The Neural Representation of Mood in the Primate Insula

Compared to other brain functions (like remembering and deciding), how the brain reflects mood is much less well understood. One reason is that there is no objective right answer to the question, "How happy are you?" Consequently, mood cannot be studied by benchmarking against percent correct on behavioral tasks. While challenging, we cannot shy away from understanding mood given that ~20\% of adults will experience a mood disorder at some point, and for ~30\%, existing treatments will be ineffective. Here, we build on work that modulates, probes, and models fluctuations in subjective happiness as individuals experience wins and losses in a gambling task. A pooled mood model fit to behavioral data from eight human subjects provided reasonable predictions of happiness fluctuations for each subject's held-out data (0.43<r2<0.79). Characteristic of mood, fit mood model parameters integrated across multiple trials to estimate happiness. To determine how this operationalization of mood is reflected in the primate brain, we investigated the representation of estimated happiness in the insula of two monkeys performing the same gambling task. Here, we assume that the representation of human subjective happiness can be meaningfully studied by probing neural representations in the monkey brain, contingent on finding straightforward and robust neural correlates. We found that estimated happiness was reflected in monkey insula in the overall vigor of the population response in a manner that persisted across trials. Specifically, we found a strong and highly significant correlation between the magnitude of the insula response measured between trials and estimated happiness in both animals (r=0.56 \& r=0.49; p < 10-10). In addition, we determined via a partial correlation analysis that insula population response vigor reflected an integration of multiple trial outcomes, like behavior. These results are among the first insights into the nature of mood representations in the primate brain.