Suppressing Negative Memories In Depression

According to Beck's cognitive model of depression,compared to healthy controls,depressed individuals exhibit excessive negative biases in many aspects of cognitive processes such as attention and memory.This negative biased information obtaining and processing plays a critical role in the etiology of depression and also in maintenance of depressive symptoms(e.g.the ruminative thinking style).However,most of the previous studies focused on the “bottom-up” deficit of selective attention in depression,leaving the role of “top-down” executive control much less concerned in the depressed population.Recently,some studies demonstrated that depressed individuals are less successful in excluding the unwanted thoughts and memories from awareness due to their deficits in the "top-down" control system;and this impaired inhibitory function may hinder recovery from depressive symptoms and induce a vicious circle of increasingly mood-congruent recurrence and persistent low mood.In order to clarify the neural mechanisms underlying the memory suppression deficits in depression,this study investigated the intentional forgetting of negative and neutral materials in both memory encoding and retrieval stages with dysphoria students.The event-related potentials were used as the main technique.We used item-method directed forgetting task to investigate the depression-related intentional forgetting deficits on memory encoding stage and its neural correlates.It is found that depressed subjects had difficulties in suppressing the memory encoding of negative words,while the memory suppression of neutral words was relatively intact.In contrary,nondepressed subjects could successfully forget both neutral and negative materials.At the meantime,depressed individuals,compared to nondepressed individuals,had enhanced word-evoked P2 and LPP for negative items and enhanced cue-evoked P1 and N2 for negative to-be-forgotten items.Based on these results,we propose that two mechanisms may contribute to the failure of forgetting negative material in depression.The two mechanisms,which influence the intentional forgetting in a negative context,are inefficient memory suppression(reflected by cue-evoked P1 and N2)and excessive preliminary processing(reflected by word-evoked P2 and LPP).To investigate the depression-related memory suppression deficits on retrieval stage,we utilized Think/No-Think paradigm with neutral and negative affective pictures.The results showed that compared with nondepressed group,depressed group recalled more negative items,irrespective of either "Think" or "No-Think" instructions.Accordingly,the frontal N2(reflecting voluntary memory inhibition)and parietal late positive component(LPC)(reflecting conscious recollection)showed deflection for negative items in depressed compared with nondepressed participants.On the one hand,the reduced N2 for negative "No-Think" items indicated that depressed individuals have low motivation to suppress negative items so intentional forgetting is less successful for mood-congruent events.On the other hand,the enhanced LPC for negative "Think" items suggested that negative memories are excessively revisited by depressed participants(compared with nondepressed ones)due to their mood-congruent and intrusive nature.Thus we demonstrated that depressed individuals show behavioral and ERP deviations from healthy controls for both voluntary suppression and conscious retrieval of negative memory;the two abnormalities of memory control together contribute to the difficulties in forgetting negative material in depression.However,the depressed participants we recruited in these two experiments are dysphoric students with high depressive tendency rather than depressed patients,therefore,making inferences based on these results should be in cautious way.