Bimanual coordination in dyslexic adults

Irun)pa,rholoq~a. Vol 33. No. 6, pp 7X1-793. 1995 CopyrIght,c 199 Elsev~er Saence Ltd Prmkd I" Great Rntain.Allrightsreserved IN3283932 95 S9.5O+l~OO

002&3932(95)00019-4

BIMANUAL LAWRENCE

COORDINATION

H. MOORE,*? WARREN DAVID C. THEBERGE*

IN DYSLEXIC

S. BROWN,*?1 and JENNIFER

ADULTS

TARYN E. MARKEE,“? C. ZVI§

*Travis Institute of Biopsychosocial Research, Fuller Graduate School of Psychology, Pasadena. California, U.S.A.; tDepartment of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, U.S.A: and #California State University, Northridge, California, U.S.A. (Rrcriord

3 May 1994; accepted

19 Junuary

1995)

Abstract-Various types of dyslexia have been associated with tactile-motor coordination deficits and inefficient transfer of information between the two cerebral hemispheres. Twenty-one dyslexic adults were compared to 21 controls on the Bimanual Coordination Task, a test of tactile-motor coordination and interhemispheric collaboration. When compared to control subjects, dyslexics showed a consistent pattern ofdeficits in bimanual motor coordination, both with and without visual feedback. In particular, dyslexics had greater difficulty relative to normals when the left hand had to move faster than the right, and when the hands had to make opposite (mirror-image) movements, suggesting problems with interhemispheric modulation of visuomotor control. In addition, accuracy on this bimanual coordination task was significantly correlated with the Block Design subtest of the WAIS-R, but not with a rhyme fluency task, suggesting some contribution of right hemisphere controlled visuospatial skill to performance. Key Words:

dyslexia;

motor

function;

corpus

callosum

INTRODUCTION The early work of Orton [49] attempted to link reading disability with lateralized neurological deficits. As research in this area has progressed, hypotheses have been based largely on the understanding of cerebral dominance, and suggest that a failure of specific mechanisms in either the left or right hemisphere may account for certain forms of reading deficit. However, more recent research suggests that deficits in interhemispheric integration may be an important contributing factor to reading deficiencies. The term dyslexia generally refers to reading disability in some form or another. However, the specific subtypes of reading disability are still a matter of debate. Early theorists believed that dyslexia was primarily a disturbance in visual processing, and used terms such as word blindness [29,45] and strephosymbolia [48] to describe the disorder that they believed caused the specific difficulty in learning to read [29,45]. Proponents of a visuospatial basis for dyslexia argue that dyslexics have an instability in their visual abilities when they are trying to read due to a developmental disorder of the right hemisphere [SS]. Additional support for this view comes from studies which show reading difficulties following injury to

IAddress N. Oakland

for reprint requests: Dr W. S. Brown, Ave.. Pasadena. CA 91101. U.S.A.

Travis

781

Institute,

Fuller Graduate

School

of Psychology.

