Interhemispheric functional differences in prefrontal cortex of monkeys.

Monkeys had nonpolarizable electrodes implanted bilaterally in prefrontal (principal sulcus), precentral, and occipital cortex. They were trained on a spatial delayed-response (DR) task (8-sec intratrial delay), while cortical potentials were recorded. Three groups of monkeys were trained to 90% criterion: (A) 4 monkeys with only the right hand (the left wrist was attached to the testing chair); (B) 2 monkeys with only the left hand; and (C) 2 monkeys with the left and right hands on alternate sessions. Intermanual transfer tests were then given. Averaged steady potential (SP) shifts of several seconds duration were found in prefrontal cortex during cue presentation and the early portion of the intratrial delay and from the precentral area during the choice response. Evaluations of these SP shift magnitudes indicated: (1) Training with only one hand resulted in substantially larger SP shifts in the prefrontal and precentral areas contralateral to the responding hand; (2) alternate hand training resulted in somewhat larger prefrontal SP shifts in the right hemisphere; (3) intermanual transfer had marked effects on the precentral SP shifts, with larger magnitudes in the hemisphere contralateral to the responding hand, but had little effect on the magnitudes of both prefrontal SP shifts. (4) Subsequent training of Group C monkeys with only one hand resulted in greater SP shifts in the prefrontal area contralateral to the responding hand and in decreased SP shifts in the ipsilateral prefrontal area; and (5) additional intermanual transfer tests had no effects on SP shift magnitudes from both prefrontal areas. These findings indicate a dissociation in interhemispheric functions between the precentral and prefrontal cortical areas, with the formal implicated in motor organization for the contralateral limb, and the latter in mediation of mnemonic processes, primarily in one hemisphere. This hemispheric specialization is affected by the hand-training procedure, but other endogenous or experimential factors may be involved.