Animals were tested for anteromedial and dorsomedial thalamic nucleus lesion effects on three activity tests and on a passive avoidance task. Insignificant effects were obtained on the activity tests and there was no correlation between activity scores and passive avoidance performance. The anteromedial thalamic lesioned group were retarded on passive avoidance performance while surgical controls and the dorsomedial thalamic nucleus lesioned group rapidly learned the avoidance response. ProblemIn a recent review of much of the neurological literature (Gerbrandt, 1964) it was concluded that the passive and active avoidance tasks represented fundamentally different problem solving situations in which the operation of distinct brain systems is critical. On the one hand, the passive avoidance task was used as a prototype for situations where the correct task solution was of a low probability of elicitation and in competition with a standing response of a higher probability of elicitation. The active avoidance task, however, was used as a prototype for situations necessitating the immediate elicitation of a high probability response where lower probability mediating responses might interfere. Lesions within a circuit similar to that suggested by Papez (1937) seemed to interfere with r a is e pro b a b i Ii t Y functions, while lesions within much of the medial and nonspecific thalamic nuclei and their projection fields seemed to interfere with h i g h pro b a b iIi t y functions.This investigation is intended to fill in two of the gaps which were left in the research relevant to the anatomical generalizations previously made. First, although it is well known that the anteromedial thalamic nucleus is an important link in the Papez circuit, this nucleus has not been specifically lesioned and the animal tested for functioning on a passive avoidance task. Second, the dorsomedial thalamic nucleus has been implicated as functioning on active avoidance tasks (Thompson, 1963) but these lesioned Ss were not tested for passive avoidance performance. In the present study Ss are lesioned in the dorsomedial and anteromedial thalamic nuclei and tested for passive avoidance performance. MethodA total of 30 male hooded rats at 120 days of age were divided into groups of 10 each for surgical treatments. All Ss were anesthetized with Nembutal, stereotaxically mounted, and were surgically prepared for electrode trephining. The surgical control (SC) Ss were sutured PI)'Chon. Sci., 1965, VoL 2.

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