Anomalous bimanual coordination among dyslexic boys.

Wesleyan University and Haskins Laboratories New Haven, Connecticut Dyslexia has been associated with lc•bemisphere dysfunctions; however, recent studies also suggest interhemispheric difficulties. To test this, we assessed bimanual coordination in dyslexic and nondis- abled boys using an Etch-a-Sketch-like task. Group performance was equivalent for parallel hand movements (both clockwise). However, the dyslexics showed significant impairments on mirror movements, particularly in their left hands, and often unknowingly reverted to parallel movements when visual feedback was removed. Although these difficulties generally suggest impaired interhemi- spheric coordination, specific mirror movement deficits have never before been observed in any population. We propose that deficient interhemispheric collaboration combined with anomalous ipsilateral manual control may account for the dyslexics' performance. Recent theoretical proposals suggest that efficient interhemi- spheric integration is important in reading development (Glad- stone & Best, 1985; Kershner, 1985a) and that dyslexic children may have deficits in interhemispheric collaboration (Davidson, Leslie, & Saron, 1988; Denckla, 1986; Gladstone & Best, 1985; Kershner, 1985b). A number of empirical investigations lend support to this proposal. Compared with normal readers, read- ing impaired children show less interhemispheric electroen- cephalogram (EEG) coherence (SEar, Hanley, & Simmons, 1973). Anatomical measures of the corpus callosum, the major neocortical commissure, have been reported for two dyslexic brains; both showed abnormal thinning (Drake, 1968; Witelson in McGuinness, 1985). Behaviorally, certain tachistoscopic (Gross-Glenn & Rothberg, 1984; Yeni-Komshian, Isenberg, & Goldberg, 1974) and dichotic (Kershner, Henninger, & Cooke, 1984; Obrzut, Obrzut, Hynd, & Pirozzolo, 1981) studies of reading disabled children have also suggested abnormal inter- This article is based on research conducted for the partial fulfillment of the doctorate degree requirements by Marshall Gladstone. The work and manuscript preparation were supported by March of Dimes Birth Defects Foundation Grant 12-165 awarded to Richard J. Davidson and by National Institute of Child Health and Human Development Grant HD- 1994 to Haskins Laboratories. We thank the following people for their invaluable help on this proj- ect: Barry H. Cohen, for statistical and methodological support; Clifford Saron, for technical support; Susan Leslie and Orit Batey, for neuropsy- chological testing; Martha Denckla, for her thoughtful comments on an earlier draft of the article; Mary Beth Hickey and Nina Huza, for their assistance in typing the paper; and particularly our subjects and their parents, for their participation in the research. Correspondence concerning this article should be addressed to either Marshall Gladstone, Department of Clinical Psychology, Institute of Living, 400 Washington Street, Hartford, Connecticut 06106, or Rich- ard J. Davidson, Department of Psychology, University of Wisconsin, 1202 West Johnson Street, Madison, Wisconsin 53706. hemispheric sharing of sensory information. Whereas normal age-matched children display a stable right-ear advantage on verbal dichotic tests, reading disabled children switch the direc- tion of ear advantage as a function of which ear is attended to (Obrzut et at., 1981) and whether the response mode is oral or written (Kershner et at., 1984). This "attentional switching" a phenomenon usually seen only among younger normal chil- dren, has been attributed to the inadequate regulation of atten- tional resources between the hemispheres (Hiscock & Kins- bourne, 1980). Thus, although contemporary views of dyslexia have more commonly attributed reading failure to left-hemisphere impair- ment (Rudel, 1985), recent evidence has also implicated deficient interhemispheric attentional regulation and sensory integration. However, a number of questions about interhemi- spheric coordination among disabled readers remain unan- swered. For example, relatively little is known about the role of interhemispheric processes in the bilateral motor anomalies that have been clinically observed for many years in reading disordered children (Denckla, 1985; Tomaino, in press). Re- cently, Denckla (1985) has stated that "Orton's [ 1937] concept of mixed dominance may be important not in terms of 'com- peting engrams' in the visual field, but rather in terms of [inter- hemispherically] competing motor coordination systems in the anterior portions of the brain" (p. 193). In light of the theory and data suggesting impaired interhemispheric communication among dyslexics, it would be important to determine whether their atypical motor performance is also indicative of anoma- lous interhemispheric processes. The motor anomalies of "pure dyslexic" children (i.e., chil- dren with primary reading impairment but without other sig- nificant language, neurodevelopmental, or emotional disorders) appear to be largely confined to fine or repetitive movements of the distal musculature of the fingers and hands. For example, impairment in rapid alternating finger movements was the only 236