Risky decision-making involves weighing good and bad outcomes against their probabilities in order to determine the relative values of candidate actions. Although human decision-making sometimes conforms to rational models of how this weighting is achieved, irrational (or nonnormative) patterns of risky choice, including shifts between risk-averse and risk-seeking choices involving equivalent-value gambles (the “reflection effect”), are frequently observed. In the present experiment, we investigated the role of serotonin in decision-making under conditions of uncertainty. Fifteen healthy adult volunteers received a treatment of 3 g per day of the serotonin precursor, tryptophan, in the form of dietary supplements over a 14-day period, whereas 15 age- and IQ-matched control volunteers received a matched placebo substance. At test, all participants completed a risky decision-making task involving a series of choices between two simultaneously presented gambles, differing in the magnitude of their possible gains, the magnitude of their possible losses, and the probabilities with which these outcomes were delivered. Tryptophan supplements were associated with alterations in the weighting of gains and small losses perhaps reflecting reduced loss-aversion, and a marked and significant diminution of the reflection effect. We conclude that serotonin activity plays a significant role in nonnormative risky decision-making under conditions of uncertainty.

Using transcranial magnetic stimulation (TMS), we have tested the time needed for the perceived onset of spontaneous motor intention to be fully determined. We found that TMS applied over the presupplementary motor area after the execution of a simple spontaneous action shifted the perceived onset of the motor intention backward in time, and shifted the perceived time of action execution forward in time. The size of the effect was similar regardless of whether TMS was applied immediately after the action or 200 msec after. The results of three control studies suggest that this effect is time-limited, specific to modality, and also specific to the anatomical site of stimulation. We conclude that the perceived onset of intention depends, at least in part, on neural activity that takes place after the execution of action. A model, which is based on the mechanism of cue integration under the presence of noise, is offered to explain the results. The implications for the conscious control of spontaneous actions are discussed.