The Cognitive And Neural Mechanism Of Memory Suppression And Its Intervention In Dysphoria

Memory suppression is a psychological process in which people voluntarily forget unwanted or unpleasant memories to reduce memory accessibility.Memory suppression of negative memory is particularly important for maintaining mental health.However,the research results are inconsistent on whether and how valance affect memory inhibition function in different individuals.The behavioral and neural mechanisms of memory inhibition function in dysphoria who are at high risk for depression need to be clarified.Moreover,memory suppression works by suppressing memory retrieval,which can be divided into two processes: recollection and familiarity.Nevertheless,researches on how memory suppression effect these two processes mainly used directed forgetting paradigm,which has been criticized for failing to separate voluntary forgetting and involuntary forgetting.Therefore,this current study would use the Think/No-Think paradigm in a functional Near-Infrared Spectroscopy(f NIRS)study and a transcranial Direct Current Stimulation(t DCS),respectively,to further examine the effect of active memory suppression on recollection-and familiarity-based memory retrieval in dysphoric individuals.In study 1,dysphoric(29)and non-dysphoric individuals(27)were recruited to complete the modified TNT task,and the f NIRS was used to investigate the neural mechanism of memory inhibition.For behavioral results,a repeated-measures ANOVA was performed over the recall accuracy,and a significant three-way interaction of memory instruction(T,NT,baseline)× valence(neutral,negative)× group(dysphoria,nondysphoria)was found on recall acccuracy,which showed that in the dysphoric group,no difference on recall accuracy between the NT and the baseline conditions was found,either for neutral stimuli or negative stimuli.In the non-dysphoric group,the recall accuracy of the NT condition was lower than the baseline condition for neutral stimuli,but no difference between these two conditions was found for negative stimuli.Then,response type(remember,know)was further included as an independent variable,a significant fourway interaction of memory instruction × valence × response type × group was found on recall accuracy,which showed that,in dysphoric group,no difference on remember-based and know-base recall accuracy between the NT and the baseline conditions was found,either for neutral stimuli or negative stimuli.In non-dysphoric group,the remember-based recall accuracy of the NT condition was lower than the baseline condition for neutral stimuli,but no difference between these two conditions was found for negative stimuli.In addition,in non-dysphoric group,no difference on know-base recall accuracy between these two conditions was found,either for neutral stimuli or negative stimuli.In f NIRS results,first,a repeated-measures ANOVA was performed over the parameter estimates(beta),there was no three-way interaction of memory instruction(T,NT)× valence × group.And a significant two-way interaction of memory instruction × valence was found at the superior frontal gyrus and the middle frontal gyrus.For negative stimuli,the NT condition induced greater activation at the middle frontal gyrus and the superior frontal gyrus than the T condition.For neutral stimuli,there was no difference in brain activation between the NT and T conditions.Furthermore,including the response type(remember,know,forget)as an independent variable,a significant interaction of memory instruction × valence ×response type × group was found at the preforntal cortex and the temporoparietal junction.The results showed that the interaction of memory instruction × group in neutral stimuli was significant on forget response.Then an independent sample t test of brain activity on differences between the NT and T conditions showed that dysphoric group had greater activation at the left inferior frontal gyrus than non-dysphoric group.In study 2,fifty dysphoric participants were recruited and randomly assigned into anodal stimulation group(25)and sham stimulation group(25)to complete the TNT task,and t DCS was used to investigate the effect of stimulating the right dorsolateral prefrontal cortex on improving memory inhibition function in dysphoric individuals.A repeatedmeasures ANOVA was performed over the recall accuracy,and a significant three-way interaction of memory instruction × valence × group(anodal stimulation,sham stimulation)was found,which showed that in the anodal stimulation group,the recall accuracy of the NT condition was lower than the baseline condition both for neutral and negative stimuli,and the suppression-induced forgetting effect of neutral stimuli was higher than that of negative stimuli.In the sham stimulation group,no difference on recall accuracy between the NT and the baseline conditions was found,either for neutral stimuli or negative stimuli.Therefore,the present study showed that:1.Negative stimuli are more difficult to suppress than neutral stimuli.2.The suppression-induced forgetting effect was only found in non-dysphoric individuals for neutral stimuli.No suppression-induced forgetting effect was found in dysphoric individuals,either for neutral stimuli or negative stimuli.3.The suppression-induced forgetting effect was found in conscious memory retrieval(recollection),but not unconscious memory retrieval(familiarity).4.Suppressing(no-think)negative stimuli induced greater activation at the superior frontal gyrus and the middle frontal gyrus than thinking negative stimuli.5.When suppressing neutral stimuli,dysphoric individuals had greater activation at the inferior frontal gyrus than non-dysphoric individuals.6.Anodal stimulation of the right dorsolateral prefrontal cortex with t DCS improved the memory suppression performance in dysphoric individuals,and improvements in suppressing negative stimuli were lower than suppressing neutral stimuli.In summary,the current study demonstrated that dysphoric individuals showed behavioral and neural mechanism abnormalities in memory inhibition,and further verified that sitmulating the right dorsolateral prefrontal cortex can effectively improve memory inhibition function in dysphoric individuals.The findings will help clarify the mechanism of impaired inhibition in depressed individuals,and provide a direction and possibility for designing interventions targeting at improving inhibiting function in depressed individuals.