Single unit recording from macaque orbitofrontal cortex during a social decision making task. (A) Left, trial structure. Monkeys fixated a central stimulus, then shifted gaze to T1, resulting in a fluid reward, or T2, resulting in a fluid reward and the subsequent presentation of an image. The volume of fluid reward and the image category displayed varied in a nested block design []. The social category of the image pool remained stationary for superblocks of approximately 60–100 trials, whereas the amount of fluid reward associated with the two targets remained stationary for blocks of ~20 trials. Right, exemplars of social image categories. (B) Monkeys differentially value distinct classes of social images. Left, image values for each image category as derived from session-by-session choice behavior. The y-axis represents the percent of juice supplement paired with T1 necessary to induce indifference between T1 and T2. The x-axis indicates the image category paired with T2. Positive values indicate that animals sacrifice juice to view an image category, whereas negative values indicate that monkeys must be “bribed” to view an image. Right, viewing time durations for each image category for the trials in which the monkey viewed an image display. Y-axis represents the percent of samples for which the monkey’s eye position fell within the confines of the presented image while that image was being displayed. *indicates p<0.05 ** indicates p<0.001. (C) Location of recording sites from all three monkeys, depicted on coronal cross sections from a macaque brain atlas []. Each dot represents one neuron. Cool-colored dots represent those neurons grouped into Areas 14/25 (medial OFC), and hot-colored dots represent neurons grouped into Areas 11/13 (lateral/orbital OFC). Different colors within each shad represent individual monkeys.

OFC neurons respond to both fluid and visual social rewards. (A). Average population response. Note transient increases in firing rate time locked to the onset of juice delivery and the onset of image. PSTH represents population average response of all neurons aligned to task events. Left panel, aligned to initial fixation (trial start); middle panels, aligned to saccade and juice reward onset; right panel, aligned to image onset. Shaded gray boxes indicate the three epochs (saccade, juice delivery, image display) used for subsequent firing rate analysis. Firing rate was normalized by subtracting baseline (ITI) firing rate on each trial. PSTH is collapsed across all juice reward sizes and all image categories. Only trials in which the monkey chose to view the image are included. (B) Percentage of recorded neurons with firing rates significantly modulated by social image category (black bar), fluid amount (gray bar), or their interaction (white bar) for three different monkeys. The number of neurons modulated by image is significantly larger than that modulated by fluid or their interaction for all three monkeys. (C) Top and middle panel: representative neurons with firing significantly modulated by social image category (left), but not by fluid reward magnitude (right). Bottom panel displays an example neuron whose firing was significantly modulated by both image category (left) and fluid amount (right). For all example neurons, time zero is aligned to image reward onset in left panel and juice reward onset in right panel. Gray shaded boxes indicate the epoch relevant to the depicted statistical results. For illustration purposes PSTHs depict non-normalized firing rates smoothed with a 200 ms boxcar during trials for which the monkey chose to view an image.

OFC neurons signal fluid and social image value. (A) Correlation coefficients between normalized firing rates and fluid rewards (left) and image rewards (right) for the juice delivery epoch and image epoch, respectively. Significant neurons are highlighted in red. Black arrowheads designate the r-values of the two neurons depicted in (B). Social r-values were calculated using firing rates measured while fluid reward was held constant (i.e. only trials yielding the medium-sized fluid reward), and juice r-values were calculated using normalized firing rates measured while image was held constant (i.e. only trials displaying the gray square). Both forced choice and free choice trials resulting in image display were included. (B) Representative neurons with firing modulated by fluid reward magnitude (left) and social image value (right). Black, cyan and magenta represent small, medium, and large fluid amounts, respectively; Green, orange, yellow, and red represent different image categories ordered by value (as derived from choice behavior), ranked in the order listed. Time zero is aligned to juice reward onset (left) and image reward onset (right). Gray shaded boxes indicate the epoch relevant to the depicted statistical results. Histograms are smoothed with a 200 ms boxcar. For illustration purposes, plots depicted in (B) are derived from non-normalized firing rates during trials for which the monkey chose to view an image, and are collapsed across all image categories (left) or juice amounts (right). (C) OFC neurons signal social interest. Left top panel, mean looking durations for the four image categories during a sample session. G, gray square; D, dominant faces; S, subordinate faces; P, perinea. Left bottom panel, mean firing rates of the example neuron as a function of looking duration. Right panel, non-normalized firing rate of a neuron recorded during the same session, segregated by image category. PSTH is aligned to image onset; black bar indicates the period during which the image was displayed. Histogram is smoothed with a 200 ms boxcar.