BEHAVIORAL EFFECTS OF MAMMILLOTHALAMIC TRACTOTOMY IN CATS

Abstract

Effects of mammillo-thalamic tractotomy on behavior were evaluated with three tests. Lesions were produced by stereotaxically positioned focused ultrasound and by d.c. electrolytic fulguration. Cats that sustained complete bilateral mammillothalamic tractotomy showed marked deficit in retention and relearning of preoperatively learned avoidance responses compared with the performance of sham-operated control cats or cats in which the lesion failed to interrupt the tracts completely. No disturbance of postoperative retention of a positively reinforced visual discrimination habit was found after bilateral mammillothalamic tractotomy. When naive cats receive avoidance conditioning after mammillothalamic tractotomy, no statistically significant loss in learning was observed. However, some tractotomized cats (4 out of 11) were severely impaired and failed to reach learning criterion. It is suggested that the lesions do not directly affect neural mechanisms subserving associative or storage aspects of memory, but the impairment of retention in the double-grill box is thought to derive from lesion-induced enhancement of an innate defensive response of "freezing" which is incompatible with an active avoidance response to the conditioned stimuli.