5-HT modulation by acute tryptophan depletion of human instrumental contingency judgements

The concept of 'depressive realism', that depression leads to more accurate perception of causal control, has been influential in the field of depression research, but remains controversial. Recent work testing contingency learning has suggested that contextual processing might determine realism-like effects. Serotonin (5-hydroxytryptamine, (5-HT)), which is implicated in the pathophysiology of depression, might also influence contextual processing. Using acute tryptophan depletion (ATD), we tested the hypothesis that dysfunctional serotoninergic neurotransmission influences contingency judgements in dysphoric subjects via an effect on contextual processing. We employed a novel contingency learning task to obtain separate measures (ratings) of the causal effect of partcipants' responses and efficacy of the background context over an outcome. Participants, without a history of depression, completed this task on and off ATD in a double-blind, placebo-controlled, within-subjects design. As with other work on contingency learning, the effects of ATD were related to baseline mood levels. Although no overall effects of ATD were observed, the subgroup of participants with low Beck depression inventory (BDI) scores showed reduced ratings of contextual control and improved accuracy of contingency judgements under positive contingencies following ATD, compared to placebo. High BDI participants demonstrated low accuracy in contingency judgements, regardless of serotoninergic status. No effect of ATD on contingency judgements was observed in the group as a whole, but effects were observed in a subgroup of participants with low BDI scores. We discuss these data in light of the context processing hypothesis, and prior research on 5-HT and depressive realism.