Neuropsychologia,
1975,
Vol.
13, pp. 449 to 454.
Pcrgamon
Press.
Printed
in England.
TACTILE PERCEPTION OF DIRECTION IN RELATION TO HANDEDNESS AND FAMILIAL HANDEDNESS NILS R. VARNEY and ARTHUR L. BENTON Neurosensory
Center and Departments of Neurology and Psychology, University of Iowa, Iowa City, U.S.A. (Received 9 January 1975)
Abstract-The accuracy of tactile perception of the direction of tactile stimulation of brief duration applied to the palms of the hands was investigated in relation to hand preference and familial background in normal subjects. in confirmation of previous findings, perception of direction was significantly more accurate on the left hand than on the right in righthanded subjects. Lefthanded subjects showed no trend toward superior performance on either hand. Familial background in respect to handedness was found to be a significant and independent determinant 07 lateral pattern of performance. Righthanded subjects with a purely’dextral familial background showed clear suoerioritv on the left hand while those who had a lefthanded paren: showed no lateral asymmetry ;n performance. Lefthanded subjects with righthanded parents show no lateral asymmetry in performance but those with a lefthanded parent show clear superiority on the right hand. The tindings underscore the importance of familial hackground as a determinant of performances reflecting asymmetry in hemispheric function.
INTRODUCTION A NUMBER of studies of tactile-spatial
perception in both normal subjects and patients with brain disease have generated results leading to the conclusion that the right hemisphere plays the same distinctively important role of subserving spatial thinking in the tactile modality that it does in the sphere of vision [l--6]. These studies have dealt almost exclusively with righthanded subjects who presumably are left-hemisphere dominant for language functions. Thus the generalization that the right hemisphere is in some sense “dominant” for tactile-spatial thinking can be considered to be valid only for such subjects. The present investigation was addressed to two questions. The first concerned the relationship of handedness to performance pattern in tactile-spatial tasks. The specific question raised was whether or not normal subjects who are not righthanded would show the same pattern of performance (indicative of right hemisphere “dominance”) in the tactile perception of direction as that shown by righthanded subjects [5]. It was hoped that the findings would shed light on the character of the cerebral organization of spatial abilities in persons who are other than righthanded. The second question arose from the results of the study of BENTON, LEVIN and VARNEY [S] indicating that normal righthanded subjects were more accurate in identifying the direction of linear tactile stimulation applied to the palm of the left hand than to the right hand. Mean recognition score was significantly higher on the left hand than on the right and a majority of subjects (71%) showed superior accuracy on the left hand as compared to the right. These findings of left hand superiority (analogous to right ear superiority for verbal material in the dichotic listening condition) provided the basis for inferring right hemisphere “dominance” in mediating the performance. However, just as is typically the 449
450
NILS R. VARNEYand ARTHURL. BENTON
case in dichotic listening studies, a significant minority of righthanded subjects did not conform to the dominant trend. Specifically, while 17 subjects showed left hand superiority in accuracy of directional perception, 5 showed right hand superiority and 2 showed equal accuracy in the two hands. Obviously a generalization that fails to hold for 29% of the subjects in a defined group must be considered to be rather weak. One possibility that suggests itself as a means of increasing accuracy of prediction is to define righthandedness not only in terms of manual preference but also in terms of familial background in respect to manual preference. Thus the second question that was posed was whether or not righthanded subjects with a purely dextral familial background would differ in performance pattern from those who had lefthanded parents or lefthanded siblings.
