Functional development of the prefrontal cortex in early life and the problem of neuronal plasticity

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Monkeys given selective orbital or dorsolateral prefrontal lesions in infancy were compared with those given identical lesions later in life on a battery of tests known to be sensitive to prefrontal disorders. The monkeys were initially tested when they were 12–18 months of age and were later reassessed on one of the prefrontal measures at 24 months of age. At the earlier testing ages, the monkeys given orbital lesions in infancy were as impaired as monkeys given identical lesions as juveniles, while the monkeys given dorsolateral resections as infants exhibited sparing of function in comparison with those operated upon in the juvenile period. At the later testing age, however, the two groups of monkeys that were operated upon in infancy reversed their standing: the group with orbital lesions now gave evidence of virtually complete recovery whereas the extent of recovery after the dorsolateral lesions was less impressive. These findings suggest two types of functional sparing after early brain damage: one, exemplified in the case of early dorsolateral lesions, in which the sparing becomes increasingly less impressive as the cortical area in question attains functional maturity; and the other, revealed in the monkeys with the early orbital ablations, in which a residual area may come to subsume the functions of the damaged cortex. This latter reparative process, it is proposed, is likely to occur when certain essential conditions are met: there is a region remaining intact after surgery which is functionally related to the damaged cortex; and, this region is relatively “uncommitted” to its own functional development at the time of brain injury.