Activity of deep cerebellar nuclear cells during classical conditioning of nictitating membrane extension in rabbits.

The activity of neurons in the interposed and dentate nuclei of the cerebellum was investigated during differential classical conditioning of the rabbit eye blink nictitating membrane response. Forty-seven percent of the 165 cells in the study responded to the orbital stimulation used as the unconditioned stimulus (US). The latency distribution of US-elicited responses was bimodal with peaks at 7 and 19 ms. Twenty-one percent of the cells responded with short latencies to the tones used as conditioned stimuli (CSs). These cells typically responded to both the reinforced and nonreinforced CSs. Forty-one percent of the cells responded on conditioned response (CR) trials but not on trials without CRs. The average lead of the neural response to the CR was 71.4 ms. Cells that responded on CR trials were more likely to respond to the CSs, or to the CSs and the US, than cells that did not respond on CR trials. For about half of the cells that responded on CR trials the latency of response followed trial-by-trial variations of CR latency. For the remainder, the response was time-locked to CS-onset. Cells whose responses paralleled the CR may be involved in the initiation or modulation of the CR, while those whose responses were time-locked to the CS may be involved in sensory processing underlying the initiation of the movement. The pathways that may underlie the US- and CS-elicited responses are also discussed.