DECREMENTS IN AVOIDANCE BEHAVIOR FOLLOWING MAMMILLOTHALAMIC TRACTOTOMY IN CATS

Abstract

The present study is the 2nd in a series of studies investigating the function of the mammillothalamic system. The first study described a large decrement in cats'' retention of a shuttle-box conditioned, avoidance response following lesions of the mammillothalamic tract (MTT). Experiment 1 of the present study verified these findings and extended them to a slightly different shuttle-box training procedure. A less severe, but highly significant, decrement was also found following partial MTT lesions. This decrement was highly correlated with the amount of MTT damage (-.79). By utilizing small lesions produced by focused ultrasonic radiation, it was possible to demonstrate that lesions placed entirely within the MTT were sufficient to produce such a decrement. It was argued that the substantial decrements in avoidance retention following unilateral MTT lesions implicated the crossed fibers from the lateral mammillary nucleus to the anterodorsal nuclei of the thalamus as the crucial structure in producing the avoidance deficit. In Experiment 2 the cats were returned to the left side of the avoidance apparatus after every response and thus received shock in only the left compartment while the right compartment was always "safe". Under these conditions, MTT lesions had no significant deleterious effect on the retention of the avoidance response. Several hypotheses to explain the difference in avoidance decrements between the 2 procedures were discussed.