Callosal transfer of finger localization information in phonologically dyslexic adults.

Dyslexia, particularly phonological dyslexia, has been hypothesized to be associated with deficits in interhemispheric interactions mediated by the corpus callosum. Twenty-one dyslexic subjects were compared to 21 controls on the Finger Localization Test in order to observe patterns of tactile-motor integration and interhemispheric collaboration. When compared to control subjects, dyslexics showed consistent deficits in finger localization, particularly when more complex trials had to be completed. When subjects were re-grouped according to phonological processing ability (regardless of diagnostic group membership), subjects with lower phonological processing scores were found to perform poorly on complex trials of finger localization, particularly on those trials which reflect callosal transfer of finger localization information from one hemisphere (hand) to the other. These results suggest that interhemispheric transfer of information is critical to efficient phonological processing.

Bimanual coordination in dyslexic adults.

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 of deficits 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.