Response selection properties of food and brain-stimulation reinforcers in rats.

Rats were exposed to a differential conditioning procedure in which one lever-stimulus predicted a food UCS (for one group) or a brain-stimulation UCS (for 2 other groups) while a second lever-stimulus was explicitly unpaired with the UCS. A fourth group received differential training where one lever-stimulus signalled food and the other brain-stimulation. The animals reliably approached and contracted the predictive stimuli with there being no significant difference in this tendency between groups. There was, however, a significant difference in the form of the stimulus contact behavior as a function of the quality of the UCS. Rats gnawed the stimulus when the UCS was food but sniffed it when the UCS was brain-stimulation. This difference was independent of the level of food deprivation and occurred within animals in the group which received both kinds of UCS. The results indicate that properties specific to the reinforcer are important in response performance. Such effects should be considered in the analysis of performance on instrumental tasks, especially when concern is focussed on performance differences which are correlated with qualitatively different rewards.