CONDITIONED ENHANCEMENT OF THE EARLY COMPONENT OF THE RAT EYEBLINK REFLEX

Conditioned enhancement of the rat eyeblink reflex was studied using a s a response measure the electromyogram (EMG) in the orbicularis oculi (oo) muscle, which is responsible for the active force generating eye lid closure. During a reflex eyeblink, the EMG evidences both a short- latency (R1) and a long-latency (R2) component, mediated by different circuits. The R2 response exhibits several experience- or use-dependen t modifications. We were interested in the modifiability of the neurop hysiologically simpler R1 response. Experiments were designed to deter mine whether the R1 response can be enhanced by a conditioned stimulus (CS) that has been explicitly paired with an aversive unconditioned s timulus (US). The R1 response was elicited by electrical stimulation o f the supraorbital branch of the trigeminal nerve. Following long-dela y conditioning, the CS produced a significant R1 enhancement and laten cy decrease that were dependent upon explicit CS-US pairings. The CS b y itself produced no significant EMG response, consistent with a modul atory rather than additive effect. This is the first demonstration tha t the R1 response can be associatively modulated. Based on other evide nce, we hypothesize that the CS-produced enhancement results from a re latively direct projection from the amygdala to the R1 circuit. As an index of aversive conditioning, R1 enhancement may prove to be a usefu l expression system because it offers high temporal resolution and the underlying circuitry is relatively simple-the shortest path from the fifth nerve to the muscle consisting of only two central synapses. (C) 1996 Academic Press, Inc.