The decision value computations in the vmPFC and striatum use a relative value code that is guided by visual attention

Abstract

There is a growing consensus in behavioral neuroscience that the brain makes simple choices by first assigning a value to the options under consideration and then comparing them. Two important open questions are whether the brain encodes absolute or relative value signals, and what role attention might play in these computations. We investigated these questions using a human fMRI experiment with a binary choice task in which the fixations to both stimuli were exogenously manipulated to control for the role of visual attention in the valuation computation. We found that the ventromedial prefrontal cortex and the ventral striatum encoded fixation-dependent relative value signals: activity in these areas correlated with the difference in value between the attended and the unattended items. These attention-modulated relative value signals might serve as the input of a comparator system that is used to make a choice.

Figure 1.

A, Trial structure. Yellow arrows illustrate target positions of eye fixation (not shown during the actual experiment). Participants alternated their eye fixation to the item cued by a target color frame before indicating their choice by pressing a button at the end of trial. The target frame color (red or green), the initial position of eye fixation (left or right), and the duration of each eye-fixation (1–4 s) were randomized. B, Sample eye-fixation data from representative individuals. The y-axis represents eye-fixation positions (13.5 units = 1° visual angle). The center coordinates of a screen were given [0,0]. The dotted lines represent the center coordinates of left and right frames. C, Logistic psychometric choice curves by relative eye-fixation duration in two-choice trials. The dashed horizontal line represents the half-maximum response threshold that determines the PSE. D, PSE comparison as a function of relative fixation time. Error bars denote SEs.

Figure 2.

A, vmPFC activity correlated with stimulus ratings during the separate set of liking-rating trials (p < 0.05, corrected). All images and coordinates are in MNI space. B, ROI analyses of vmPFC activity during choice trials. C, vStr activity correlated with stimulus ratings during the initial liking-rating trials. D, ROI analyses of vStr activity during two-choice trials. Note that in both cases, activity correlated positively with the difference in value between the attended and unattended items. The y-axis of ROI plots represents average beta weights (estimated coefficients) for parametric regressors of the differences between left and right item values. Error bars denote SEs.

Figure 3.

A, ROI analyses of the vmPFC during choice trials. B, ROI analyses of the vStr during choice trials. The y-axis represents average beta weights (estimated coefficients) for each parametric regressors. Error bars denote SEs. att, Attended stimuli; unatt, unattended stimuli.

Figure 4.

Two (attention condition) by two (fixation side) ANOVA. A, The vStr showed a main effect of attention condition. B, Paired t test between fixation-left and fixation-right indicators showed activation in visual cortex ipsilateral to the location of overt fixations. p < 0.05 corrected.

Figure 5.

The relative value signals were not affected by the choice. A, The vmPFC activity of left-choice trials during choice trials. B, The vmPFC activity of right-choice trials during choice trials. C, The vStr activity of left-choice trials during choice trials. D, The vStr activity of right-choice trials during choice trials. The y-axis of ROI plots represents average beta weights (estimated coefficients) for parametric regressors of the differences between left and right item values. Error bars denote SEs. V^C, Value of chosen items; V^UC, value of unchosen items.

Figure 6.

Value signals for attended and unattended items. A, The vmPFC activity for attended and unattended item values. B, The vStr activity for attended and unattended item values. The y-axis of ROI plots represents average beta weights (estimated coefficients) for parametric regressors. Error bars denote SEs. C, Frequency histogram of F statistics for the vmPFC ROI comparing nested models with and without the V^unatt regressor. D, Similar plot for vStr. The vertical dashed lines indicate the significance threshold at p < 0.05.

Figure 7.

A, Bilateral STG/MTG showed task-related functional connectivity (task vs test) with the vmPFC seed. B, A main effect of attention condition of two-by-two ANOVA showed significant activations in the vmPFC. The vmPFC circled in purple was used as a seed region in PPI analyses. C, Left STG/MTG showed functional connectivity with the vmPFC seed during the fixation-left condition. D, Right STG/MTG showed functional connectivity with the vmPFC seed during the fixation-right condition. p < 0.05, corrected.