Neural responses to incongruency in a blocked-trial Stroop fMRI task in major depressive disorder

Background: Patients with major depressive disorder (MDD) perform poorly on the Stroop task, which is a measure of the executive control of attention, with impaired interference resolution. The neural correlates of this deficit are not well described. To examine how this deficit relates to pathophysiological abnormalities in MDD, we conducted an fMRI Stroop study comparing MDD subjects to controls. Methods: Forty-two unmedicated patients with current MDD and 17 control subjects underwent fMRI scanning with a color-word Stroop task. Subjects assessed font color during alternating color identification (e.g., 'XXXX in blue) and incongruent color/word blocks (e.g., the word 'red in blue). We examined neural activation that was greater in incongruent than color identification blocks (Z>2.3 and corrected p<0.05), controlling for trial-by-trial reaction time. Results: Compared to controls, MDD subjects exhibited lower activation during incongruent blocks across multiple brain regions, including middle frontal gyrus, paracingulate and posterior cingulate, precuneus, occipital regions, and brain stem. No brain regions were identified in which MDD subjects were more active than controls during incongruent blocks. Limitations: Not all MDD subjects were antidepressant-naïve. Conclusions: Brain regions related to executive function, visual processing, and semantic processing are less active during processing of incongruent stimuli in MDD subjects as compared to controls. Deficits of attention in MDD may be the product of a failure to maintain activity across a distributed network in a sustained manner, as is required over the sequential trials in this block design. Further studies may clarify whether the abnormalities represent a trait or state deficit.