Callosal and cortical contribution to procedural learning

Acallosal and callosotomized subjects usually show impairments on tasks req uiring bilateral interdependent motor control. However, few studies have as sessed the ability of these subjects to learn a skill that requires the sim ultaneous contribution of each hemisphere in its acquisition. The present s tudy examined whether acallosal and callosotomized subjects could learn a v isuomotor skill that involved a motor control from either both or a single hemisphere. Eleven adult patients, six acallosal and five callosotomized, p articipated in this study. Seven of these patients had epileptic foci locat ed in the frontal and/or temporal areas and one of the acallosal patients s howed bilateral prefrontal atrophy following surgical removal of an orbitof rontal cyst. The performance of the experimental subjects was compared with that of 11 matched control subjects, on a modified version of a serial rea ction time task developed by Nissen and Bullemer (Cogn Psychol 1987; 19:1-3 2), This skill acquisition task involved bimanual or unimanual key-pressing responses to a sequence of 10 visual stimuli that was repeated 160 times. A declarative memory task was then performed to assess explicit knowledge o f the sequence, None of the experimental subjects learned the task in the b imanual condition. Patients with frontal epileptic foci or orbitofrontal da mage also failed to learn the task in the unimanual condition when they wer e using the hand contralateral to the damaged hemisphere. All other subject s, including the acallosal and callosotomized patients with temporal foci, learned the visuomotor skill as well as their controls in the unimanual con dition. In spite of the absence of transfer and interhemispheric integratio n of procedural learning, some of the acallosal and callosotomized patients were able to learn the sequence explicitly These findings indicate that th e corpus callosum and the frontal cortical areas are important for procedur al learning of a visuomotor skill, They also confirm the dissociation descr ibed by Squire (Science 1986; 232:1612-9 and J Cogn Neurosci 1992; 4:232-43 ) between the declarative and procedural memory systems and extend this dis sociation to processes involving simultaneous bihemispheric co-operation.