Serotonin affects association of aversive outcomes to past actions.

Impairment in the serotonergic system has been linked to action choices that are less advantageous in a long run. Such impulsive choices can be caused by a deficit in linking a given reward or punishment with past actions. Here, we tested the effect of manipulation of the serotonergic system by tryptophan depletion and loading on learning the association of current rewards and punishments with past actions. We observed slower associative learning when actions were followed by a delayed punishment in the low serotonergic condition. Furthermore, a model-based analysis revealed a positive correlation between the length of the memory trace for aversive choices and subjects' blood tryptophan concentration. Our results suggest that the serotonergic system regulates the time scale of retrospective association of punishments to past actions.