Rumination might be defined as dwelling on negative events or having persistent negative thoughts. In the present study, investigators attempted to measure rumination levels using a patient self-evaluation dealing with the tendency to react to negative emotions with ruminative, or repetitive thoughts; however, in patients with MDD, persistent negative thoughts are a hallmark feature, and there are challenges associated with such subjective measures.

Thus, in the present study by Schwert and colleagues, the aim was to further characterize this relationship between rumination and cognitive impairment. Results of this study indicated that cognitive impairments in MDD are linked to rumination in the domain of processing speed. Indeed, this was the only cognitive domain that was influenced by rumination after investigators included other predictors in the hierarchical regression analysis. However, ultimately, after hierarchical linear regression, an effect of high- and low-level rumination status on executive function could not be established. Indeed, data are limited on the significance of a single, select cognitive deficit, its relevance to executive functions, and its impact on functional outcomes in patients with MDD. There are, however, data to support a prime influence of cognitive function on depressive symptom severity. For instance, Siegle and colleagues found that use of the Paced Auditory Serial Addition Test increased metacognitive skills, decreased rumination, and improved depressive symptom severity.

Investigators proposed that rumination could be an underlying mechanism for the connection between processing speed and cognitive impairments in MDD, while acknowledging that further research is needed to assess this relationship.