A disconnection syndrome due to agenesis of the corpus callosum: disturbance of unilateral synchronization

Recently, interhemispheric disconnection syndromes have been noted in patients with agenesis of the corpus callosum (ACC) during the performance of certain tasks. However, few studies have demonstrated an asymmetric disconnection syndrome. In this report, we present just such a syndrome in a patient with ACC, who manifested ambidexterity (but with a left-hand tendency) and had high intelligence, no neurological deficits, and no associated malformations. In a comparison with similar subjects (amateur musician), we studied her asymmetric deficits using four tasks: (1) simple reaction time for visual stimuli, (2) paced finger tapping in synchrony with visual or auditory stimuli, (3) paced finger tapping without an external reference, and (4) rhythmical finger tapping in synchrony with visual or auditory stimuli. While the comparable subjects displayed no significant difference between hands, and the patient showed no significant difference between hands in the auditory paradigm, her tapping performance deteriorated significantly when asked to synchronize the left hand with timed visual stimuli, irrespective of whether finger tapping was paced or rhythmical. We believe that this phenomenon constitutes a novel asymmetrical disconnection syndrome in an ACC subject; these results suggest that synchronization of multimodal temporal information was lateralized in the left hemisphere (in this case), which is something that the ACC patient could not compensate for.