Opposite effects of unilateral forebrain ablations on ipsilateral and contralateral hypothalamic self-stimulation

Unilateral ablations of frontal cortex, rostral striatum, nucleus accumbens, septal area and olfactory tubercle decreased ipsilateral hypothalamic self-stimulation; the same ablations had the opposite effect on contralateral self-stimulation. The ablations shifted the function relating response rate to stimulation frequency (rate-frequency function) to the right for ipsilateral self-stimulation and to the left for contralateral self-stimulation, suggesting a reduction and an augmentation, respectively, of the rewarding impact of the stimulation. The inhibition of ipsilateral self-stimulation was neither total nor permanent; 20-30% shifts in threshold were seen at first, but behavior returned to near-normal levels over a period of several weeks. In contrast, the augmentation of contralateral self-stimulation showed no significant change over the same period; in this case the 20-30% shifts in threshold were immediate and permanent. The degree of change in ipsilateral threshold was positively correlated with lesion size; the degree of change in the contralateral threshold was not. Ablations restricted to cortical tissue caused a lesser degree of augmentation of contralateral self-stimulation and had no effect on ipsilateral self-stimulation. The small effects of large ablations on ipsilateral self-stimulation confirm similar observations of Huston and Stellar and their co-workers and raise questions for current theories regarding the role of dopamine in brain stimulation reward. The facilitation of contralateral self-stimulation indicates that brain stimulation reward does not involve a completely lateralized mechanism.