Classical Conditioning Mediated by the Red Nucleus in the Cat

Summary

We have attempted to develop a behavioral and neuronal model for the study of classical conditioning using the corticorubrospinal system. A conditioned stimulus (CS) was applied to the cerebral peduncle (CP) in cats which had lesions that interrrupted the corticofugal fibers caudal to the red nucleus. The unconditioned stimulus (US) was an electric shock to the skin of the forelimb that produced flexion of the limb. After pairing of the CS and US in close temporal association, an initially ineffective stimulus to the cerebral peduncle was found to give rise to flexion of the elbow. Extinction of this conditioned response could be achieved by applying the CS alone, or by reversing the sequence of the stimuli (US-CS:backward pairing). The US when used alone did not produce an increase in the effectiveness of the CS, and pairing a fixed CS with a US at random intervals did not produce any increase in performance in response to the CS. In these respects, the observed behavioral modification has the features of associative conditioning.

Because both the threshold for and strength of elbow flexion induced by stimulation of the nucleus interpositus of the cerebellum were identical in experimental and control animals, it is unlikely that the interpositorubrospinal system is the neuronal site of behavioral change. Instead, since the conditioned response is most probably mediated the corticorubrospinal system, it is likely that a modification of the corticorubral synapses underlies the change in behavior. Extracellular recording from red nucleus cells in unanesthetized cats has shown that stimulation of the CP induces a greater degree of excitation in the red nucleus in animals that have undergone conditioning than control animals. (The control group contained cats which had received no training, or those which had received backwardly paired stimulation, or random CS-US pairing).