Value computations in ventral medial prefrontal cortex during charitable decision making incorporate input from regions involved in social cognition

Abstract

Little is known about the neural networks supporting value computation during complex social decisions. We investigated this question using functional magnetic resonance imaging while subjects made donations to different charities. We found that the blood oxygenation level-dependent signal in ventral medial prefrontal cortex (VMPFC) correlated with the subjective value of voluntary donations. Furthermore, the region of the VMPFC identified showed considerable overlap with regions that have been shown to encode for the value of basic rewards at the time of choice, suggesting that it might serve as a common valuation system during decision making. In addition, functional connectivity analyses indicated that the value signal in VMPFC might integrate inputs from networks, including the anterior insula and posterior superior temporal cortex, that are thought to be involved in social cognition.

Figure 1.

Experimental timeline and behavioral results. A, The two trial types, free and forced donation trials, were identical except that in forced donation trials subjects were told the dollar amount that they had to donate to the charity. Each charity was presented twice, once as a free trial and once as a forced trial. Charity, trial type, and forced donation amount were fully randomized within subjects. B, Frequency of donations in free trials for the entire group. C, Donations as a function of deservingness ratings. Error bars represent the SEM.

Figure 2.

VMPFC activity correlated with the subjective value of charitable donations at the time of decision making. A, Region of VMPFC in which activity correlates with the parametric modulator for the value of the charitable donations. Color scale denotes Z-scores. Statistical parametric maps are whole brain corrected for multiple comparisons at p < 0.05. B, Overlay map showing the results of conjunction analyses identifying regions of VMPFC in which activity is correlated with the value of charitable donations in the current study, and with the value of primary rewards in other decision-making studies. Red, Conjunctions with value signals for primary food rewards described by Plassmann et al. (2007). Green, Conjunctions with value signals for primary food rewards described by Hare et al. (2008). Each contrast used in the conjunction analyses was thresholded at p < 0.005 unc.

Figure 3.

Activity in pSTC correlated with subjects' willingness-to-give. A, Region of the right pSTC in which the difference in average activity between free and forced trials was was positively correlated with willingness-to-give. Statistical parametric maps are whole brain corrected for multiple comparisons at p < 0.05. Color scale denotes Z-scores. B, Scatter plot showing the correlation between a behavioral measure of overall willingness-to-give and average activity in the area of pSTC showed in Figure 3A. The scatter plot is for the purpose of display only and was not the basis of any inferences in this study. Each point represents one subject.

Figure 4.

Regions exhibiting PPI with VMPFC during all trials. A, Regions exhibiting an increased correlation with VMPFC activity during the charity evaluation period at p < 0.05 corrected. The anterior insula is circled in white. B, Conjunction analysis showing areas of IFG that exhibit functional connectivity with both pSTC and VMPFC during the decision period. A threshold of p < 0.005 uncorrected was used for the conjunction. Color scale denotes Z-scores.

Figure 5.

Results of the mediation analysis testing the relationship between pSTC, IFG, and VMPFC activity. A, Diagram of the path model showing the coefficients ± SEM, significant at ***p < 0.001. B, Histogram of the bootstrapped distribution of the mediation effect (a*b). The lighter gray color denotes the 95% confidence interval.

Figure 6.

Differences in VMPFC task related connectivity during charitable donation and food reward decisions. Dark gray bars represent mean ± SEM of the VMPFC PPI coefficients in right pSTC and both the left and right anterior insula (AI). Light gray bars represent the VMPFC PPI coefficients for the same regions during decisions about purchasing a snack food item for oneself in a separate study (Plassmann et al., 2007). Regions where the difference in PPI between decision tasks is significant are indicated as *p < 0.05, **p < 0.005, and ^†p < 0.1. PPI coefficients are not significantly different from zero during the food decision task in any of the three regions.