METHOD The subjects in the study were 102 University students (42 men; 60 women). Each subject was assigned to 1 of 6 possible groups with respect to direction and strength of manual preference and the presence or absence of lefthandedness in his family on the basis of his responses to the inventory shown in Table I. The groups were as follows: I. R(R) Strongly righthanded with dextral familial background (15 men; 19 women) 11. R(LS) Strongly righthanded with lefthanded sibling (12 men; 12 women) III. R(LP) Strongly righthanded with lefthanded parent (3 men; 7 women) IV. NR(R) Not righthanded with dextral familial background (6 men; 11 women) V. NR(LS) Not righthanded with lefthanded sibling (1 man; 6 women) VI. NR(LP) Not righthanded with lefthanded parent (5 men; 5 women). A subject was classified as strongly righthanded and assigned to Group I, II, or III if he was a self-declared strong righthandcr and if he indicated right hand preference for at least 9 of the 10 activities listed in the inventory. A subject was classified as not righthanded if he indicated left or mixed lateral preference for three or more activities listed in the inventory. Subjects who indicated left or mixed lateral preference for two activities listed in the inventory were excluded from the study in order to differentiate the groups clearly from each other. If a subject indicated that one or both of his parents was lefthanded, he was assigned to the R(LP) or NR(LP) group. If he indicated that a sibling (but not a parent) was lefthanded, he was assigned to R(LS) or NR(LS) group. If a subject had at least 4 relatives (i.e., 2 parents, 2 siblings) who were righthanded and no lefthanded siblings, he was assigned to the R(R) or NR(R) group. Subjects with exclusively righthanded relatives but less than two siblings were excluded from the study because of the risk that the small family size might conceal a trend toward lefthandedness. Procedure
The apparatus used to control the application of linear punctate stimulation to the palms of the hands was the electromechanical stimulator of CARMONand D~solv [7] which has been described in detail in previous reports [2,4, 51. The essential stimulus was a linear array of 3 stainless steel rods, each having a flat circular surface of 0.8 mm2 and a weight of 20 g the distance between the rods being 15 mm. The stimulus was positioned 2 mm above the palm and presented in a fixed order in 1 of 4 directions: vertical, horizontal and either 45” diagonal. The duration of stimulation was 1 set and the intertrial interval was 10 sec. The direction of stimulation was guided by radial lines printed on a grid stamped on the subject’s palms prior to testing. Each subject was run on 4 blocks of 12 trials each in an ABBA order, i.e., 12 stimulations of one palm, 24 stimulations of the other palm, 12 stimulations of the first palm. Choice of the starting hand was made by the experimenter so that each group was roughly balanced with respect to the starting hand. One practice trial was given to familiarize the subject with the procedure. A black curtain obscured the subject’s view of his hand and a visual display showing the 4 different directions of stimulation was placed before him so that he could conveniently point to it with either his left or right hand. The 24 test trials were then presented to each palm and the subject’s responses were recorded. Performance level on each hand was defined as the number of correct identifications made on the 24 trials. Each subject was then classified as “Left Hand Superior” (higher score for the left hand than the right), “No Hand Difference”, or “Right Hand Superior” (higher score for the right hand than for the left).
TACTILE
PERCEPTION
OF DIRECTION
IN RELATION
Table 1. Neurosensory A.
Are you righthanded
B.
Do you consider ately or weakly
TO HANDEDNESS
Center handedness
AND FAMILIAL
451
HANDEDNESS
inventory
Left
Right
or lef?hanled'!
yourself to be strongly, inodcrrighthanded (or lefthanded)?
S'zrong
?Lixcd
Moderate
Weak
1.
With which
hand do you write?
Right
Left
Either
2.
With which
hand do you use a tennis racquet?
Right
Left
Either
3.
With which hand do you use a screwdriver?
Right
Left
Either
4.
With which
Right
Left
Either
5.
With
Right
Left
Either
6.
With which hand do you use a hammer:
Right
Left
Either
7.
With which
Right
Left
Either
8.
With which hand do you use a toothbrusli!
Right
Left
Either
9.
With which
Right
Left
Either
Right
Left
Either
10.
hand do you throw a ball?
which hand
do you use a needle
hand do you light a
in sewing?
match?
hand do you deal cards?
With which hand do you hold a knife when
1.
Is your
2.
Is your mother
3.
If you have any siblings (brothers ness of each (write in more blanks
father righthanded righthanded
carving meat?
or lelthanded?
Right
Left
Nixed
Don't
Know
or lecthanded?
