Abstract:

Major Depressive Disorder (MDD) is associated with mood-congruent processing biases towards negative information in working memory (WM), which is considered as the core manifest of cognitive vulnerability of MDD. This review provides an overview of the biased processing of emotional information of depression in three executive components of WM (i.e., updating, inhibition, and shifting). Patients with MDD are slower in updating negative contents in WM and have difficulties in disengaging from task-irrelevant negative information within WM. Neuroimaging studies show that the dorsolateral prefrontal cortex (dlPFC) and dorsal anterior cingulate cortex (dACC) are hyperactivated in patient with MDD relative to healthy controls. Moreover, patients with MDD perform poorer than healthy controls in updating positive contents in WM and they have smaller occipital P1 in response to positive materials, which indicates an insensitivity to positive items on early encoding stage of WM updating in depression, thus depression is associated with negative enhancement and positive attenuation in updating emotional contents in WM. In regards to inhibition, patients with MDD are found to exhibit impairments in suppressing irrelevant negative information and cannot effectively prevent the irrelevant negative information entering WM during inhibition. These impairments in inhibition have been suggested to be associated with altered brain activations of the dlPFC and ACC. Relative to healthy controls, patients with MDD show decreased activation of the dlPFC and increased activation of the rostral anterior cingulate cortex (rACC) and posterior cingulate cortex (PCC). Abnormal functional connectivity within the frontoparietal network has also been suggested to underlie deficits in cognitive control of depression. The findings of emotion-specific dysfunctions of shifting function in depression are inconsistent, which might be related to emotional valence of task materials and confounded by other symptoms, such as rumination. The current review of biased processing of emotional information of depression in WM provides a new perspective for understanding cognitive bias of depression, which helps unravel the cognitive and neural mechanisms that underlie the cognitive information processing of emotional contents within WM of depression. Importantly, this review also offers important clues for future research on vulnerability factors implicated in the onset and maintenance of depression and expands our understanding of cognitive models of depression from a cognitive neuroscience perspective. Based upon this literature review, we therefore have identified some open questions and future research directions in this important research area. Firstly, additional research is needed to investigate the unity and diversity of the biased processing in three WM components, and assess their distinctive contribution to depressive symptoms, as well as the mediating effect of other cognitive biases (attention, memory and interpretation biases) and emotion regulation strategies on this association. It is worth noting that the combined cognitive bias hypothesis has been proposed to formulate the interactions among cognitive biases in depression. This hypothesis is arguing that cognitive biases do not operate in isolation, but are interdependent and work together to influence symptoms of depression. Secondly, future research is needed to examine dysfunctions of WM bias across divergent samples (the at-risk sample, the current MDD, the remitted MDD) to uncover the impact of biased processing in different stages of MDD. Thirdly, the induction effects of different emotional materials (affective words, emotional face, or emotional pictorial materials) on biased processing should be explored in future studies. Moreover, unraveling the neurophysiological mechanisms underlying the process of emotional materials in WM could help resolve the inconsistency of previous findings. Finally, by examining the effect of WM training on reducing WM bias and depressive symptoms, future research could provide direct evidence for the causal relationship between cognitive bias and depression and benefit future development of cognitive bias modification interventions for biased processing in WM of depression. 

Key words: working memory, the central executive system, emotional stimuli, depression