Classical conditioning of the rabbit (Oryctolagus cuniculus) nictitating membrane response, with electrical brain stimulation as the unconditioned stimulus.

Rabbits were given classical conditioning training by using paired tone CS (conditioned stimulus) and brain-shock UCS (unconditioned stimulus), with the stimulating electrode localized in the vicinity of the abducens (6th nerve) nucleus such that the electrical stimulus elicited a low-threshold nictitating membrane (NM) extension response as the unconditioned response. Eight of the 27 animals developed clear conditioned NM responses to the tone CS. Control procedures, e.g., subsequent explicity unpaired training, argue against sensitization, pseudoconditioning, and "kindling" as possible explanations. There was no clear-cut anatomical differentiation of electrode-tip locations between learners and nonlearners. Both learners and nonlearners were subsequently trained with paired tone-corneal air puff, and neuronal unit activity was recorded from the stimulating electrode. Animals that had learned with the prior brain-shock UCS exhibited substantially greater increases in neuronal unit activity during air-puff training, which suggests the existence of a differentially effective anatomical-physiological substrate for conditionability.