Right
Left
Mixed
Don't
Know
or sisters), if needed).
give the sex, age and handed-
1)
Sex
A&e
Handedness:
Right
Left
Mixed
Don't
Know
2)
Sex
Age
Har.dedness:
Right
Left
Mixed
Don't
Know
3)
Sex
Age
Handedness:
Right
Left
Mixed
Don't Know
4)
Sex
Age
Handedness:
Right
Left
Mixed
Don't
5)
Sex
A&- ___
Handedness:
Right
Left
FIixad
Don't :,now
Know
RESULTS The distribution of subjects in the 6 handedness-familial background groups in respect to their classifications as “Left Hand Superior”, “No Hand Difference” or “Right Hand Superior” is shown in Table 2 and graphically represented in Fig. 1. By inspection it is apparent that the righthanded and lefthanded subjects showed different patterns of performance. A majority (66%) of the righthanded subjects showed superior accuracy of recognition in the Zeft hand while a slight majority (56%) of non-righthanded subjects showed superior accuracy of recognition in the right hand. Statistical analysis of the difference between the combined handedness groups [R(R) + R(S) + R(P) vs N(R) + N(S) + N(P)] by means of the proportions test of DIXON and MASSEY [l 1, p. 2491 indicated a significant main effect of handedness (P < O-02) on the between-hands performance pattern. The Wilcoxon signed-rank test showed that a significantly higher number of subjects in the combined righthanded group performed better with their left hand than
NILS
452
R. VARNEYand ARTHURL. BENTON
with their right hand (z score = 2.76, P -C0.005). The same test showed that the combined non-righthanded group failed to show significant asymmetry in performance on the two hands (z score = 1.30). Table 2. Distribution of subjects showing right or left hand superiority in relation to handedness and familial background
Cr~Y.lp~:
R(R) R(S)
Left Superior
25 (67%) (74%) 16
Right Superior
77 (29%) (21%) I 26%
66%
*R = righthanded; N -r not righthanded; (P) = lefthanded parent.
h'ollifference
(R) = dextral familial background;
(S) = lefthanded sibling;
100 I
CLASSIFICATION = l : (RI= IS): (P): l
FIG. 1. Proportion
Hlghihonded Not Righlhonded FcmM Dextral Lefthanded Slblmg Lefthanded Parent
of subjects in each subgroup showing superior accuracy of recognition in the left hand.
Inspection of Table 2 and Fig, 1 also discloses an apparent influence of familial background performance pattern. As will be seen, the proportion of righthanded subjects showing superior recognition in the left hand is associated with the presence or absence of lefthandedness in the family. Righthanded subjects with a purely dextral familial background showed a relatively high frequency of left hand superiority (74x), those with a lefthanded sibling showed a less pronounced trend toward left hand superiority (67X), and those with a lefthanded parent showed no trend toward superior performance in either hand. Similarly, a high proportion (80%) of lefthanded subjects with a lefthanded parent
TACTILE
PERCEPTION
OF DIRECTION
IN RELATION
TO HANDEDNESS
AND FAMILIAL
HANDEDNESS
453
showed superior recognition in the right hand while those with only a lefthanded sibling or from a purely dextral familial background showed no bias toward lateral asymmetry in performance. Statistical analysis (proportions test) of the difference between the combined familial groups [R(R) + N(R) vs R(S) + N(S) vs R(P) + N(P)] indicated a significant main effect of familial background (P < O-01) on performance pattern. The same analysis showed no interaction between handedness and familial background, indicating that these factors were exerting independent influences in the same direction on the subjects’ performances.
