The relationship between reward and probability: Evidence that exploration may be intrinsically rewarding

The probability of an event occurring and the reward associated with the event can both modulate behaviour. Response times are decreased to stimuli that are either more rewarding or more likely. These two factors can be combined to give an Expected Value (EV) associated with the event (i.e. probability of the event x reward magnitude). In four experiments we investigate the effect of reward and probability on both saccadic and manual responses. When tested separately we find evidence for both a reward and probability effect across response types. When manipulations of reward magnitude and probability of the event were combined, the probability modulations dominated and these data were not well accounted for by the EV. However, a post-hoc model that included an additional intrinsic reward associated with responding provided an excellent account for the data. We argue that reward consists of both an explicit and intrinsic component. In our task, the saccadic and manual responses are linked to the information provided by the targets and the goals of the task, and successful completion of these is in itself rewarding. As a result targets associated with a higher probability of being presented have a higher intrinsic reward.