Supplementary Figure 8: Activity in bilateral hippocampus scales with entropy over scene reinstatement evidence.

We investigated the hypothesis that hippocampal activity could reflect retrieval of memories in support of decisions [1,2]. We used the scene-specific reinstatement weights to investigate activity related to memory retrieval. In previous studies, we observed that hippocampal activity increases along with uncertainty about an action’s outcome, both for simple sequential responses and goal-directed planning decisions [3,4]. We interpreted those findings as consistent with hippocampus’ known role in memory retrieval. In the context of action evaluation, memory retrievals could constitute evidence about the outcome of the actions under consideration (similar to how forward trajectories are “replayed” as rodents make navigation decisions [5]). In this task, greater uncertainty about the associated context should lead to a wider range of next-step outcomes to evaluate. We therefore reasoned that activity in hippocampus at choice might scale with uncertainty about the probed item’s context. We computed the evidence that participants reinstated each context image on each trial, by taking the correlation between per-scene template patterns and PPA activity on that trial. The resulting six numbers were then normalized to create a probability distribution for that trial, reflecting the relative likelihood that each scene was being remembered. The entropy over this distribution can thus be considered the uncertainty over the reinstated context. Consistent with a role for hippocampus in retrieving memories that are used to evaluate outcomes, this entropy value is reliably correlated with hippocampal activity at the time of choice (mean R=0.0670, SEM 0.0261, t(29)=2.5733, P=0.0155). Error bars are +/- 1 SEM.