DISCUSSION The findings of the study show that handedness is indeed a significant determinant of lateral pattern of performance on a tactile-spatial perceptual task. The strongly righthanded subjects showed the same trend toward superior accuracy of recognition in the left hand that was found in the earlier study of BENTON, LEVIN and VARNEY [5]. In contrast, the lefthanded subjects showed a slight and nonsignificant trend in the opposite direction, i.e., superior accuracy of recognition in the right hand, the difference between the directional trends of the two groups being statistically significant. Evidently the inference derived from studies of righthanded subjects, both normal and brain-diseased, that the right hemisphere plays a distinctively important role in mediating spatial perception in the tactile modality does not hold for lefthanded subjects. The results for the combined group of righthanded subjects confirmed those of our earlier study [5] not only in respect to the occurrence of superior accuracy of perception in the left hand for these subjects as a group but also in respect to the finding of a relatively high proportion of subjects who did not conform to this rule (29 % in the previous study; 34 “//,in the present study). That familial background with respect to handedness is also a determinant of lateral pattern of performance is clearly indicated by the findings. In the case of righthanded subjects, those with a purely dextral familial background showed a stronger tendency toward left hand superiority than did those with lefthanded relatives. Hand preference of the parents appears to be a particularly strong determinant of performance pattern, a finding which is accord with genetic theory. The importance of familial background with respect to handedness was as clearly demonstrable in the subjects who were not righthanded as in those that were. As a group, these subjects who were not righthanded showed no trend toward asymmetry in lateral pattern of performance. However, formation of subgroups that were homogeneous with respect to familial background showed that, while those with righthanded parents continued to manifest the dominant group trend, those with a lefthanded parent showed significantly superior performance in the right hand. Thus the findings indicate that familial background in respect to handedness is a significant determinant of hemispheric asymmetry of function as reflected in lateral performance pattern on this tactile-spatial perceptual task. The results are consistent with those of other investigators who have explored the question with different task performances. ZURIF and BRYDEN [8] found that lefthanded subjects with a sinistral familial background differed in lateral pattern of performance on tachistoscopic and dichotic listening tasks from lefthanded subjects with a purely dextral familial background. In a study of patients with unilateral brain disease, H~CAEN and SAUGUET [9] found that aphasic disorder was associated with lesions of the left hemisphere in lefthanded patients with a dextral
454
NILS
R. VARNEYand ARTHURL. BENTON
familial background while a lesion in either hemisphere was likely to cause aphasic disorder in lefthanded patients with a sinistral familial background. The fact remains that, even when homogeneous groups of subjects defined in terms of both hand preference and familial background were formed, some subjects do not show the predicted performance pattern. For example, the proportion of exceptional subjects in the righthanded-familial dextral group was smaller than when familial background was not taken into account but it was still large enough to pose problems of interpretation. One possible reason for the occurrence of these “exceptional” cases in the present study is the modest reliability of our perceptual task. Using the split-half procedure for the first and second trial blocks on each hand, the reliability of the task was computed to be 0.71. Conceivably the application of a longer, more reliable task would lead to a reduction of the number of “exceptional” cases. Parenthetically, it may be noted that the reliability of procedures employed to investigate asymmetry of hemispheric function, such as dichotic listening and tachistoscopic stimulation of visual fields, is rarely assessed. The recent finding of PIZZAMIGLIO, DE PASCALIS and VIGNATI [lo] that an apparently typical dichotic listening task yielded only marginally stable measures for individual subjects indicates that the reliability factor cannot be neglected. The demonstration in this study as well as in those of ZURIF and BRYDEN [8] and HBCAEN and SAUGUET [9] of the importance of familial background as a determinant of hemispheric asymmetry of function has obvious methodological implications for investigative work in this area. Classification of subjects in terms of both hand preference and familial background is clearly a prerequisite for securing consistent and interpretable results. Acknowledgement-This investigation was supported by Research Grant NS-00616 and Program-Project Grant NS-03354 from the National Institute of Neurological Diseases and Stroke. Neurosensory Center Publication No. 329.
