Search

Small lesions in the most posterior portion of the globus pallidus and adjacent internal capsule virtually eliminated fear or defensive reactions in wild Norway rats. Flight to an approaching experimenter, startle reactions to dorsal and vibrissae contacts, defensive attack to a conspecific and reactions to handling by human experimenters were grossly reduced following surgery and for a period of 30 days thereafter. After tail shock, however, some defensive behaviors reappeared. This pattern of behavior changes suggests a striking similarity between effects of the present small lesions and the “amygdala-lesion syndrome” of reduced defensiveness to nonpainful stimuli. It is also similar but not identical to the reduced defensiveness which follows damage to the midbrain central gray, with the difference between the two lesions suggesting that more complex fear-eliciting stimuli are processed at the higher site.

Experiments were carried out to study the effects of electrical stimulation of the hypothalamus on the performance of switch-off behavior motivated by electrical stimulation of the midbrain central gray. Stimulation of the medial hypothalamus paired with central gray stimulation facilitated the performance, resulting in a decrease in response time. Stimulation of the lateral hypothalamus except the midlateral portion inhibited the performance, thus increasing the response time. Stimulation of the midlateral hypothalamus facilitated the performance. It appears that stimulation of the medial hypothalamus enhances the motivation induced by central gray stimulation, and that stimulation of the lateral hypothalamus inhibits the performance with the induction of a different motivation. Midlateral stimulation facilitates the performance probably by elicited go-mechanism.

Rats trained on separate levers to escape and avoid electric shock received dorsal mesencephalic lesions. Experimental group animals were deficient in postoperative performance. Control animals showed no behavioral deficit. The lesions of the experimental group typically included the ventrolateral edge of the lateral periaqueductal gray matter (PGM) and the ventral PGM bilaterally. Control group lesions were medial to and dorsal to the experimental group lesions. The results indicate that the vicinity of the ventral and lateral PGM is functionally important for maintenance of conditioned responding to shock. Anatomical connections involving the PGM, lateral spinothalamic tract and rostral limbic regions are discussed relative to the role of the PGM in mediating the conditioned behavior.