Neocortical asymmetry and open-field behavior in the rat.

The total volume of the neocortex in each cerebral hemisphere was measured in male and female rats that had been exposed to one of two early experience treatments and subsequently tested behaviorally. Rats were either handled in infancy and raised in enriched environments or not handled in infancy and raised in standard lab cages. In adulthood they were tested in the open field for activity, for initial direction of movement out of the start box, and for laterality of wall-hugging behavior. Early experience did not have an effect on neocortical asymmetry, but a gender difference was disclosed. The right neocortex of males was larger than the left by a mean of 1.5%. Females rats did not show a significant asymmetry, although when asymmetry was present it was in favor of the left side. Cortical asymmetry correlated with open-field behavior. The degree of activity decreased with increasing neocortical asymmetry, and an interesting gender effect was present. Also, there was a positive correlation between neocortical asymmetry and direction of wall-hugging behavior, but only in animals with volume differences of less than 1%. These findings represent the first documentation of behavioral correlations with anatomic brain asymmetries in nonhuman species.