REFERENCES 1. CORKIN, S. Tactually-guided maze learning in man: effects of unilateral cortical excisions and bilateral hippocampal lesions. Neuropsychologiu 3,339-351, 1965. 2. CARMON, A. and BENTON,A. L. Tactile perception of direction and number in patients with unilateral cerebral disease. NerrroloXy 19, 525-532, 1969. 3. GAXANIGA, M. S. The Bisected Bruin. Appleton-Century-Crofts, New York, 1970. 4. FONIENOT, D. J. and BEECTON, A. L. Tactile perception of direction in relation to hemispheric locus of lesion. Neuropsychologia 9, 83-88, 1971. 5. BENTON,A. L., LEVIN, H. S. and VAR~TEY, N. R. Tactile perception of direction in normal subjects. Neurology 23, 1248-1250, 1973. 6. WITELSON,S. F. Hemispheric specialization for linguistic and nonlinguistic tactual perception using a dichotomous stimulation technique. Cortex 10,3-17, 1974. 7. CARMON,A. and DYSON,J. A. New instrumentation for research on tactile sensitivity and discrimination. Cortex 3,406-418, 1967. 8. ZURIF, E. G. and BRYDEN,M. P. Familial handedness and left-right differences in auditory and visual perception. Neuropsychologia 7, 179-188, 1969. 9. HBCAEN,H. and SAUGUET,J. Cerebral dominance in left handed subjects. C0rfe.u 7, 19-48, 1971. 10. PIZZAMIGLIO,L., DE PASCALIS,C. and VIGNATI,A. Stability of dichotic listening test. Cortex 10,203-205, 1974. 11. DIXON, W. J. and MASSEY,F. J. Introduction to Statktical Analysis. McGraw-Hill, New York, 1969.
1975,
Vol.
13, pp. 449 to 454.
Pcrgamon
Press.
Printed
in England.
TACTILE PERCEPTION OF DIRECTION IN RELATION TO HANDEDNESS AND FAMILIAL HANDEDNESS NILS R. VARNEY and ARTHUR L. BENTON Neurosensory
Center and Departments of Neurology and Psychology, University of Iowa, Iowa City, U.S.A. (Received 9 January 1975)
Abstract-The accuracy of tactile perception of the direction of tactile stimulation of brief duration applied to the palms of the hands was investigated in relation to hand preference and familial background in normal subjects. in confirmation of previous findings, perception of direction was significantly more accurate on the left hand than on the right in righthanded subjects. Lefthanded subjects showed no trend toward superior performance on either hand. Familial background in respect to handedness was found to be a significant and independent determinant 07 lateral pattern of performance. Righthanded subjects with a purely’dextral familial background showed clear suoerioritv on the left hand while those who had a lefthanded paren: showed no lateral asymmetry ;n performance. Lefthanded subjects with righthanded parents show no lateral asymmetry in performance but those with a lefthanded parent show clear superiority on the right hand. The tindings underscore the importance of familial hackground as a determinant of performances reflecting asymmetry in hemispheric function.
INTRODUCTION A NUMBER of studies of tactile-spatial
perception in both normal subjects and patients with brain disease have generated results leading to the conclusion that the right hemisphere plays the same distinctively important role of subserving spatial thinking in the tactile modality that it does in the sphere of vision [l--6]. These studies have dealt almost exclusively with righthanded subjects who presumably are left-hemisphere dominant for language functions. Thus the generalization that the right hemisphere is in some sense “dominant” for tactile-spatial thinking can be considered to be valid only for such subjects. The present investigation was addressed to two questions. The first concerned the relationship of handedness to performance pattern in tactile-spatial tasks. The specific question raised was whether or not normal subjects who are not righthanded would show the same pattern of performance (indicative of right hemisphere “dominance”) in the tactile perception of direction as that shown by righthanded subjects [5]. It was hoped that the findings would shed light on the character of the cerebral organization of spatial abilities in persons who are other than righthanded. The second question arose from the results of the study of BENTON, LEVIN and VARNEY [S] indicating that normal righthanded subjects were more accurate in identifying the direction of linear tactile stimulation applied to the palm of the left hand than to the right hand. Mean recognition score was significantly higher on the left hand than on the right and a majority of subjects (71%) showed superior accuracy on the left hand as compared to the right. These findings of left hand superiority (analogous to right ear superiority for verbal material in the dichotic listening condition) provided the basis for inferring right hemisphere “dominance” in mediating the performance. However, just as is typically the 449
450
NILS R. VARNEYand ARTHURL. BENTON
case in dichotic listening studies, a significant minority of righthanded subjects did not conform to the dominant trend. Specifically, while 17 subjects showed left hand superiority in accuracy of directional perception, 5 showed right hand superiority and 2 showed equal accuracy in the two hands. Obviously a generalization that fails to hold for 29% of the subjects in a defined group must be considered to be rather weak. One possibility that suggests itself as a means of increasing accuracy of prediction is to define righthandedness not only in terms of manual preference but also in terms of familial background in respect to manual preference. Thus the second question that was posed was whether or not righthanded subjects with a purely dextral familial background would differ in performance pattern from those who had lefthanded parents or lefthanded siblings.
