The effect of counterfactual information on outcome value signal encoding: Evidence for fully-adaptive coding along the rostrocaudal axis of the medial prefrontal cortex

OBJECTIVE: When we make a decision, usually we experience its consequences but we have no informa0on about potential outcomes of alternative decisions we could have made. Yet, this additional information is useful to improve learning (e.g., Palminteri et al., 2015). In this study, we analyzed brain activity related to outcome processing in loss and gain contexts to explore the neural coding of value for both factual and counterfactual outcomes, when either partial or complete feedback about the outcomes is provided. The aim of the experiment is two-fold: 1) to investigate where factual and counterfactual information is represented and 2) to unravel how outcome value is encoded in different feedback conditions. We hypothesize that counterfactual information will produce rescaling of value signal depending on the context, such that the value of a neutral outcome becomes positive in a loss context (as absence of punishment) and nega0ve in a gain context (as absence of reward), thus reflecting fully-adaptive coding of value signals. METHODS: Twenty-eight participants performed a probabilistic instrumental learning task while undergoing functional magnetic resonance imaging scanning. On each trial, they had to choose between two symbols probabilistically associated with a certain reward (or punishment). Then, the outcome of the decision was revealed. Importantly, on half of the trials participants received feedback only about the outcome of their decision (partial feedback condition), while in the other half they were informed about both the factual and the counterfactual outcome (complete feedback condition). We used univariate analysis as well as multivariate pattern classification (Haynes, 2015) to explore outcome value processing and encoding of factual and counterfactual outcomes, in trials with either complete or par0al feedback. Furthermore, we assessed neural coding of outcome value in these two conditions. RESULTS: Our results show that: 1) not only factual but also counterfactual information can be decoded from local patterns of brain activity, 2) brain activation in regions along the rostrocaudal axis of the medial prefrontal cortex (mPFC) increases for processing of factual outcomes but decreases for counterfactual informa0on processing, 3) outcome decoding is significant in less rostral regions of mPFC, and 4) outcome value is represented with a fully-adaptive code when complete feedback is provided, while it is encoded in an absolute way when feedback is partial. CONCLUSION: These findings suggest that outcome value processing is implemented through multiple coding mechanisms flexibly activated depending on the specific choice setting at hand.