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the right hemisphere [26], and neuroanatomical evidence of right hemisphere abnormalities in dyslexic brains [ 181. Hypotheses of lateralized differences in dyslexics have also focused on dysfunction in the left hemisphere [lo, 211, specifically in the region of the left angular gyrus [16, 551, or possibly the planum temporale [ 191. Such models have arisen largely from an early study of an alexic adult who lost the ability to read following an injury to the left cerebral hemisphere [20]. Causal mechanisms which involve left hemisphere insult have subsequently been hypothesized for children with developmental reading deficits as well. In further support of this hypothesis, studies of dyslexic children have noted performance deficits on tasks thought to rely upon specialized skills of the left hemisphere, such as sequencing ability [a]. Dyslexia may also be associated with deficits in either or both hemispheres for different individuals. Recent research suggests that approximately 60-70% of dyslexics suffer problems with both right and left hemisphere mediated functions [57]. According to Stein [57], perhaps only 15% of dyslexics can be called pure ‘visual’, while another 15% suffer dysfunction primarily involving difficulties in phonological processing. Recent attention has shifted to the exploration of the integration and/or transfer of information between the cerebral hemispheres as a critical factor in reading ability. It is argued that efficient interhemispheric integration is important in reading development [Zl , 22, 351 and that dyslexic children may be deficient in interhemispheric collaboration as compared to normal readers [12]. EEG studies have shown that reading impaired children have less interhemispheric coherence [56], and anatomical measures of the corpus callosum, the major neocortical commissure, have shown abnormal thinning for two dyslexic brains [15]. Tachistoscopic [24] and dichotic listening [3, 35, 461 studies of reading disabled children have also suggested abnormal transfer of sensory information, while other studies have demonstrated abnormal evoked potential interhemispheric transfer times in dyslexia [12, 13,421. Bimanual motor skill and speed have also been employed to further understand the nature of interhemispheric collaboration in the dyslexic brain. An early study of boys with reading disability utilized a bimanual tapping test and found that boys with reading disability had greater difficulty than normal boys in maintaining a steady tapping rate with the left hand when tapping with two hands in alternation [ 11. Ability in unimanual and bimanual finger tapping was also explored in a group of disabled readers of varying intelligence [37] and those with above average intelligence [68]. These authors found that the reading disabled groups had consistently greater difficulty on tasks of interhand coordination than on unimanual tapping as compared to normal reading groups. Differences between dyslexic and normal subjects in performance on the Purdue Pegboard were found by Leslie et al. [38] and were interpreted as indicating deficits in interhemispheric interactions. Deficiencies in interhemispheric visuomotor coordination were also seen in dyslexics on a test of the latency of eye-tracking when stimuli were presented bilaterally [28]. While individuals with callosal agenesis (developmental absence of a corpus callosum) have particular difficulty in localizing tactile stimuli of the fingertips and transferring that information intermanually, significant deficiencies in interhand transfer have also been seen in dyslexic children [24]. Preilowski [51-533 was one of the first to demonstrate the relationship of bimanual coordination to functioning of the corpus callosum. With the use of an X-Y plotter he was able to demonstrate the importance of the anterior portions of the corpus callosum for the direct interhemispheric integration required for fine regulation of bimanually coordinated

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motor activity. In particular, Preilowski’s subjects who had surgical disconnection of the anterior portion of the corpus callosum showed marked deterioration of performance when they were denied visual feedback during performance of the task. Subsequent studies by Jeeves and colleagues [30-321 have also demonstrated that subjects with callosal agenesis are severely impaired in performance of a bimanual coordination task when denied visual feedback. Results ofthese studies lend support to Preilowski’s presumption that performance on a test of bimanual coordination involves interhemispheric processes mediated through the corpus callosum. The Bimanual Coordination Task has also been implemented by several investigators using an Etch-A-Sketch toy [7,64, 701, and interhemispheric coordination of tactile-motor activity has been studied in dyslexic boys utilizing an Etch-A-Sketch-like task [22]. While these authors found that group performance was equivalent for parallel hand movements, the dyslexic group showed significant impairments on mirror movements (especially left hand movements), often reverting back to parallel movements when visual feedback was removed. A similar pilot study of bimanual coordination in dyslexic children revealed clear performance deficits on an Etch-A-Sketch task [44]. A significant body of evidence suggests that a critical component of reading difficulty involves disabilities in phonological processing [9,41,59,65]. Phonological processing has been broadly defined as the ability to manipulate speech sounds in spoken or written language [SO] or as the use of phonological information (i.e. the sounds of one’s language) in processing written and oral language [66]. During reading, this type of processing involves sequential decoding of spelling units (including both letters and letter clusters) into the phonemes, or primary units, of language [17]. Manis et 01. [41], in a study of40 normal readers and 50 dyslexic children who were given a battery of various visual and reading tests, found that the most revealing difference between the groups was in the level of phonological processing abilities. Based on a review of the literature and on their own results, these authors concluded that most developmental dyslexics have a specific language disorder that involves some aspect of phonological processing. This result is consistent with the work of Tallal [59] who has demonstrated deficits in the perception of rapid auditory transitions necessary for phonemic perception among dyslexic individuals. Similarly, Catts [9] argues that research should not rely solely upon traditional definitions of dyslexia (e.g. as a discrepancy between reading achievement and chronological age), but should also understand developmental dyslexia more comprehensively as involving deficits in phonological processing. Despite the traditional conceptualization that phonological abilities are tied specifically to the left hemisphere, as in the case ofdyslexia, recent research has suggested that phonological reading is mediated to some degree through interhemispheric transfer functions of the corpus callosum [33,62,63]. In several studies of callosal agenesis, Temple and colleagues C33.62, 631 found that the only language impairment that has been consistently observed was a deficit in the retrieval of words from rhyming cues. Thus, while acallosal patients are not typically labeled dyslexic, the pattern of reading in acallosal subjects is comparable to developmental phonological dyslexia in that these subjects were impaired on tasks ofexplicit phonological processing involving both recognition and production of rhyme. Thus, poor hemispheric communication processes may underlie various forms of reading disability. Deficits in performance on tactile-motor and visumotor measures of interhemispheric coordination have been found in dyslexic children and in acallosal patients who also manifest phonological processing deficits. The aim of the present study was to determine

