Neonatal Frontal Lesions in the rat: sparing of learned but not species-typical behavior in the presence of reduced brain weight and cortical thickness.

Rats with complete removal of the cortex anterior to bregma in adulthood (frontal cortex) were compared behaviorally and neuroanatomically with rats with similar removals at 7 or 25 days of age. Excision of the frontal cortex in adult rats produced transient aphagia, chronic motor abnormalities in feeding, a chronic drop in body weight, increased activity in running wheels, impaired performance at a spatial reversal learning task, and chronic abnormalities in a variety of species-typical behaviors, including swimming, food hoarding, and defensive burying. In contrast, similar lesions in infant rats failed to produce aphagia, a chronic drop in body weight, increased activity, or impaired learning of a spatial reversal task. Infant lesions did not allow sparing of complex species-typical behaviors, however, such as those involved in feeding, swimming, hoarding, or defensive burying. Furthermore, when the brains of neonatally operated rats were compared with those of control rats or rats operated on in adulthood, there were striking differences. The cerebral hemispheres of the neonatal operates were smaller both in surface dimensions and weight, the thalamus was smaller, and the cerebral cortex was thinner. These data imply that there may be substantially less sparing of function following frontal cortex lesions in infancy than previously believed and that neonatal frontal lesions in rats have significant effects on brain development in regions far removed from the actual site of surgical excision.