Paradoxical effects of proximal unconditioned stimulus preexposure: interference with and conditioning of a taste aversion.

Taste aversions were conditioned in rats by giving the subjects an injection of lithium following exposure to saccharin. Treatment with lithium shortly (30 to 60 min.) before such a conditioning trial disrupted the taste-aversion learning produced by the postsaccharin drug injection. In Experiment 1, this interference effect was found to be a monotonically decreasing function of the preexposure-to-conditioning interval. The greatest disruption of conditioning was observed with a 30-min. preexposure-to-conditioning interval, and no interference occurred if the preexposure was administered 1 or 2 days before conditioning. These results suggest that in addition to the more durable effects of unconditioned stimulus (US) preexposure emphasized by previous investigators, US preexposure also activates temporary mechanisms that likewise attenuate subsequent conditioning. Experiment 2 demonstrated that lithium given before saccharin exposure disrupts the conditioning produced by a postsaccharin drug injection even if the presaccharin injection would itself otherwise be effective in producing a "backward" conditioned saccharin aversion. This outcome indicates that proximal US preexposure can have two opposite and paradoxical effects: It can interfere with as well as condition a taste aversion.