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

Abstract

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.

Fig. 1.

Affective reward image preference task and post experimental pleasantness rating results. Percentages displayed in each of the pie charts relate to the proportion of participants who preferred that type of reward. Mean ratings of pleasantness for each of the participant’s choices (1st-5th) can be viewed on the right side of the figure. These bar plots reveal ratings of pleasantness decrease in tandem with the choice rank from the affective reward image preference task. Interestingly, participants rate their 5th choice as less pleasant as our neutral image class (“0”), supporting our use of the 1st and 4th image class choice.

Fig. 2.

Experiment 1: ERPs time-locked to trial feedback (a). Bar plots (b) represent mean ERP potentials from the Rew-P temporal window (grey box). Shaded regions represent standard error of the mean. Point reward appears to instantiate a Rew-P while picture rewards do not. However, this lack of Rew-P for picture reward may be related to the inflated P2 evoked during the reward cue incentive screen (c), in which the signal of salience may migrate to the first instance of prediction.

Fig. 3.

Experiment 2: ERPs time-locked to trial feedback. Similar to Experiment 1, point feedback invoked a Rew-P (a₁), but picture feedback evoked an N2 instead (a₂). Bar plots (b) display mean amplitudes during the feedback time window (grey box). Comparison ERP plots for points versus pictures for high (c₁) and low (c₂) rewarding feedback clearly reveals that reward points evoke a Rew-P while rewarding images evoke an enhanced N2. ERPs for point and picture lose conditions were not different (c₃). P2 modulations for incentivized reward cues (d) did not differ between reward type incentives.

Fig. 4.

Experiment 3: ERPs time-locked to trial feedback. ERPs by condition for rewarding feedback for wins and their corresponding loses (a). As expected Rew-Ps for wins were larger for each of the reward conditions (all t’s > 2.2, all p’s < 0.03). Topographic maps for rewards (scaled: ± 1.25 μV) reveal activation related to emotional visual stimuli (see Foti et al., 2009) at 250 ms. Surprisingly, these plots reveal scalp level activation seen repeated for feedback processing. This activation appears to visually scale with the magnitude of point reward. The bar plot (b) reveals a boost in Rew-P expression when point feedback was presented in the context of a preferred image. Black boxes inlaid within the colored bars represent the mean ERP responses for no-win conditions for each respective condition.