Stroop performance in major depression: Selective attention impairment or psychomotor slowness?

Background: Numerous neuropsychological studies reported impaired Stroop performance in major depressive disorder (MDD) patients. Methods: The present study attempted to identify possible neuropsychological mechanisms involved in this impairment in untreated MDD outpatients (n = 75) as compared to healthy subjects (n = 83). Inspection Time, Finger Tapping, Simple and Choice Reaction Time were considered as measures of perceptual, motor, psychomotor speed, and response selection, respectively. Results: MDD patients performed significantly slower than healthy controls in the neutral and the congruent conditions, but not in the incongruent ones. In order to identify predictors of Stroop performance, linear hierarchical regressions analyses were performed. Age, motor and psychomotor speed were predictors of response time and accuracy on Stroop performance. Significant correlations between response time and the number of errors in all three Stroop conditions were found in MDD patients, while such a correlation was obtained in the healthy controls only in the incongruent condition. Limitations: Although education was included as a covariate in our analyses, suggesting that the observed effects could not be ascribed to education differences, further testing with education-matched samples is warranted. Conclusions: Our study shows that the Stroop task performance is affected by both aging and MDD. Impairment in the Stroop performance can be predicted by psychomotor slowness and by vigilance level in MDD outpatients, but not by impairment of selective attention per se.