Purkinje Cell Activity and the Conditioned Nictitating Membrane Response

Abstract

In 1980 our colleague John E. Desmond (Desmond et al., 1981) discovered that a small unilateral lesion of the dorsolateral pons (DLP) could completely and irretrievably eliminate a previously acquired conditioned response (CR). As in all our behavioural studies, the CR was the classically conditioned nictitating membrane (NM) response of the rabbit. The NM response is a passive consequence of defensive or protective retraction of the eyeball. In a typical experiment, the animal is given 100 pairings/day of a tone conditioned stimulus (CS) with mild electrostimulation of one eye as an unconditioned stimulus (US). The onsets of the two stimuli are separated by a few hundred msec, and these pairings or trials are separated by 15-30 sec. Conditioned responses are extensions of the NM occurring within the CS-US interval. A CR typically consists of a graded movement of the membrane in a naso-temporal direction with full extension coinciding with US onset. CRs typically begin to emerge within a hundred or so trials. Desmond’s discovery was accidental because he was attempting to lesion the locus coeruleus, a structure lying just medial to the lesion that eliminated CRs. Earlier lesion and ablation studies had shown that brain regions rostral to the midbrain are not essential for NM conditioning (Moore, 1979; Moore et al., 1980; see also Mauk and Thompson, 1987).