Neural correlates and effects of increased cognitive load on performance during the n-back task in major depressive disorder
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Background: Major depressive disorder (MDD) is a heterogenous disease that includes debilitating cognitive deficits, including working memory (WM). The n-back task used to examine WM as it requires continuous updating and recall. The present study examined the effects of MDD and cognitive load during the n-back task (0- and 2-back). The following were predicted, as cognitive load increased: (1) participants would display decreased accuracy and increased reaction time; (2) increased activity would be found in regions associated with WM; and (3) MDD symptoms and accuracy would be correlated with functional activity in these regions. Methods: MDD patients (n = 29) completed the n-back task during a functional magnetic resonance imaging scan. Behavioural and imaging data were analysed between conditions. Montgomery-Åsberg Depression Rating Scale (MADRS) scores and task accuracy were correlated with functional activity for each participant. Results: Compared to 0-back, participants demonstrated decreased accuracy and longer reaction times in the 2-back condition. During the 0-back condition, activity was seen in the bilateral insula and the middle occipital gyrus; in the 2-back condition, activity was seen in the middle and medial frontal gyri and the inferior parietal lobule. MADRS scores were not significantly correlated with activity in either condition. Activity in all regions identified in the 2-back condition were positively correlated with performance accuracy. Discussion: In the 0-back condition, activated regions are associated with resting state activities and the processing of visual stimuli. Compared to the 0-back condition, higher levels of activity were found during 2-back in the middle and medial frontal gyri and inferior parietal lobule, which are associated with WM, visual processing, and complex cognitive processes, indicating that participants required more neural resources during the more difficult task. Significant and mid-sized effects were found in the 2-back condition between accuracy and activity in the middle and medial frontal gyri and inferior parietal lobule, suggesting that MDD patients require more effort to perform well in cognitively demanding tasks. Limitations include the small and largely female sample, which would be improved by including more participants and considering covariates.