Signaling the unconditioned stimulus during the preexposure phase does not attenuate the unconditioned stimulus preexposure effect in preweanling rats

The unconditioned stimulus preexposure effect (US‐PE) is defined as an attenuation of the conditioned response after preexposure to the US prior to conditioning. Evidence exists that this effect can be weakened or eliminated by the presence of a signal predicting the US during the preexposure phase. This evidence has been found consistently across a variety of procedures in adult rats. The aim of the present study was to evaluate whether, in infant rats, signaling the US (LiCl) during preexposure with a salient cue (almond odor) attenuates the US‐PE. During the preexposure phase, preweanling rats received three (Experiment 1) or one (Experiment 2) preexposures to LiCl, preceded by exposure to almond odor. Appropriate control groups were also included in these experiments. After preexposure, two conditioning trials were carried out in which subjects were given LiCl after saccharin consumption. During preexposure, three (Experiment 1a), although not one (Experiment 2a), contingent exposures to almond odor and LiCl resulted in a strong odor aversion. Extinction of the learned taste aversion was facilitated by prior experience with LiCl (Experiments 1b and 2b). This effect was observed regardless of whether or not LiCl was signaled by the almond odor. These results do not coincide with the associative hypotheses proposed to explain the US‐PE, nor are they concurrent with alternative explanations based on the learned helplessness phenomenon. © 2011 Wiley Periodicals,Inc. Dev Psychobiol 54: 808–817, 2012