Category-dependent and category-independent goal-value codes in human ventromedial prefrontal cortex

Abstract

To choose between manifestly distinct options, it is suggested that the brain assigns values to goals using a common currency. Although previous studies have reported activity in ventromedial prefrontal cortex (vmPFC) correlating with the value of different goal stimuli, it remains unclear whether such goal-value representations are independent of the associated stimulus categorization, as required by a common currency. Using multivoxel pattern analyses on functional magnetic resonance imaging (fMRI) data, we found a region of medial prefrontal cortex to contain a distributed goal-value code that is independent of stimulus category. More ventrally in the vmPFC, we found spatially distinct areas of the medial orbitofrontal cortex to contain unique category-dependent distributed value codes for food and consumer items. These results implicate the medial prefrontal cortex in the implementation of a common currency and suggest a ventral versus dorsal topographical organization of value signals in the vmPFC.

Figure 1

Task, univariate value signals and behavioral results. (a) Illustration of experiment time course and data extraction. Subjects were presented with an 80% chance of obtaining a stimulus drawn from a pool of 120 stimuli evenly divided into three categories (food, money and ‘trinkets’) and they responded with an integer WTP value between 0 and 4 euros (approximately $5.45) inclusive (Methods). In preparation for the multivariate analyses, we extracted a sample of neural data at the bid time point in each trial (with a shift of 5 s to account for hemodynamic delay). For a given bid, the two volumes closest in time (one before and one after) to the shifted time point were averaged to create a single sample. (b) A region of the vmPFC, overlapping with a previous similar result11, was parametrically modulated by the chosen bid value at the time of decision, peak coordinates (x, y, z = 0, 35, −7), t = 3.14, P < 0.05 SVFDR (results presented at P < 0.005, uncorrected). (c) Distribution of WTP bids per category (similar to those obtained previously). The average bid was €1.47 (s.d., €1.28) for food items, €1.91 (s.d., €1.3) for monetary sums, €1.97 (s.d., €1.56) for trinkets. There was a difference between the mean bids of the three categories (ANOVA, P < 0.001). The average bids were significantly greater than zero for all three classes (P < 0.001). The majority of bids were nonzero (71% for food, 82% for money and 74% for trinkets).

Figure 2

Distributed category-dependent value codes in the mOFC for food and trinkets. (a) Stimulus value represented in distributed codes in the mOFC for food and trinket categories. The peak classification accuracy t-scores were at the following coordinates: food, (x, y, z = −9, 17, −22), t = 3.05; trinkets, (x, y, z = −3, 41, −11), t = 3.86; P < 0.005 SVFDR (results presented at P < 0.005, uncorrected). (b) Plot of MVPA second-level voxel t-scores versus y-axis location. Food and trinket MVPA value t-scores are plotted in blue and red, respectively. Gray dashed line indicates P < 0.005 uncorrected significance threshold. Large dots indicate peak t-scores.

Figure 3

Organization of univariate and distributed value signals in the vmPFC distinguished by coding mechanism and stimulus information content. (a) A sagittal view of the vmPFC, showing that univariate and multivariate category-independent value representations are concentrated in the mPFC whereas category-dependent value signals (for the food and trinket categories) are located more ventrally in the OFC. Peak of the category-independent value decoding conjunction was at (x, y, z = −3, 41, 3), t = 2.40, P < 0.05 SVFDR (results presented at P < 0.005, uncorrected). (b) Bootstrap results for univariate/multivariate value correlations performed for each combination of category and vmPFC subregion.

Figure 4

Comparisons of univariate and multivariate value signal strengths across vmPFC subregions. (a) For the food and trinket categories, univariate and within-category MVPA second-level voxel t-scores are plotted as a function of the voxel’s z coordinate. The t-scores in the univariate brain maps exhibited a significantly greater tendency to increase along the z axis (P < 0.05). (b) Difference between the within-category MVPA and univariate value t-scores across voxels for the food and trinkets item categories in the mPFC and the mOFC. Error bars, s.e.m.

Figure 5

Stimulus category coding. In the frontal lobe, the central OFC (peak (x, y, z =−21, 38, −11), t = 11.14), the mFPC (peak (x, y, z = 6, 65, −11), t = 6.89) and the dorsolateral PFC (peak (x, y, z = −60, 17, 14), t = 11.34) contained distributed neural patterns pertaining to the identity of the stimulus under consideration. Toward the posterior, regions of the temporal lobes including the fusiform, inferior temporal and parahippocampal gyri, and areas around the intraparietal sulci also reflected category-discriminating activity (Supplementary Table 1). Results are presented at P < 0.005 FDR.