Relative interval duration and the US pre-exposure effect.

Three experiments examined the role of CS (conditioned stimulus) duration in the unconditioned stimulus (US) preexposure effect. Rats received preexposure to unsignalled food pellets that were delivered on a fixed-time 90-s schedule and magazine entry responses were recorded. In Experiment 1, there was no evidence of retardation of conditioning to a 15- or 60-s CS when rats that received US preexposure were compared to unexposed control groups. Experiment 2 revealed a US-preexposure effect with a 90-s CS, but only when the rats were given a 31.5-min wait in the experimental chambers prior to the onset of US exposure. In Experiment 3, it was discovered that the magnitude of US preexposure was related to CS duration, with longer CS durations demonstrating progressively greater retardation in conditioning. The results are discussed in light of recent time-based accounts of classical conditioning.