Cortical motor neuron activity in the cat during classical conditioning with central stimulation as the CS and the US

The neuronal basis of associative conditioning in the cat was investigated by pairing stimulation of thalamocortical pathways as the conditioned stimulus (CS) with antidromic activation of pericruciate pyramidal tract (PT) cells as the unconditioned stimulus in a differential classical conditioning paradigm. Contrary to expectations, based both on the available literature and on theoretical considerations (i.e., a pairing or activation hypothesis), thalamic stimulation was not an effective CS. The response of PT cells to thalamic stimulation did not change as a function of reinforcement with PT stimulation. These results led to a reconsideration of the hypothesis that the simple pairing of any two neural events is the essential mechanism underlying associative conditioning changes. The results did suggest that the combined activation of specific and nonspecific thalamic nuclei may be important in producing increases in responsiveness of PT neurons.