Right-sided population bias and lateralization of activity in normal rats.

Of 602 rats tested for amphetamine-induced or nocturnal rotation (circling behavior), 54.8% (P less than 0.025) had right side biases. Of 292 rats tested in two-lever operant situations, 57.5% (P less than 0.02) preferred the right lever. Right-biased rats were significantly more active and had stronger side preferences than left-biased rats. It is suggested that the increase in right-sided population bias in humans, as compared to rodents, is related to a cortical asymmetry and the phylogenetic increase in cortical modulation of subcortical structures.