If ACC causally mediates model-based but not model-free RL, inhibiting ACC in a task in which these strategies give similar recommendations should have little effect. To test this, we performed the same ACC manipulation in a probabilistic reversal learning task, in which model-based and model-free RL are expected to generate qualitatively similar behavior (n = 10 JAWS mice, 202 sessions, 78,041 trials, n = 10 GFP mice, 202 sessions, 67,009 trials; Figure S8). Inhibiting ACC from trial outcome to subsequent choice produced a very subtle (but significant) reduction in the influence of the most recent outcome on the subsequent choice (Figure S8D; permutation test p = 0.024, Bonferroni corrected for six predictors, stimulation-by-group interaction p = 0.014). Directly comparing effect sizes between the two tasks is challenging, because in the structurally simpler reversal learning task, subjects adapt much faster to reversals (Figures 1E and S8C) and hence recent trials have a stronger influence on choices. However, the small effect in the reversal learning task relative to the influence of previous outcome on non-stimulated trials, suggests that in this simpler task, in which model-based and model-free RL both recommend repeating rewarded choices, other regions could largely compensate for ACC inhibition.
