Latency of the two-way avoidance response in rats: inhibition of delay

Avoidance and escape latencies were examined in 24 rats trained in two-way avoidance response to auditory and visual conditioned stimuli (CS). In rats trained with darkness CS the escape latencies decreased within the first 50-trial session but later were stable over the course of nine sessions. Avoidance latencies shortened within sessions as the response-eliciting properties of the conditioned stimuli increased. On the contrary, median daily avoidance latencies lengthened during training. Training with the more salient auditory stimulus resulted in a decrease in the probability of avoidance responses early in the CS-US interval, reflecting the development of inhibition of delay. Acquired inhibition of delay was eliminated by an increase in the fear-inducing properties of situational cues. When presented in a compound, the less salient darkness stimulus was fully overshadowed by the noise stimulus.