Rewarding images do not invoke the reward positivity: They inflate it.

Increasing evidence suggests that the reward positivity conforms to an axiomatic reward prediction error - that is, it closely follows the rule-like encoding of surprising reinforcers. However, a major limitation in these EEG studies is the over-reliance on a single class of secondary rewards like points or money, constraining dimensionality and limiting generalizability. In the current suite of studies we address this limitation by leveraging different classes of rewards outcomes, specifically emotionally pleasant pictures. Over a series of three experiments, participants were able to choose idiosyncratically preferred pictures as rewards. During the first two experiments, participants were rewarded with either high or low points or high or low preferred pictures. The reward positivity was modulated by points, but not by pictures (regardless of preference), which instead evoked enhanced N2 amplitudes. In a third study that paired high/low points and preferred/non-preferred pictures, the point-induced reward positivity was inflated by the presence of a preferred picture. In line with past research stating the reward positivity is primarily sensitive to positive reward prediction error, this report finds that it is also influenced by a liking dimension, which possibly acts as an affective state to frame the motivational aspect of extrinsic rewards.