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whether adults with general reading difficulties and/or specific phonological difficulties perform more poorly on measures of tactile bimanual coordination.

processing

METHOD Subjects

Subjects were 42 right-handed junior college students between the ages of 18 and 40. Twenty-one students were identified as dyslexic by the college Learning Resource Center, while 21 non-dyslexic students from the same school served as controls. A brief preliminary telephone interview was conducted to see if the potential subject met any of the exclusion criteria. Ifnone were present, an appointment was made for testing. At the time of the appointment each subject was given an explanation of the study and informed consent was obtained. At this time a more thorough screening was conducted to exclude those subjects who did not meet the following criteria: (a) right-handed, as established by the Edinburgh Handedness Inventory (primary use of the right hand for at least 90% of activities) [47]; (b) no uncorrected vision or hearing problems; (c) no history of neurological disorder (e.g. epilepsy, MS, etc.): (d) no head trauma with loss of consciousness greater than I hr; (e) English as first and primary language; (f) no significant past or present substance abuse; (g) no prescription medications which affect central nervous system functions; and (h) normal intelligence. For experimental subjects, consent was obtained to use existing test scores obtained from the Learning Resource Center, and a Full Scale WAIS-R IQ score of 80 or higher was necessary for inclusion into the study. For control subjects, a shortened version of the WAIS-R was administered 16, 11,543 and an estimated Full Scale IQ score of 80 or above was necessary for inclusion. Reading ability was measured by the Woodcock-Johnson Psychoeducational Battery (WJPB). All experimental subjects had been tested with the original version of the WJPB and were included in the study if they met eligibility requirements for services in the Learning Resource Center. Eligibility for services was determined by performance scores on aptitude and achievement tests including the WJPB and WAIS-R, and was calculated according to the guidelines for identifying community college students in California with learning disabilities [8]. For all experimental subjects, a significant discrepancy (as outlined by the Community College Assessment System) was noted between reading scores on the WJPB and performance on the WAIS-R. In addition, each experimental subject had a primary diagnosis ofdyslexia as determined by Reading Cluster scores from the WJPB. Experimental subjects were not included if they had been given an additional disagnosis of dyscalculia, dysgraphia, attention deficit hyperactivity disorder, or other non-reading related disability. All control subjects were screened using the three reading subtests of the WJPB-Revised to rule out reading disability. For inclusion in the study, the mean of the three standard scores of the WJPB-R reading tests could not be below one standard deviation (i.e. 85), nor could any one of the subtests’ standard scores be below 80. Instruments

Coordination Task (BCT) is an adaptation of the Preilowski Bimanual Coordination Task. The Bimanual bimanual task [Sl] which uses a modified Etch-A-Sketch toy 171. A series of three separate overlays were used on the Etch-A-Sketch which contained paths which the subject was to negotiate with a cursor, the vertical movement of which was controlled by the right hand, and the horizontal by the left hand (see Fig. I ). One rotation of a knob moved the cursor 33 mm vertically or horizontally. Each path was 120 mm long from beginning to end, and was 10 mm wide. Subjects began by performing unimanual trials. Subjects were asked to complete two timed trials on a vertical path using their right hand only, then two timed trials on a horizontal path using their left hand only. Following the unimanual trials, a series of bimanual trials were presented. A total of six bimanual angles were attempted, with two timed trials performed on each angle. The first three angles (22.5, 45 and 67.5 deg) all required the clockwise movement of the Etch-A-Sketch knobs by both the right and left hands. Subjects began with one untimed practice trial on the 45 deg angle (which required both hands to move simultaneously at the same speeds), and then performed two timed trials at this angle. Subjects then alternated two timed trialseach at angles 22.5 (which required the left hand knob to be rotated at twice the speed of the right) and 67.5 deg (which required right hand rotation at twice the speed of the left). The second three angles (112.5, 135 and 157.5 deg) all required movement of the Etch-A-Sketch knobs in opposite, or mirror directions (i.e. left hand turned counterclockwise while right hand turned clockwise). Subjects began with one untimed practice trial at 135 deg (which required equally fast simultaneous movement), then performed two timed trials at this angle. Subjects then alternated two timed trials each at angles 112.5 (which required faster right hand knob rotation) and 157.5 deg (which required faster left hand rotation). Time to completion and length of the line drawn by the cursor were averaged over each of the two attempts at each path. To initiate the task, the examiner gave a command of “Ready? Begin”. Timing was begun when the subject began the cursor movement and ended when the cursor crossed the end line of the path. Line length was measured using a hand-held planimeter, beginning at the starting point of the cursor and ending at the end line of the path (or its extension). The more accurate the performance, the shorter the line.

