A role for conditioned stimulus duration in toxiphobia conditioning.

The effect of conditioned stimulus (CS) exposure duration on toxiphobia conditioning was investigated. When a relatively long (20--30 min) CS exposure was administered on the conditioning trial, rats subsequently displayed a reduced conditioned aversion in comparison with when CS exposure was relatively brief (5--10 min). This result was obtained in an odor-aversion learning task with neonatal rats as subjects (Experiments 1 and 2) and in a flavor-aversion learning task with adult rats (Experiment 3). This phenomenon was further investigated in the odor-aversion task by presenting neonates with two discrete 10-min exposures to the odor CS and by varying both the temporal proximity of the two CS presentations to each other and to the occurrence of the unconditioned stimulus (US). Two exposures to the CS reduced conditioning in comparison with a single exposure but only when the second CS presentation preceded US occurrence by at least 5 min (Experiments 4 and 5). Both nonassociative and associative interpretations of the data were discussed.