Rats do react to stimulus omission.

1. The great majority of data supporting the hypothesis of a system capable of comparing current sensorial inputs with an internal representation of the environment comes from studies about exploratory activity to new stimuli or to manipulation of features of a familiar stimulus. On the other hand, these data could also be explained simply by arousal constructs. In this context, demonstrations of exploratory behavior to the absence of a previously presented stimulus (i.e., stimulus omission) would provide stronger support for the idea of a comparator. 2. To test the reaction of rats to the absence of a stimulus, rats were submitted to 7 exploratory trials in an open-field. In the 1st trial there were only two patterns on the apparatus wall. In trials 2-6 a stimulus was presented in a designated area of the field. Finally, in the 7th trial this stimulus was omitted. Results showed that the animals reacted to the stimulus omission by spending more time in the stimulus presentation place during the 7th trial than 1) in the 1st trial (also without stimulus), 2) in the 6th trial (last trial with a stimulus present), and 3) in 3 neutral sectors of the same size as the stimulus presentation place, during the 7th trial. 3. These data indicate that rats do react to the absence of a familiar stimulus and provide strong support for the existence of a Comparator System since the rats responded to "something that wasn't there anymore", a response that could only be due to a reaction triggered by a mismatch between internal representation of the environment and its present state.