Distinct value signals in anterior and posterior ventromedial prefrontal cortex

Abstract

The core feature of an economic exchange is a decision to trade one good for another, based on a comparison of relative value. Economists have long recognized, however, that the value an individual ascribes to a good during decision making (i.e., their relative willingness to trade for that good) does not always map onto the reward they actually experience. Here, we show that experienced value and decision value are represented in distinct regions of ventromedial prefrontal cortex (VMPFC) during the passive consumption of rewards. Participants viewed two categories of rewards-images of faces that varied in their attractiveness and monetary gains and losses-while being scanned using functional magnetic resonance imaging. An independent market task, in which participants exchanged some of the money that they had earned for brief views of attractive faces, determined the relative decision value associated with each category. We found that activation of anterior VMPFC increased with increasing experienced value, but not decision value, for both reward categories. In contrast, activation of posterior VMPFC predicted each individual's relative decision value for face and monetary stimuli. These results indicate not only that experienced value and decision value are represented in distinct regions of VMPFC, but also that decision value signals are evident even in the absence of an overt choice task. We conclude that decisions are made by comparing neural representations of the value of different goods encoded in posterior VMPFC in a common, relative currency.

Figure 1.

Experimental tasks. A, Trial structure for the multimodal reward task. Male, heterosexual young adults passively viewed a randomized sequence of images of female faces and monetary rewards (2 s event duration; 2 s fixation interval). The face images varied in valence from very attractive to very unattractive, based on ratings from an independent group of participants. The monetary rewards, whose value ranged from $5 to −$5, influenced the participant's overall payout from the experiment. To ensure task engagement, participants responded to infrequent visual targets that appeared as small yellow borders around the image. B, Trial structure for the economic exchange task. Trials began with a decision phase (lasting 4 s) in which the participant was forced to spend a small amount of money to view a face. Participants could choose to spend more money to view a more attractive face or less money to view a less attractive face. Following the decision phase, there was an anticipation phase that lasted either 2 or 4 s. Then a single face, randomly selected from the chosen attractiveness category, was displayed for 2 s. Trials were separated by a variable interval whose duration ranged from 2 to 6 s.

Figure 2.

Neurometric measures of monetary and social experienced value. A, To identify brain regions whose activation tracked monetary value, we contrasted large monetary gains ($5 and $2) minus large monetary losses (−$5 and −$2). Within anterior medial prefrontal cortex [x 12, y 58, z 2] and adjacent frontopolar cortex [x −2, y 68, z −2] there was a clear effect of the valence of monetary rewards (shown in green). We additionally identified brain regions whose activation tracked social value by contrasting the response to faces drawn from our most-attractive category (4-star) and that to faces drawn from our least-attractive category (1-star). Activation was modulated by the valence of social rewards in the ventral striatum [vSTR; x −8, y 18, z −6] and ventromedial prefrontal cortex [VMPFC; x 0, y 48, z −8] (shown in red). Using a conjunction analysis, we identified a small region of anterior VMPFC [aVMPFC; x 0, y 46, z −8] with activation that increased to increasing value for both forms of rewards (shown in yellow), and survived an additional cluster correction of 15 contiguous voxels. All areas of activation passed a cluster significance threshold of z > 2.3, with whole-brain cluster-correction at p < 0.05. B, Interrogation of the aVMPFC region that responded to both rewards revealed that activation within this area increased with increasing monetary reward. Error bars indicate SEM (for display purposes, error bars were calculated without correcting for normalized values). C, Within the aVMPFC region, activation increased with increasing attractiveness, with least response to the most unattractive faces and greatest response to the most attractive faces. Error bars indicate SEM (for display purposes, error bars were calculated without correcting for normalized values).

Figure 3.

Posterior VMPFC encodes information about decision value. A, We investigated whether neurometric representations of value (in the multimodal reward task) could predict whether a given individual was likely to subsequently trade money to view faces (in the economic exchange task). To do this, we introduced exchange rate (as a proportion of total opportunities) as a factor in an across-participants analyses. We found one region within the posterior VMPFC (pVMPFC) [x 6, y 26, z −14] in which differential neurometric representations of value, defined as social value (4-star minus 1-star) minus monetary value (large monetary gains minus large monetary losses), predicted exchanges. Shown are voxels passing a significance threshold of p < 0.001 (uncorrected); the pVMPFC peak of activation was highly significant (z = 3.24). B, Within pVMPFC, increasing neurometric value for social compared with monetary rewards predicted increasing likelihood of economic exchanges (r = 0.72). That is, those individuals who had the greatest neurometric value for social compared with monetary rewards, as defined by a double subtraction of social value minus monetary value, readily exchanged money to view more-attractive faces. In contrast, those individuals with the least neurometric value for social compared with monetary rewards were unlikely to sacrifice that money for a better social reward. The line represents the least-squares fit to the data points; each point reflects the average response within pVMPFC for each participant.