Stimulus modality effects on transformations of conditioned enhancement and conditioned suppression in rats.

Unconditioned and conditioned effects of house-light offset and acoustic white noise on barpressing behavior maintained by intermittent food reinforcement were tested in male hooded rats. Presentations of these stimuli prior to their acquiring signal value initially tended to depress and then to enhance barpressing rate, but generally the rate of the on-going barpressing was lower during the light offset then during noise presentations. Subsequently, one stimulus was used to signal continuous food reinforcement, evoking conditioned enhancement, and the other to signal inescapable footshock eliciting conditioned suppression of barpressing. The enhancement was acquired more rapidly than the suppression, independent of the stimulus modality used. The stimulus modality effect emerged when the continuous food reinforcement was withdrawn, since enhancement elicited by light offset extinguished more rapidly than elicited by noise. The stimulus modality effect was stronger when the other stimulus continued to signal pain and was attenuated when conditioned suppression was also extinguished. During the next stage of the experiment, the signal values of the conditioned stimuli were reversed, resulting in easy transformation of conditioned suppression into conditioned enhancement and vice versa. The stimuli used and changes in their signal values exerted clear effects on the rate of barpressing during intertrial intervals and this, in turn, somewhat modulated the behavioral effects of the conditioned stimuli. These results indicate that unconditioned effects of the stimuli on the behavior interact with their properties acquired in the course of conditioning.