “different” discriminations. 2. The rejection criterion was made more stringent; Ss who had difficulty reaching or maintaining the accuracy criterion of 94% were rejected; so were Ss who had reaction times consistently longer than 650 msec. 3. Individual long reaction times (l-2 set) to a trial were not accepted. Cards producing such long RTs were reintroduced at the end of the test batch as long as the total number of errors plus long RTs did not exceed a 90% criterion (i.e. 3 cards in a test batch of 32). 4. A shorter testing time was introduced, to avoid any variability produced by fatigue. This condition was satisfied since only 3 test batches needed to be used per subject in Experiment II as Ss were now tested in only one condition (“same” or “different”).
HEMISPHERIC
DIFFERENCES
IN
THE
DISCRIMINATION
OF
CURVATURE
199
5. Two response keys, one in each hand, were used in an effort to exclude any bias due to output processing. Ss were instructed to press the keys simultaneously and the faster response cut off the digital counter used to measure RTs.
Results The median reaction times for individual subjects are presented in Table 3. There is a LVF advantage of 9.2 msec for “same” and 14.7 msec for “different” judgements overall. The group performing “same” judgements did not show a significant field effect. But, those performing “different” judgements showed a significant LVF advantage (t = 3.6, df = 9, P < O-01). Table 3. The Median reaction times for individual subjects Median
Reaction
%.Wlle~ iudwments
Times "DifferentH
RWF
LWf
(R-L)
M.4
547
533
t14
KS
537
509
TH
484
JB
524
RR
iudqements
RVF
LVF
(R-L)
DC
555
528
+30
+20
Jb4
489
484
+5
525
-41
AN
510
500
+lO
543
-19
JE
522
500
+22
506
475
+31
PM
499
461
l30
SN
619
592
+27
TB
514
432
+22
BP
596
564
l32
JC
596
590
+6
LP
521
532
-11
RS
540
526
+I4
JP
525
503
+22
EJ
602
607
-5
AB
576
571
+5
Further analysis on a Friedman two-way analysis of variance revealed that on the “different” judgements the difficulty of the discrimination signScantly influenced RTs, (P = O-042 for X2r = 6 when K = 4, N = 2). This was not the case for “same” judgements. However, RTs for judging difference of curvature did not decrease as the difference in arc lengths or heights of the curve pairs increased, as one might expect on a simple matchmismatch hypothesis. Thus some other parameter of curvature is likely to be used in detecting differences. Discussion A left visual field superiority in reaction time was found with respect to “different” judgements. There was no significant effect of field of presentation on “same” judgements, although 6 out of 9 subjects again produced faster reaction times for curves presented in the left visual field. These results are somewhat surprising in view of the results of the first experiment where, although there were no significant differences, “same” judgements produced more consistent left visual field advantages. It is possible that this is due to the operation of unwanted range effects inherent in the within-subject design of the first experiment. Thus subjects
200
K. LONGDEN. C. ELLIS
and
SUSAN D.
IVERSEN
performed batches of “press for same” alternatively with batches of “press for different” on the go-no-go paradigm, and there was a total of 6 batches in all. Subjects seemed to find no difficulty in switching from one strategy to another and examination of the results showed no consistent trends with respect to which condition came first. However, POULTON [14] has shown that even when each subject receives a number of experimental conditions in random order, unwanted range effects can revise the true rank order of the conditions, and hence give rise to erroneous results. In the second experiment, not only did each subject perform only one of the conditions but he was also ignorant of the existence of the alternative condition and received no practice in it. It is possible that subjects in the first experiment developed a set for the “press for same” condition despite equal preliminary training on both conditions. Indeed, many people seemed to find the “press for same” condition easier though this was not always borne out with respect to speed in the experimental results. Thus whereas previous experiments have found that “same” judgements are performed consistently faster this was not the case with several of our subjects. A second difference between the two experiments was in the mode of responding. In the first experiment, subjects held a single button between finger and thumb of the right hand. Because of contralateral control of distal musculature this biases against any right hemisphere advantage in perceptual processing. In the second experiment subjects held two buttons, one in either hand, and were instructed to respond with both. It was felt that this would equalize any laterality differences in the speed of producing a response enabling us to ascribe any laterality differences found to differences in perceptual processing. Without, however, analysis of hand-field combinations it is not possible to distinguish between dominance and asymmetry and the laterality of differences due to interhemispheric crossing times. Furthermore, it is possible that testing in the first experiment was too long for a reaction time study. Despite several breaks after batches of stimuli, each subject was presented with 192 cards plus the training required to reach an accurate level of performance. In the second experiment each subject received only half that number and did not have the added problem of dealing with two conditions. It was essential that subjects were accurate but to avoid any speed-accuracy trade ofl’, subjects were also encouraged to react as quickly as possible within those limits. Analysis of the errors indicated that there had been no substantial trade-off since there were no more errors in one half of the visual field than the other. There was, however, a wide variation in mean reaction time between subjects irrespective of condition. This may be indicative of the operation of different mechanisms in different subjects. Indeed, in the second experiment we rejected any subject who consistently reacted with a speed of over 650 msec, since this was well outside the “normal” range. The analysis of variance of the first experiment indicated that inter-subject variance was large enough to obscure any individual’s laterality differences, and possibly any “population” laterality effect with respect to a single mechanism of processing. Population trends in cerebral asymmetry appear to be very fragile, and it may be better to investigate individuals intensively and test the consistency of their results rather than trying to form “across-population” conclusions. There is some evidence that laterality differences become less marked with practice, and if this were so the conclusions based upon limited testing of many subjects may well be misleading. Since little is known of the mechanisms of perception of curvature with respect to “same” and “different” judgements, it is difficult to make firm prior hypotheses as to
201
HEMISPHERIC DIFFERENCES IN THE DISCRIMINATION OF CURVATURE
expected laterality effects. Previous experiments have suggested that “same” judgements mediated preferentially by the left hemisphere, may be carried out by “global gestalt” processes while “different” judgements are more analytic in nature and depend on the right hemisphere. In our experiments there was a tendency to left field advantage for both kinds of judgemerits. But the fact that the “different” judgement in Experiment II gave the only significant effect would support a special role for the right hemisphere in such judgements.