METHOD The subjects in the study were 102 University students (42 men; 60 women). Each subject was assigned to 1 of 6 possible groups with respect to direction and strength of manual preference and the presence or absence of lefthandedness in his family on the basis of his responses to the inventory shown in Table I. The groups were as follows: I. R(R) Strongly righthanded with dextral familial background (15 men; 19 women) 11. R(LS) Strongly righthanded with lefthanded sibling (12 men; 12 women) III. R(LP) Strongly righthanded with lefthanded parent (3 men; 7 women) IV. NR(R) Not righthanded with dextral familial background (6 men; 11 women) V. NR(LS) Not righthanded with lefthanded sibling (1 man; 6 women) VI. NR(LP) Not righthanded with lefthanded parent (5 men; 5 women). A subject was classified as strongly righthanded and assigned to Group I, II, or III if he was a self-declared strong righthandcr and if he indicated right hand preference for at least 9 of the 10 activities listed in the inventory. A subject was classified as not righthanded if he indicated left or mixed lateral preference for three or more activities listed in the inventory. Subjects who indicated left or mixed lateral preference for two activities listed in the inventory were excluded from the study in order to differentiate the groups clearly from each other. If a subject indicated that one or both of his parents was lefthanded, he was assigned to the R(LP) or NR(LP) group. If he indicated that a sibling (but not a parent) was lefthanded, he was assigned to R(LS) or NR(LS) group. If a subject had at least 4 relatives (i.e., 2 parents, 2 siblings) who were righthanded and no lefthanded siblings, he was assigned to the R(R) or NR(R) group. Subjects with exclusively righthanded relatives but less than two siblings were excluded from the study because of the risk that the small family size might conceal a trend toward lefthandedness. Procedure
The apparatus used to control the application of linear punctate stimulation to the palms of the hands was the electromechanical stimulator of CARMONand D~solv [7] which has been described in detail in previous reports [2,4, 51. The essential stimulus was a linear array of 3 stainless steel rods, each having a flat circular surface of 0.8 mm2 and a weight of 20 g the distance between the rods being 15 mm. The stimulus was positioned 2 mm above the palm and presented in a fixed order in 1 of 4 directions: vertical, horizontal and either 45” diagonal. The duration of stimulation was 1 set and the intertrial interval was 10 sec. The direction of stimulation was guided by radial lines printed on a grid stamped on the subject’s palms prior to testing. Each subject was run on 4 blocks of 12 trials each in an ABBA order, i.e., 12 stimulations of one palm, 24 stimulations of the other palm, 12 stimulations of the first palm. Choice of the starting hand was made by the experimenter so that each group was roughly balanced with respect to the starting hand. One practice trial was given to familiarize the subject with the procedure. A black curtain obscured the subject’s view of his hand and a visual display showing the 4 different directions of stimulation was placed before him so that he could conveniently point to it with either his left or right hand. The 24 test trials were then presented to each palm and the subject’s responses were recorded. Performance level on each hand was defined as the number of correct identifications made on the 24 trials. Each subject was then classified as “Left Hand Superior” (higher score for the left hand than the right), “No Hand Difference”, or “Right Hand Superior” (higher score for the right hand than for the left).
TACTILE
PERCEPTION
OF DIRECTION
IN RELATION
Table 1. Neurosensory A.
Are you righthanded
B.
Do you consider ately or weakly
TO HANDEDNESS
Center handedness
AND FAMILIAL
451
HANDEDNESS
inventory
Left
Right
or lef?hanled'!
yourself to be strongly, inodcrrighthanded (or lefthanded)?
S'zrong
?Lixcd
Moderate
Weak
1.
With which
hand do you write?
Right
Left
Either
2.
With which
hand do you use a tennis racquet?
Right
Left
Either
3.
With which hand do you use a screwdriver?
Right
Left
Either
4.
With which
Right
Left
Either
5.
