Absence of interhemispheric transfer of unilateral visuomotor learning in young children and individuals with agenesis of the corpus callosum.

This study was undertaken to investigate the role of the corpus callosum in interhemispheric transfer of unilateral visuomotor learning. In the first experiment, the cross-manual performance of 4 callosal agenesis participants was compared to that of 4 age- and IQ-matched controls. In the second experiment, normal children of different ages (6-7 and 11-12 years) and adults were submitted to the same task to assess the impact of callosal maturation on interhemispheric transfer. Participants had to make aiming movements from a starting position toward either a central or a lateral target on the same side as the hand used, while maintaining central fixation. Prior to training, a pretest was performed with the hand contralateral to the hand used during learning. Participants were then submitted to a posttest with the untrained hand. All participants learned the unilateral aiming task in the learning phase, as evidenced by a reduction in spatial errors with an increasing number of practice trials. However, acallosal participants and children aged 6 to 7 years failed to transfer the acquired skill from the trained to the untrained hemisphere. These findings suggest that interhemispheric transfer of visuomotor skills cannot be assumed by other structures in the case of agenesis or morphological immaturity of the corpus callosum. The results further indicate that unilateral visuomotor learning leads to the formation of a single, unihemispheric engram in the absence, whether functional or anatomical, of the corpus callosum.