The recovery of forelimb-placing behavior in rats with neonatal unilateral cortical damage involves the remaining hemisphere.

Following unilateral lesions of the somatic sensorimotor cortex (SMC) in neonatal, but not adult, rats, an aberrant ipsilateral corticospinal projection originates from the undamaged hemisphere (Hicks and D'Amato, 1970; Leong and Lund, 1973; Castro, 1975). We have evaluated the contribution of the hemisphere contralateral to a unilateral lesion of the SMC in the recovery of tactile forelimb-placing behavior. Neither adult-lesioned or neonatally lesioned animals show evidence for placing deficits with either forelimb when tested 30 or 42 d after the lesion. However, in adult-lesioned animals, a subsequent lesion of the undamaged SMC on postlesion day 42 produces placing deficits only with the forelimb contralateral to the second lesion, while such a second lesion in the neonatally lesioned rats results in placing deficits with both forelimbs. Anatomical observations in the animals used for behavioral analyses confirm previous reports of a substantial ipsilateral corticospinal projection in rats with unilateral SMC damage as neonates and demonstrate that many of these aberrant fibers recross the midline within the spinal cord to arborize extensively within the ipsilateral spinal gray. These findings indicate that, following unilateral SMC lesions in neonates, the contralateral hemisphere mediates some aspects of the recovery of forelimb placing. The aberrant ipsilateral corticospinal projection may provide the anatomical substrate through which the cortex effects this recovery.