Cognitive inhibition and working memory in unipolar depression

Over the past decade, evidence has accumulated to suggest that people suffering from Major Depressive Disorder (MDD) present impairment in attention, working memory, executive function, including cognitive inhibition, problem- and task-planning. The aim of the current study was to assess inhibitory mechanisms within working memory with emotionally neutral material in a group of patients suffering from MDD. We hypothesized that impairment in cognitive inhibition is global and not only due to the emotional valence of the stimuli employed for the tasks.

Twenty patients with MDD (DSM-IV) and 20 healthy controls were recruited. To assess cognitive inhibition, we used neutral material, in the form of the Prose Distraction Task (PDT) (Connelly SL, 1991), Trail Making Test (TMT), Modified Card Sorting Test (MCST), Rule Shift Cards (RSC), Stroop test and Hayling Sentence Completion test (HSC). The Modified 6 elements test, the Brixton Spatial Anticipation test, the dual task performance and the verbal fluencies test were also used to assess other executive function such as flexibility, planning tasks and memory.

Individuals with depression showed impairment in cognitive inhibition. They made more errors on the PDT, alongside slower response times. Slower response times were also observed on the Stroop, TMT and RSC. The MDD group made more errors in HSC and performed worse than controls in the semantic part of verbal fluency and Modified 6 elements tasks. The impairment of access function was significantly associated with the level of depression.

Depressed patients showed inability to inhibit neutral information access to working memory, restrain and delete irrelevant information. This impairment in cognitive inhibition could underlie cognitive slowness and attentional deficits in depression.