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B

Fig. 1.Examples of the three Etch-A-Sketch overlays and target paths. Top: Two unimanual paths for right hand (A =90 deg) and left hand (B =0 deg) performance. Middle: Three right angled paths for bimanual performance (C =45 deg, D = 67.5 deg, E = 22.5 deg). Bottom: Three left angle target paths for bimanual performance (F = 135 deg, G = 112.5 deg, H = 157.5 deg). Ratio of target path width to length is not to scale.

One additional untimed trial on each of the six angles was performed in a condition whereby visual feedback was denied the subject for the second half of the trial. For this set of trials, a barrier was placed over the second half of the path which obstructed the subject’s view. The subject was able to monitor performance with visual feedback during the first half of the trial, but had to rely solely upon tactile and kinesthetic feedback of movements to successfully complete the second half of the trial. Length of the line drawn was used as the measure of performance for these trials.

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Rhyme Fluency Test. A series of rhyming tests developed by Temple [60,61] were used to measure phonological processing ability in both the experimental and control groups. Administration procedures were as follows: A verbal fluency task was administered in which the subject was asked to generate as many words as possible which begin with a specified letter (e.g. ‘F’, ‘A’, ‘s’) in a 1 min period. Temple’s Rhyme Fluency Test [60,61] was then given which asked subjects to generate as many words as they could in a 1 min period which rhyme with each of I2 stimulus words (e.g. fear, ring, nine). In scoring this task, both the total number ofcorrect rhymes and the number of incorrect responses were recorded. A Rhyme Fluency Ratio score was derived by dividing the total number of rhymes generated on the Rhyme Fluency Test by the total number of words generated on the verbal fluency test. Data analysis Two separate forms of analysis were run, each using a different measure of basic reading skill. The first analysis compared BCT performance by group based upon a diagnosis of dyslexic or non-dyslexic using group-by-condition ANOVAs. Analyses were run for both BCT time and BCT line length. Average time and line length scores were derived for the two attempts at each bimanual angle, as well as average times for unimanual paths. In order to minimize the number of dependent variables, however, bimanual scores for overall time and line length were averaged for right angles (i2.5, 45 and 67.5), left angles (112.5, 135 and 157.5), angles requiring right-hand facilitation (67.5 and 112.5). and angles reauirine. left-hand facilitation (22.5 and 157.5). Each averaee score was also computed separately for l&e length on trials completed without visual feedback. The second analysis involved correlations between BCT performance and performance on the Rhyme Fluency Test and the Block Design subtest of the WAIS-R. These correlations were computed in order to test relationships between BCT performance and predominantly left hemisphere (Rhyme Fluency Test) vs right hemisphere (Block Design) functioning.

RESULTS Table 1 contains descriptive information for the two groups, including reading and rhyming scores. There were no statistically significant group differences for age, IQ or gender within the present sample. However, significant differences by group were observed for reading ability on all three reading subtests of the WJPB, as well as for verbal fluency. Despite significant differences between the groups on all WJPB scores, the range of scores revealed significant between-group overlap on individual subtests (i.e. group affiliation was not necessarily discernible from one subtest alone). The dyslexic group had a lower mean Rhyme Fluency score, but the difference was not significant, nor was the ratio different for the two groups. Similarly, despite lower scores for dyslexics, the groups did not differ significantly in scores on the Block Design subtest of the WAIS-R. Table 1. Subject characteristics

M

Age IQ* Block design Male/Female WJPBt scores: LWID** WA** PC** Rhyme fluency Verbal fluency** Rhyme fluency/verbal

Dyslexic (S.D.)

