Medial prefrontal cortex and striatum mediate the influence of social comparison on the decision process

Abstract

We compared private and social decision making to investigate the neural underpinnings of the effect of social comparison on risky choices. We measured brain activity using functional MRI while participants chose between two lotteries: in the private condition, they observed the outcome of the unchosen lottery, and in the social condition, the outcome of the lottery chosen by another person. The striatum, a reward-related brain structure, showed higher activity when participants won more than their counterpart (social gains) compared with winning in isolation and lower activity when they won less than their counterpart (social loss) compared with private loss. The medial prefrontal cortex, implicated in social reasoning, was more activated by social gains than all other events. Sensitivity to social gains influenced both brain activity and behavior during subsequent choices. Specifically, striatal activity associated with social gains predicted medial prefrontal cortex activity during social choices, and experienced social gains induced more risky and competitive behavior in later trials. These results show that interplay between reward and social reasoning networks mediates the influence of social comparison on the decision process.

Fig. 1.

Experimental paradigm. (A) Task and time course. The time courses of the private and social conditions are displayed above and below the time line, respectively. Pairs of lotteries were displayed, with numbers indicating the possible outcomes and probabilities represented by colored sectors of a circle. Lotteries were surrounded by green dotted squares representing the participant's possible choices, plus yellow dotted squares representing the counterpart, in the social condition only. The presence of the yellow dotted square, thus, indicated to the participant that they would see their counterpart's choice. (B) Experimental design. At the time of choice, the Blood-oxygen-level dependent (BOLD) signal was analyzed to test the effect of the decision context that could be private or social. During the outcome period, the BOLD signal and skin conductance data were analyzed to test the effect of two factors: valence and outcome context.

Fig. 2.

Behavioral effect of experienced outcomes. (Left) The scatter plot represents the across-subjects correlation between the distance of subject's risk behavior to its counterpart (in sessions 2 and 3) and the cumulated experienced social gains in early trials (session 1). (Right) The plot shows the same correlation for the experienced social losses. The distance between the subject's and counterpart's risk attitude was computed as the difference between the subject's and counterpart's dSD individual coefficients given by the logit regression. In a single trial, experienced social loss and social gain were measured as the difference between the obtained outcome and the outcome of the lottery chosen by the other player. We then summed these differences to compute the total value of each experienced outcome. One data point was excluded from the two correlations analyses, because participant's risk difference score was >3 SD from the grand mean. When including the outlier, r = 0.49 and P < 0.02 for early social gains, and r = 0.05 and P > 0.5 for early social losses.

Fig. 3.

mPFC activity related to social gains. (Upper) mPFC activity discriminates between the three outcome conditions (P, SSC, and SDC), when the outcomes of the two lotteries are revealed. F maps projected on the subjects’ averaged brain. (Lower) Time course in the mPFC (0, 54, 9) for the six possible outcomes. The mPFC is more activated for social gain than for all other events.

Fig. 4.

(A) Striatal activity encoding outcomes’ relative valence and outcome context during the outcome evaluation period. The coronal slice shows the interaction effect between the valence and outcome context factors. The bar graphs indicate the percent of signal change (± SEM) for the left (−9, 9, −3) and right (9, 12, −3) caudate (areas of interest defined from the interaction analysis). (B) Mean skin conductance responses (± SEM) for the six outcome events. Responses are reported in microsiemens. (C) Emotional evaluations [on a scale from −50 (extremely negative) through 0 (neither positive nor negative) to +50 (extremely positive)] given by 42 participants for the six outcome events in a previous behavioral study. Adapted from Bault et al. (41).

Fig. 5.

Effect of experienced outcomes on choice-related brain activity. Functional connectivity analysis. The map shows the voxels where activity during the choice period t is correlated with activity of the striatum during the outcome evaluation of the previous trial t − 1. The seed regions are the left and right ventral striatum regions that showed an interaction between the factors valence and outcome context (Fig. 4A).