Extension of the CS past the US can facilitate conditioning of the rabbit's nictitating membrane response

Five experiments were conducted in which the onset of a tone conditioned st imulus (CS) preceded the unconditioned stimulus (US) by 500 ms. Across expe riments, the offset of the CS was extended past the offset of the US by val ues ranging from 0 ms to 40000 ms. Extensions of the CS of 2000 ms or great er produced acquisition of a conditioned response (CR) that was as fast or faster than in the no-extension condition (0 ms). While extension of a forw ard tone CS after the US enhanced excitatory conditioning, insertion of ano ther CS(light) in a purely backward relationship with the US passed only a retardation test, indicative of latent inhibition, and not a summation test needed for conditioned inhibition. The results add to the evidence that ex citatory and inhibitory processes are both engaged following US offset. Alt ernative theories of CS processing are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.