24.76 99.38 10.85

fluency

90.43 87.57 89.00 62.29 34.52 1.90

(5.4) (10.5) (2.5) 10/l 1 (9.9) (12.7) (10.0) (16.6) (7.5) (0.68)

Control M 23.81 104.90 I I .48

112.52 110.90 108.95 75.33 42.24 1.79

(SD.) (5.9) (13.7) (2.5) IO/l I (15.9) (17.2) (16.1) (27.4) (10.6) (0.56)

Note: LWID = Letter-Word Identification; WA = Word Attack; PC = Passage Comprehension. *IQ is full scale for dyslexics and estimated for controls. tWJPB scores are original version for dyslexics and revised version for controls. **P
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Analysis of Bimanual Coordination Task performance by group was first made based on unimanual time scores. A significant group difference was obtained when a group-by-hand ANOVA was performed [F (1,40) = 6.97, P < 0.0121, and a significant effect of hand was also found [F (1,40)=4.76, PcO.0351. There was no group-by-hand interaction. In all cases, dyslexics were slower than non-dyslexics. Although bimanual time scores were also generally slower in the dyslexic group, a groupby-angle (left vs right) ANOVA yielded a non significant main effect for group. There was a significant effect for angle [F (1,40) = 4.58, P < 0.041, which was presumably due to the effect of presentation order. A group-by-hand facilitation ANOVA of bimanual time yielded no significant main effects or interaction. ANOVAs of the length of the line traversed in completing each path (a measure of accuracy or error) were carried out. The results of a group-by-angle (right vs left) ANOVA of line length revealed significant main effects for group [F (1,40) = 13.79, P-cO.OOl] and for angle [F (1,40)= 5.01, P
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Table 2. Analyses

Dependent

of bimanual

coordination task average line length for angled paths and hand facilitation by group

variables

M (mm)

Right angles Dyslexic Control

131.06 127.25

Left angles Dyslexic Control

130.30 125.22

Right angles WOVF Dyslexic Control

128.27 125.48

Left angles WOVF Dyslexic Control

126.24 123.70

Right-hand Dyslexic Control

132.84 128.02

facilitation

Left-hand facilitation Dyslexic Control Right-hand Dyslexic Control

et al.

d.f.

7.49

1

0.005

15.58

I

0.0005

5.03

1

0.015

17.16

I

0.00025

6.34

I

0.01

17.85

1

0.00025

3.65

I

0.03

12.35

I

0.0025

132.08 125.73

facilitation

Left-hand facilitation Dyslexic Control

P

F

WOVF 127.25 124.71 WOVF

Note: WOVF = without

128.78 124.71 visual feedback.

(I= -0.23). When comparing Block Design performance with BCT line length scores that are based upon hand facilitation, significant correlations were noted for both right and left hand facilitation when visual feedback was given (right: r = - 0.39, P < 0.01; left: r = - 0.39, PcO.01). Once again, for trials without visual feedback, significant correlations were found only on the condition which required left hand facilitation (left: I = - 0.42, P < 0.005; right: Y= -0.12, n.s.). Significant negative correlations suggest that less accurate BCT performance (longer line length) is associated with lower Block Design scores.

DISCUSSION The data from this report demonstrate that dyslexic and non-dyslexic individuals, differentiated on the basis of psychoeducational diagnostic criteria, showed the following patterns of performance on the BCT: (1) dyslexics were somewhat slower in unimanual knob-turning speed with both their left and right hands; (2) the groups did not differ significantly in speed of bimanual responding; (3) dyslexics were markedly less accurate in bimanual performance; (4) group differences in bimanual performance were most significant when the hands were required to make opposite (mirror-image) turning movements (i.e. left angles), and when the left hand was required to make faster turning motions than the right hand (i.e. left-hand facilitation); (5) group differences in performance were still apparent,

