Implicit Counterfactual Effect in Partial Feedback Reinforcement Learning: Behavioral and Modeling Approach

Context by distorting values of options with respect to the distribution of available alternatives, remarkably affects learning behavior. Providing an explicit counterfactual component, outcome of unchosen option alongside with the chosen one (Complete feedback), would increase the contextual effect by inducing comparison-based strategy during learning. But It is not clear in the conditions where the context consists only of the juxtaposition of a series of options, and there is no such explicit counterfactual component (Partial feedback), whether and how the relativity will be emerged. Here for investigating whether and how implicit and explicit counterfactual components can affect reinforcement learning, we used two Partial and Complete feedback paradigms, in which options were associated with some reward distributions. Our modeling analysis illustrates that the model which uses the outcome of chosen option for updating values of both chosen and unchosen options, which is in line with diffusive function of dopamine on the striatum, can better account for the behavioral data. We also observed that size of this bias depends on the involved systems in the brain, such that this effect is larger in the transfer phase where subcortical systems are more involved, and is smaller in the deliberative value estimation phase where cortical system is more needed. Furthermore, our data shows that contextual effect is not only limited to probabilistic reward but also it extends to reward with amplitude. These results show that by extending counterfactual concept, we can better account for why there is contextual effect in a condition where there is no extra information of unchosen outcome.