~4cknowledgenzerrts-We thank Prof. 0. L. ZANGWILL helpful comments on the manuscript.
for support of our work and Dr. G. BERLUCCHIfor
REFERENCES 1. MILNER,B. Interhemispheric Bull. 27,272-271,
differences in the localization of psychological
processes in man. Br. rned
1971.
2. BLAKEMORE, C., IVERSEN,S. D. and ZANGWILL,0. L. Brain Functions.
A. Rev. Psychol. 23,413-456, 1972. 3. UMILTA,C., FROST, N. and HUMAN, R. Interhemispheric effects on choice reaction times to one- twoand three-letter displays. J. exp. Psychol. 93, 198-204, 1972. 4. STUDDERT-KENNEDY,M. and SHANKWEILER,D. Hemispheric specialization for speech perception. J. acoust. Sot. Am. 48, 579-594, 1970. 5. DARWIN, C. J. Ear differences in the recall of fricatives and vowels. Q. JI. exp. Psyclzol. 23,46-62, 1971. 6. UMILTA,C., RIZZOLATTI,G., MARZI, C. A., ZAMBONI,G., FRANZINI,C., CAMARDA,R. and BERLUCCIII, G. Hemispheric differences in the discrimination of line orientation. Neuropsychologia 12,165-174,1974. 7. COLTHEART,M. Visual feature-analyzers and after-effects of tilt and curvature. Psycho/. Rev. 78,114-121, 1971. 8. RIGGS, L. A. Curvature as a feature of pattern vision. Science 181, 1070-1072, 1973. 9. MCCOLLOUGH,C. Colour adaptation of edge detectors in the human visual system. Science 149,11151116, 1965. IO.HUBEL, D. H. and WIESEL,T. N. Receptive fields and functional architecture in two non-striate visual areas (18 and 19) of the cat. J. Neurophysiol. 28,229-289, 1965. Il. BLAKEMORE,C. and OVER, R. Curvature detectors in human vision. Perception 3, 3-7, 1974. 12. EGETH, H. and EPSTEIN,J. Differential specialization of the cerebral hemispheres for the perception of sameness and difference. Percept. Psychophys. 12, 218-220, 1972. 13. ATKINSON,J. and EGETH, H. Right hemisphere superiority in visual orientation matching. Can. J. Psychol.
27,152-158,
1973.
I4 POULTON,E. C. The new psychophysics:
six models for magnitude estimation.
Psychol. Bull. 69, 1-19,
1968.
ResumtS
:
paires
de courbes
On a pr&sentd
b droite
en demandant
que possible
si les courbes
rentes"..:ans
l'experience
ses
"differentes"
une
tendance
niveau separes
de
et
3 un avantage
de sujets
pour
ce cas, un avantage jugements
de la paire 1,
les
"identiques"
signification.
t@ de la discrimination.
temps
Les
gauche
similaires mais
blen
gauche
RT 6taier.t aussi
oc "diffeles repon-
cyu'il exista
qui n'attcignait
et
des
rapidement
(RT) pour
2, on a utilisf
"differents"
du champ
de fixation aussi
"identiquer"
de r@action
l'cxp,&rience
les jugements
du point
d'indiquer
Etaient
dtaient
du champ
Dans
siqnificatif
'differents".
et 2 qauche
aux sujets
pas
"identiaues".
Ftait
obtenu
en relation
Ic
des c[roupcs
avec
Dans les
a la difficul-
202
K. LONGDEN,C.ELLISand SUSAND. IVERSEN Deutschsprachige Zusammenfassung: Rechts und links vom Fixierpunkt w&den Kurvenpaare dargeboten. Versuchspersonen wurden gebeten, so schnell wie moglich anzugeben, welche Kurven eines Paares identisch oder diverent waren. In einem ersten Experiment waren die Reaktionszeiten (RI')fiir lfdifferentell und "identische" Antworten ahnlich, allerdings war eine gewisse Linksfeldiiberlegenheitzu erkennen, sie war aber nicht signifikant. In einem zweiten Experiment wurden getrennte Gruppen fiir die Beurteilung "different" und "identisch'lverwandt. In diesem Fall wurde eine signifikante tfberlegenheitfiir die linke Gesichtsfeldhalfte bei der Beurteilung lldifferentlf nachgewiesen. Die Reaktionszeiten entsprachen demnach dem Schwierigkeitsgrad der Unterscheidung.