Abstract:

When confronted with reminders of an unpleasant memory, people often try to prevent the unwanted memory from coming to mind. Suppression-induced forgetting (SIF) means that the attempt to prevent unwanted memories from entering awareness results in a decrease in the long-term accessibility of these memories. Previous studies indicated that the suppression of retrieval is accomplished by control mechanisms that inhibit unwanted memories. Suppressing retrieval increased engagement of the right dorsolateral prefrontal cortex and middle frontal gyrus and concomitantly decreased engagement of the hippocampus. The degree of SIF is affected by the emotionality of information and an individual’s emotional state and training. 
From the perspective of research materials, current studies on the SIF effect of emotional memories mostly use neutral cues to match emotional materials, seldomly assess the emotional valence of cues during the learning process or after retrieval inhibition phase. More attention is paid to the memory association between cue and target event, but ignores the affective connection of them. In reality, due to the close connection between events and cues in the process of forming traumatic memories, trauma-related cues are no longer completely neutral but would have negative emotional valence. Therefore, future studies should explore the impacts of SIF on the basis of focusing on affective connection between cue and event. 
By individual pathological state, existing studies have investigated the SIF effect in depression and post-traumatic stress disorder (PTSD) groups, but not reached consistent conclusions. This may be related to heterogeneity of mental disorders. The people who are diagnosed with PTSD or depression will may have different etiology, symptoms and underlying neural mechanisms. Future studies should distinguish these patients into different subtypes and conduct tests in groups with the same subtypes and the same symptoms, so as to further compare the SIF effect and its neural mechanisms among different subtypes. Given that poor inhibition in patients with depression and PTSD is often associated with difficulty in activation of the right middle frontal gyrus and right dorsolateral prefrontal cortex, stimulation of the right dorsolateral prefrontal cortex may improve core symptoms in patients with PTSD. Therefore, repetitive transcranial magnetic stimulation may be used in the future to improve the activity of relevant brain regions and enhance the inhibitory control. In addition, in reality, due to the impact of stress hormones on memory consolidation, unpleasant memories are often strong and profound. Future studies may also consider the influence of stress hormones on SIF. 
Finally, repeated suppression training can gradually weaken the right dorsolateral prefrontal cortex-hippocampal negative coupling connections, leading to more effective control of invasive memories, suggesting the feasibility of training intervention in memory control. It has been found that mindfulness-based strategy was to facilitate the process of forgetting by releasing the tension due to the resistance to forget and aiding the reallocation of attentional resources away from the unwanted items. Therefore, future studies should consider whether the mindfulness-based forgetting strategies can improve the effect of SIF, and whether individuals who have engaged in long-term mindfulness training (such as meditation) show a better ability of retrieval suppression than those who are naïve to mindfulness practices. 
Future studies should investigate ways to improve the therapeutic effects of SIF on clinical pathological memory based on an in-depth understanding of the neural mechanisms of SIF.

Key words: suppression-induced forgetting, emotional valence, emotional state, training