Conditioned pleasure attenuates the startle response in rats.

The acoustic startle response of rats was found to be attenuated if elicited in the presence of a conditioned stimulus predicting reward. During conditioning, animals received a total of 21 pairings of light with palatable food and sucrose solution, whereas controls received food and sucrose in the absence of light. The amplitude of the acoustic startle response was significantly reduced in the presence of light in conditioned animals, but not in controls. It is assumed that a conditioned response to light is the activation of a central state of pleasure. We therefore suggest that "pleasure-attenuated startle" reflects a mechanism by which a defensive or aversive response is attenuated during a pleasant, hedonic state.