Major depressive disorder (MDD) has extensive ties to motivation, including impaired response time (RT) performance. Average RT, however, conflates response speed and variability, so RT differences can be complex. Because recent studies have shown inconsistent effects of MDD on RT variability, the present research sought to unpack RT performance with several key improvements: (1) a sample of adults (n = 78; 18 MDD, 60 Control) free of antidepressant medication; (2) an unambiguously appetitive task with appealing incentives at stake; and (3) ex-Gaussian RT modeling, which can unconfound speed and variability by estimating parameters for the mean (Mu) and standard deviation (Sigma) of the normal component and the mean of the exponential component (Tau). The groups had comparable Mu and Sigma parameters, but the MDD group had a significantly larger Tau, reflecting greater intraindividual RT variability. The findings suggest that MDD's effect on average RT can stem from greater intraindividual variability, not from overall slowness. Possible mechanisms, such as impaired executive processes in MDD and difficulties maintaining stable mental representations of incentives, are considered.
Keywords: depression; ex-Gaussian models; intraindividual variability; motivation; response times.