The effect of anterior thalamic nuclei lesions upon conditioned avoidance responses in rat.

Three groups each of 7 hooded rats had bilateral symmetric lesions of the n. anterior ventralis the n. anterior medialis and n. anterior dorsalis and were compared to two groups of 7 nonoperated control rats. After the lesions no changes in spontaneous behavior, sensory or motor functions, body weight, reaction type and thresholds to painful footshocks were observed. The postoperative acquisition of a one-way conditioned avoidance response in a simple runway task was significantly retarded in ventral and medial rats and impossible in dorsal rats. While escape reactions were not impaired, lesioned rats had troubles passing the start door early after the onset of the conditioned stimulus. During alternation training of avoidance responses, the ventral and the medial rats preferred one side of the Y-maze. When they learned to run to the illuminated exit as a high-probability stimulus after several sessions, the entire stereotype became more unstable and the percentage of avoidance responses decreased. None of the lesioned rats escaped shock in a pole-climbing test, which was characterized by very low probability of correct response in the first session. These anterior thalamic nuclei are part of the Papez circuit which may be the main substrate for learning and retrieval of problems with low probability.