With
Right
Left
Either
6.
With which hand do you use a hammer:
Right
Left
Either
7.
With which
Right
Left
Either
8.
With which hand do you use a toothbrusli!
Right
Left
Either
9.
With which
Right
Left
Either
Right
Left
Either
10.
hand do you throw a ball?
which hand
do you use a needle
hand do you light a
in sewing?
match?
hand do you deal cards?
With which hand do you hold a knife when
1.
Is your
2.
Is your mother
3.
If you have any siblings (brothers ness of each (write in more blanks
father righthanded righthanded
carving meat?
or lelthanded?
Right
Left
Nixed
Don't
Know
or lecthanded?
Right
Left
Mixed
Don't
Know
or sisters), if needed).
give the sex, age and handed-
1)
Sex
A&e
Handedness:
Right
Left
Mixed
Don't
Know
2)
Sex
Age
Har.dedness:
Right
Left
Mixed
Don't
Know
3)
Sex
Age
Handedness:
Right
Left
Mixed
Don't Know
4)
Sex
Age
Handedness:
Right
Left
Mixed
Don't
5)
Sex
A&- ___
Handedness:
Right
Left
FIixad
Don't :,now
Know
RESULTS The distribution of subjects in the 6 handedness-familial background groups in respect to their classifications as “Left Hand Superior”, “No Hand Difference” or “Right Hand Superior” is shown in Table 2 and graphically represented in Fig. 1. By inspection it is apparent that the righthanded and lefthanded subjects showed different patterns of performance. A majority (66%) of the righthanded subjects showed superior accuracy of recognition in the Zeft hand while a slight majority (56%) of non-righthanded subjects showed superior accuracy of recognition in the right hand. Statistical analysis of the difference between the combined handedness groups [R(R) + R(S) + R(P) vs N(R) + N(S) + N(P)] by means of the proportions test of DIXON and MASSEY [l 1, p. 2491 indicated a significant main effect of handedness (P < O-02) on the between-hands performance pattern. The Wilcoxon signed-rank test showed that a significantly higher number of subjects in the combined righthanded group performed better with their left hand than
NILS
452
R. VARNEYand ARTHURL. BENTON
with their right hand (z score = 2.76, P -C0.005). The same test showed that the combined non-righthanded group failed to show significant asymmetry in performance on the two hands (z score = 1.30). Table 2. Distribution of subjects showing right or left hand superiority in relation to handedness and familial background
Cr~Y.lp~:
R(R) R(S)
Left Superior
25 (67%) (74%) 16
Right Superior
77 (29%) (21%) I 26%
66%
*R = righthanded; N -r not righthanded; (P) = lefthanded parent.
h'ollifference
(R) = dextral familial background;
(S) = lefthanded sibling;
100 I
CLASSIFICATION = l : (RI= IS): (P): l
FIG. 1. Proportion
Hlghihonded Not Righlhonded FcmM Dextral Lefthanded Slblmg Lefthanded Parent
of subjects in each subgroup showing superior accuracy of recognition in the left hand.
Inspection of Table 2 and Fig, 1 also discloses an apparent influence of familial background performance pattern. As will be seen, the proportion of righthanded subjects showing superior recognition in the left hand is associated with the presence or absence of lefthandedness in the family. Righthanded subjects with a purely dextral familial background showed a relatively high frequency of left hand superiority (74x), those with a lefthanded sibling showed a less pronounced trend toward left hand superiority (67X), and those with a lefthanded parent showed no trend toward superior performance in either hand. Similarly, a high proportion (80%) of lefthanded subjects with a lefthanded parent
TACTILE
PERCEPTION
OF DIRECTION
IN RELATION
TO HANDEDNESS
AND FAMILIAL
HANDEDNESS
453
showed superior recognition in the right hand while those with only a lefthanded sibling or from a purely dextral familial background showed no bias toward lateral asymmetry in performance. Statistical analysis (proportions test) of the difference between the combined familial groups [R(R) + N(R) vs R(S) + N(S) vs R(P) + N(P)] indicated a significant main effect of familial background (P < O-01) on performance pattern. The same analysis showed no interaction between handedness and familial background, indicating that these factors were exerting independent influences in the same direction on the subjects’ performances.
