A possible social relative reward effect: Influences of outcome inequity between rats during operant responding.

Social interactions/situations have dramatic influences on motivation. Creating animal models examining these influences promotes a better understanding of the psychological and biological underpinnings of social motivation. Rodents are sensitive to social history/experience during associative conditioning and food-sharing tasks. Would reward-oriented operant behavior be sensitive to social influences by showing a negative contrast-like effect when another organism obtains a greater value outcome? We used a side-by-side arrangement of operant response chambers wherein one animal obtained consistently high reward signaled by a discrete cue. The neighboring, experimental rat experienced different combinations of high and low reward trial sequences. Control conditions included distraction from a conspecific in the neighboring chamber (rat distractor) or cue/food dispenser operating without a conspecific (program distractor) in addition to testing subjects alone. Results support an influence of the other animal actively performing the task on the experimental subject's behavior. Primarily, responding was significantly slower for the low reward trials while the neighboring rat was receiving the higher magnitude reward. The lever-press and not food-cup retrieval latency was significantly slower during exposure to a conspecific neighbor performing the operant task. The effect was not obtained in all session sequences and was more pronounced using longer series of consecutive low reward trials. The slowing effect was also obtained with the program-distractor experience in a different trial sequence. These findings suggest a social-induced negative incentive contrast effect in rats possibly mediated by an outcome inequity process that could have key similarities to complex situational-affective effects on motivation involving frustration or jealously.