Concomitant classical conditioning of the rabbit nictitating membrane and eyelid responses: correlations and implications.

Simultaneous recordings of muscle unit activity from the muscles of the left and right eyelids (M. obicularis oculi) and recordings of the movement of the left nictitating membrane (NM) were taken during classical conditioning in the rabbit using a tone CS paired with an airpuff UCS to the left cornea. The unconditioned eyelid responses were found to be bilateral. The conditioned eyelid responses were also bilateral in most animals. Both the conditioned and unconditioned eyelid responses were larger on the left side. The conditioned responses of the left and right eyelids and the left NM were found to increase in magnitude and decrease in latency from the onset of the CS over training trials in almost the exactly same manner (correlations as high as .99). Behaviorally, the three responses could occur independently, suggesting that the cranial nuclei which control them (left abducens/accessory abducens, left facial nucleus, right facial nucleus) are not strongly coupled. Thus, for the learned response, it is suggested that the three nuclei are controlled by a common central system. This finding has implications for the nature of the engram--the essential neuronal circuitry encoding the learned response.