DISCUSSION The findings of the study show that handedness is indeed a significant determinant of lateral pattern of performance on a tactile-spatial perceptual task. The strongly righthanded subjects showed the same trend toward superior accuracy of recognition in the left hand that was found in the earlier study of BENTON, LEVIN and VARNEY [5]. In contrast, the lefthanded subjects showed a slight and nonsignificant trend in the opposite direction, i.e., superior accuracy of recognition in the right hand, the difference between the directional trends of the two groups being statistically significant. Evidently the inference derived from studies of righthanded subjects, both normal and brain-diseased, that the right hemisphere plays a distinctively important role in mediating spatial perception in the tactile modality does not hold for lefthanded subjects. The results for the combined group of righthanded subjects confirmed those of our earlier study [5] not only in respect to the occurrence of superior accuracy of perception in the left hand for these subjects as a group but also in respect to the finding of a relatively high proportion of subjects who did not conform to this rule (29 % in the previous study; 34 “//,in the present study). That familial background with respect to handedness is also a determinant of lateral pattern of performance is clearly indicated by the findings. In the case of righthanded subjects, those with a purely dextral familial background showed a stronger tendency toward left hand superiority than did those with lefthanded relatives. Hand preference of the parents appears to be a particularly strong determinant of performance pattern, a finding which is accord with genetic theory. The importance of familial background with respect to handedness was as clearly demonstrable in the subjects who were not righthanded as in those that were. As a group, these subjects who were not righthanded showed no trend toward asymmetry in lateral pattern of performance. However, formation of subgroups that were homogeneous with respect to familial background showed that, while those with righthanded parents continued to manifest the dominant group trend, those with a lefthanded parent showed significantly superior performance in the right hand. Thus the findings indicate that familial background in respect to handedness is a significant determinant of hemispheric asymmetry of function as reflected in lateral performance pattern on this tactile-spatial perceptual task. The results are consistent with those of other investigators who have explored the question with different task performances. ZURIF and BRYDEN [8] found that lefthanded subjects with a sinistral familial background differed in lateral pattern of performance on tachistoscopic and dichotic listening tasks from lefthanded subjects with a purely dextral familial background. In a study of patients with unilateral brain disease, H~CAEN and SAUGUET [9] found that aphasic disorder was associated with lesions of the left hemisphere in lefthanded patients with a dextral
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familial background while a lesion in either hemisphere was likely to cause aphasic disorder in lefthanded patients with a sinistral familial background. The fact remains that, even when homogeneous groups of subjects defined in terms of both hand preference and familial background were formed, some subjects do not show the predicted performance pattern. For example, the proportion of exceptional subjects in the righthanded-familial dextral group was smaller than when familial background was not taken into account but it was still large enough to pose problems of interpretation. One possible reason for the occurrence of these “exceptional” cases in the present study is the modest reliability of our perceptual task. Using the split-half procedure for the first and second trial blocks on each hand, the reliability of the task was computed to be 0.71. Conceivably the application of a longer, more reliable task would lead to a reduction of the number of “exceptional” cases. Parenthetically, it may be noted that the reliability of procedures employed to investigate asymmetry of hemispheric function, such as dichotic listening and tachistoscopic stimulation of visual fields, is rarely assessed. The recent finding of PIZZAMIGLIO, DE PASCALIS and VIGNATI [lo] that an apparently typical dichotic listening task yielded only marginally stable measures for individual subjects indicates that the reliability factor cannot be neglected. The demonstration in this study as well as in those of ZURIF and BRYDEN [8] and HBCAEN and SAUGUET [9] of the importance of familial background as a determinant of hemispheric asymmetry of function has obvious methodological implications for investigative work in this area. Classification of subjects in terms of both hand preference and familial background is clearly a prerequisite for securing consistent and interpretable results. Acknowledgement-This investigation was supported by Research Grant NS-00616 and Program-Project Grant NS-03354 from the National Institute of Neurological Diseases and Stroke. Neurosensory Center Publication No. 329.
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