[Involvement of thalamic nuclei in the formation of conditioned avoidance responses in rats. III. Lesions of the nucleus ventralis medialis].

Bilateral lesions of the nucleus ventralis medialis thalami (VM) did not change the reactivity to sensory and painful stimuli, the preoperatively fixed motor patterns and the preoperatively elaborated conditioned avoidance response (CAR, jumping test) in hooded rats (Long-Evans-strain). The postoperative acquisition of a CAR in a runway was attained as in controls, but with significantly higher running speed. During continuation of the training in a Y-maze using a 2:2-alternation schedule the CAR decreased gradually to zero as a consequences of increasing reaction times (time from onset of the acoustic conditioned signal to the onset of running). At least the lesioned animals only started with the onset of the painful reinforcement. In spite of the non-avoidance these animals learned to escape correctly through the illuminated floor (dark-light-discrimination). The results are discussed as a consequence of a deficit in conditioning of locomotor programs.