Escape from shock versus escape from shock accompanied by a visual stimulus in rats

Two groups of 15 rats each were trained in a shuttle box to escape foot-sho ck either unsignalled or presented in compound with a visual cue: darkness. The visual cue presented in shock compartment amplifies the behavioral ten dency actually prevailing in the response repertory of the rat. During the Ist session the compound enhanced the species-specific flight resulting in shortening of the rat's escape latency. Thereafter, during subsequent sessi ons, darkness exaggerated resistance to enter the other compartment; thus e scape latencies were longer under compounded than under unsignalled procedu re. The darkness cue reduces the intertrial response rate relative to the u nsignalled group. This latter finding supports the discrimination model of the effect, since the compound helps the animals to discriminate the illumi nated "safe" period between trials from the aversive shock period. Our data seem to suggest that the darkness presented synchronously with escapable g rid-shock acquires aversive properties.