Differential effects of prefrontal cortex ablation in neonatal, juvenile, and young adult rats

Rats sustaining lesions restricted to the medial (MF) or the orbital (OF) pre-frontal cortex at various ages (10, 25, 40, or 60 days postnatally) were tested as adults on a series of behavioral tasks that are known to be sensitive to such lesions in adults. On spatial alternation learning, both the 40- and 60-day MF operates were seriously impaired, whereas neither the 10- nor the 25-day MF operates differed from control. On a hoarding task, 25- and 60-day MF operates hoarded less food than either controls or 10-day MF operates. Lesions of OF cortex in males at 40 or 60 days of age produced a significant increase in running-wheel activity; OF lesions in both sexes at 25 days of age or later produced a decrease in the rate of continuous reinforcement reacquisition (following experience with differential reinforcement of low rates of responding and bar-press extinction) relative to controls, whereas 10-day OF operates did not differ from controls on either task. Thus, rats with lesions of either frontal area at 10 days of age showed complete behavioral sparing on all measures. The effects of lesions at later ages varied with the behavioral tasks employed and with lesion locus. Although the 10-day operates received a somewhat longer postoperative recovery interval than most of the later operates, these results cannot be explained on the basis of recovery time alone. A control group of 60-day operates with the same recovery interval as the 10-day operates did not differ on any measure from the 60-day operates with shorter recovery intervals.