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although not markedly larger, when bimanual performance was attempted without visual feedback; and (6) BCT performance accuracy was correlated with the Block Design subtest of the WAIS-R, but was not correlated with the Rhyme Fluency test of phonological processing. While unimanual performance suggested differences between groups in motor speed, differences in response speed were not apparent in bimanual performance. Nevertheless, clear differences appeared in response accuracy. It is likely that, despite instructions which gave equal stress to speed and accuracy, subjects in both groups adopted a strategy of speed at the expense of accuracy. Thus, dyslexics responded to bimanual tasks with speed equivalent to normals, but only at the expense of significantly less accurate lines. Performance of dyslexic adults on the BCT in the present study was comparable to that reported by Gladstone et al. [22] for dyslexic children. While Gladstone and colleagues found group differences between dyslexic boys and controls on both speed and accuracy, the present study noted no significant bimanual time differences. Similar results were found, however, for accuracy of bimanual performance. Both studies found a significant left-hand facilitation disadvantage, a significant right angle (mirror movement) disadvantage, and a significant disadvantage on trials performed without visual feedback for dyslexics as compared to controls. The fact that the task was not highly practiced by the sujects, and thus not overlearned, suggests that performance was most likely a product of control processes, which would rely on cortical mechanisms, rather than accomplished by more automatic processes. Thus, one explanation for deficits in bimanual performance in dyslexics involves postulation of a lateralized cortical processing deficit. Many believe that developmental dyslexia can be best explained as a dysfunction of the language dominant left hemisphere [20, 591. On the basis of this hypothesis, it might be expected that other left hemisphere-controlled skills, such as fine motor manual skills [37] or motor sequencing abilities [2] would be compromised in dyslexics. Tallal [59] has argued that the primary dysfunction in dyslexia has more to do with deficits in the temporal processing of rapid sequential information by the left hemisphere. This deficit has been shown to be present in children not only for verbal output, but in motor tasks as well [34]. Thus, the current deficits noted in BCT might be explained by a deficit in the ability of the left hemisphere to modulate and control the rapid sequential motor movements required in the BCT. With regard to the current data, some of the BCT deficits in the dyslexics are consistent with left hemisphere dysfunction. For example, dyslexics were found to be impaired on such ‘left hemisphere’ measures as right-hand unimanual speed and accuracy on bimanual paths requiring right-hand facilitation. However, under a left hemisphere damage theory one would not expect the differences found in the present study for left-hand unimanual motor speed, nor would one predict the more robust group differences seen for performance on bimanual paths which demand left-hand facilitation. Because successful bimanual performance would likely place demands on complex visuospatial analysis, an argument could be made that BCT deficits reflect right hemisphere dysfunction in the dyslexic group. Deficits in spatial representation have been found in dyslexics which suggest dysfunction of the right hemisphere [57]. Indeed, significant correlations were found in the current study between a measure of visuospatial abilities (Block Design) and several line length measures on the BCT. Thus, right hemisphere visuospatial abilities must play a significant role in BCT performance. However, ifthis theory was to account for all of the BCT performance differences between dyslexics and normals,

