Developmental determinants of cortical plasticity.

A central problem in neuropsychology concerns the mechanisms by which functions are restored following injury to the brain. Functional recovery is particularly striking following damage to the cerebrum in early life and accordingly our studies deal with the effects of prefrontal cortical lesions performed on infant monkeys. We describe here two essentially different patterns of behavioral recovery following selective lesions of the dorsolateral and orbital prefrontal cortex. Following orbital lesions in infancy, the monkeys are impaired when tested initially between 12 and 18 months of age but give evidence of substantial recovery when they are retested at 24 months of age. In contrast, monkeys given dorsolateral resection early in life exhibit sparing of function during the initial period of testing but this impunity usually associated with dorsolateral injury in infancy gives way later to a picture of retarded development. We attribute these two patterns of recovery - progressive and regressive - to differential rates of development of the two prefrontal cortical regions involved. Our evidence suggests that while orbital cortex becomes functionally mature by 1 year of age and probably earlier, the elaboration of dorsolateral function is delayed well into the second year of life. We propose that the dorsolateral cortex, by virtue of its relative immaturity at the time of orbital excision, can come in time to subsume the functions of the orbital cortex but that the orbital cortex may be functionally "committed" too early in life to compensate reciprocally for injury sustained by the dorsolateral cortex.