Abstract
Previous work has shown that medial frontal lesions in adult rats produce deficits on spatial reversals, delayed responses, and active avoidance, whereas more ventrolateral (orbital) lesions have little effect on these tasks. These findings were confirmed. However, when lesions were made at 2, 5 or 9 days of age, rats tested as adults showed dramatic sparing of function on all these tasks, whereas similar lesions inflicted at 35 or 40 days of age produced deficits on delayed responses and spatial reversals. Orbital frontal lesions failed to significantly alter behavior on any task at any age. The effects of the neonatal lesions are in striking contrast to the effects of juvenile or adult lesions, even though the infant lesions were considerably larger. Histological analysis revealed no retrograde degeneration in the dorsomedial thalamic nucleus of the rats operated in infancy unless the lesions were so large that the adjacent caudate-putamen was also damaged. The contrast between these results, and results obtained after similar lesions in adults may indicate the growth of sustaining collaterals after the infant lesions which might be involved in the observed behavioral sparing.
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