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one might not also expect to find slower right-hand unimanual performance in dyslexics as was demonstrated in the present study. Results of the current study, including the relative effect sizes for the various measures of bimanual performance, might also be explained by theories of interhemispheric interaction. One version of this explanation relies on a callosal transfer model of bimanually coordinated fine motor activity. According to the connectionist model originally formulated by Liepmann [39], the left hemisphere has a unique role in fine motor control of either hand, with left-hand praxis dependent on callosal transfer. Current research reveiwed by Heilman and Rothi [27] points to callosal lesions as a probable explanation for some abnormalities of skilled movements of the left hand. For example, several studies of patients with left hemisphere lesions [14,25,36,69], as well as studies using sodium Amytal to sedate the left hemisphere [43], have shown bilateral impairment of skilled movement, implying dependence of right hemisphere (left hand) skilled motor function on the left hemisphere. More importantly, the existence of left-hand (but not right-hand) apraxia due to callosal lesions has been demonstrated in a number of studies [23,40,67]. Bogen [4,5] has suggested that callosal transfer from the right to the left hemisphere may contribute to right-hand praxis in the case of specific higher cognitive performances like drawing. Nevertheless, nondominant-hand praxic function still appeared to require some amount of interhemispheric transfer of information for the control of the fine motor activity of the left hand. Within this model, poor bimanual coordination in dyslexics might reflect deficient crosscallosal control of right hemisphere (left hand) motor activity by left hemisphere processing systems. Such a theory would explain the larger, more statistically significant group differences in bimanual performance tasks which demand faster left hand responding, or mirror image responding. Thus, deficient callosal transfer of motor information should be most apparent when the left hand must respond faster than the right, or when it must respond in a manner opposite to that of the right hand. However, not easily explained by this theory of deficient callosal transfer are the present differences found in right-hand unimanual speed. Another model which provides a satisfactory explanation for the present results is a model of callosal function which focuses on shielding from interhemispheric interference. In this model, motor function would be faster and/or most accurate on the BCT when each hemisphere is allowed to function autonomously, without interference from the other hemisphere. This may be particularly true in the case of a rather simple motor response like knob turning. In the case of right-handed subjects, the left hemisphere is more likely to interfere with right hemisphere (left hand) functioning during bimanual performance than vice-versa. The integration of hand performance necessary to draw an accurate sloping line may occur via bilaterally available visual information (at least when working under visual control), rather than callosal sharing of praxic functional control. Under this theory, the current data would indicate that interhemispheric interference is present to a greater degree in dyslexic rather than normal individuals, particularly for bimanual tasks requiring faster left hand responding. Thus, markedly deficient bimanual performance by dyslexics reflects a failure of shielding from cross-callosal interference, particularly from the interference created in right hemisphere (left hand) systems due to simultaneous left hemisphere motor activity. Slower unimanual speed in dyslexics might also be due to more bidirectional interhemispheric interference in a task best done by one hemisphere working alone. A model of callosal functioning which suggests that its function is to sheild each hemisphere from interference is consistent with the results found on the BCT task in patients

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without a callosum. In the case of complete commissurotomy patients [70], rather substantial deficits were apparent in a similar Etch-A-Sketch version of the BCT to the one used in the present study. In the case of both partial (anterior) split-brain patients [Sl] and acallosals [31], very mild (if any) disabilities were found for bimanual performance under visual guidance. However, more dramatic deficits did appear in these subjects when between hand integration, available via visual feedback, was removed. In the presence of visual feedback, callosal agenesis or surgical disconnection of the anterior corpus callosum allowed for good shielding of hemispheric activity, and, because visual input was available to each hemisphere, allowed for relatively normal performance. In the trials where visual feedback was denied, the absence of the corpus callosum prevented the interhemispheric tactile-motor interaction necessary to successfully perform the task on the basis of hemispherically integrated kinesthetic or praxic information alone. The result was clear performance deficits. Thus, an explanation of BCT deficits in dyslexic individuals which emphasizes failure of shielding and the occurrence of hemispheric interference accounts for performance under visual control. The suggestion that both reading disability and impaired bimanual tactile-motor performance may be related to callosal dysfunction has support from other studies in the literature [22, 24, 37, 683. Klicpera et al. [37], in a study that found deficits in bimanual finger-tapping in dyslexic boys, suggested that “impaired mechanisms of interhemispheric communication account for both reading retardation and motor deficits” (p. 622). However, Wolff et ul. [68] suggested, on the basis of the apparent absence of dyslexia in the majority of acallosals, that “impaired efhciency of interhemispheric communication is not of etiological significance in developmental dyslexia” (p. 357). The conclusions of Wolff et al. should be tempered, if not denied, by recent work on phonological processing deficits in acallosals [33, 62,631. If dyslexia results from uncontrolled cross-callosal interference, then one might not expect to see classically defined dyslexia in the acallosal or split-brain individual. Absence of relationships between BCT performance and specific phonological processing abilities (rhyming), in the presence of strong relationships between BCT performance and dyslexia diagnosis, is surprising. This may be related in part to the nonsignificant group differences in rhyming. Since phonological processes (e.g. rhyming ability) has been shown to be an important aspect ofdyslexia [9,41,59,65,66], these data support the hypothesis oftwo reading-related problems, one associated with performance on those tests forming the criteria for a dyslexia diagnosis in adults and associated with BCT performance deficits, and another which is rooted more specifically in phonological processes and unrelated to BCT performance. In summary, the present study has shown a clear deficit in bilaterally coordinated motor activity among dyslexic adults. A hypothesis based on a primary left hemisphere dysfunction in these dyslexics is not well supported by these data. Rather, our data support dysfunction of the right hemisphere, the corpus callosum, or both.

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