Serotonin and active avoidance following negative reward prediction errors. (a) Depiction of ‘next actions’. Responses to trial (t) were analyzed for bet(t)=(0%, (10–20%), (30–40%), (50–60%), (70–80%), and (90–100%)). For each of these six levels we examined ‘lower bet’ next actions (black arrows) and ‘hold-or-raise’ bet next actions (gray arrows). (b) The n=842 negative RPE transients presented in Figure 4a and b are represented here but separated according to next-bet decision and current betting level. These results are provided in order to explore the significant negative interaction between serotonin and current bet on predicting change in bet (Supplementary Table 2). Consistent with this negative interaction in the regression analysis, we observe that large positive 5-HT transients at large bets predict a lowering of the bet at trial t+1 (black line, right panels). While dips in 5-HT are associated with reducing one’s bet at low bet levels to even lower levels (black line, left panels). The opposite effect is observed for holding or raising one bets, grey lines (with 0% not showing any significant transient effects). Significance here is indicated for uncorrected t-tests against zero (*p<0.05, **p<0.01, ***p<0.005). (c) Applying a correlation analyses to examine the relationship between serotonin and current bet levels when the next decision is to lower one’s bet. We find that over all ‘lower bet’ decisions, serotonin, and the current bet level were positively correlated (R=0.3; p<0.00001). This indicates that market withdrawal is affected by serotonin following poor outcomes. More specifically it indicates that serotonin may prevent further withdrawal (when investment is already low) and promote withdrawal (when investment is high). (d) Correlating decisions to hold or raise bets suggests the opposite effect (but at only trend-level significance).