Serotonin and active avoidance following positive reward prediction errors. (a) Depiction of ‘next actions’ as per negative RPE analysis in Figure 5. (b) Serotonin transients (n=882) following positive reward prediction errors as per Figure 4a and b but shown here separated according to decision on next trial and current bet level (lower bet on trial (t+1): cyan line; hold-or-raise bet on trial (t+1): dark green line). Only at low bets (where counterfactual losses dominate) did we observe large transients—the direction of the transient was not discriminative however in terms of next-bet decision. (c) Unlike following negative RPES, no parametric effect, in terms of current bet level and serotonin response, was observed for the decision to lower bet following positive reward prediction errors. (d) Similarly, no parametric effects were observed for serotonin responses preceding a decision to raise or increase bet levels (following a positive reward prediction error).