Lateralization of reward in rats: differences in reinforcing thresholds.

Fourteen rats with bilaterally implanted lateral hypothalamic electrodes were allowed to self-stimulate each side of the brain during daily test sessions. Rotation (circling behavior) during self-stimulation sessions was also recorded. All rats rotated in preferential direction regardless of the side of the brain stimulated, and, in each case, the direction was the same as that subsequently determined in response to d-amphetamine. All rats had asymmetries in self-stimulation thresholds related to the direction of rotation. Thresholds were lower on the side contralateral to the direction of rotation, and entire rate-intensity functions were displaced to the left on that side. The results, discussed in terms of lateralization of affect, suggest a model in which quantitative differences in neuronal firing can be translated into apparent qualitative specialization, with the two sides of the brain appearing to be specialized for high and low mood, respectively.