Brain Structural Mechanism And Influenced Factors Of Motivated Forgetting

Forgetting is as important as memory itself.We need to remember the happiest moment of past experiences and sometimes to forget the disturbing and unpleasant memories.Immersing in unfortunate and painful memories will not only affect study,work and normal life,but also cause some individuals to suffer from mental illness.Therefore,it is meaningful for those who want to get rid of the unwanted memories by intentionally forgetting them if it is possible.The ability to intentionally suppress unwanted memories,which,ultimately,increase the possibility of forgetting,was termed as motivated forgetting which reflect the inhibitory ability of memories.Some experimental paradigms focused on whether memory encoding or memory retrieval process was the key to the memory suppression have been used to study motivated forgetting,among which “Think/No-think” paradigm was used to imitate and reveal the nature of retrieval-suppression behind motivated forgetting.With the development of fMRI technics,researchers have made great achievements in the field of retrieval-induced forgetting.There are several brain areas including cognitive control related dorsal lateral prefrontal cortex(DLPFC),anterior cingulate cortex(ACC),parietal cortex,thalami and memory related hippocampus,which are often seen in fMRI analyses.Specifically,magnitude of BOLD signal activation in DLPFC is positively correlated to the inhibitory ability,while the activation in hippocampus is negatively correlated to it.Furthermore,studies found that activation in DLPFC could predict deactivation in hippocampus,and the magnitude of functional connectivity under tasks are also positively correlated to inhibitory ability.Therefore,researchers hypothesized that cognitive control brain areas could exert an influence on the function of hippocampus which ultimately cause forgetting.By analyzing the task fMRI or functional connectivity,previous researchers mainly focused on how many brain areas were involved under a given retrieval manipulation.Although some researchers explored the individual differences of inhibitory ability from anatomical perspective,studies from structural perspectives are few.Moreover,in the anatomical study,only few subjects' hippocampus volume were analyzed,and no study had focused on the anatomical features of DLPFC.Therefore,this study wants to know whether it is possible to combine VBM,resting functional connectivity and MVPA methods to explore the structural mechanism behind the motivated forgetting and to check the possibility of using structural features as a biomarker in predicting individual inhibitory ability of memory.Finally,although the hypothesis suggest that DLPFC suppress the hippocampus during motivated forgetting,no direct evidence of anatomical or fiber tracking data in human beings were found to support it.Given that some researchers then suggest that the inhibitory passage from DLPFC need a transfer hub,such as ACC or thalamus,which are considered to be attentional related brain areas,therefore,this study also wants to know whether inhibitory ability is affected by other influential factors.Based on these,we hereinafter carry out two studies to answer the two questions above.Study 1 used combined VBM,resting functional connectivity and MVPA methods to explore the structural mechanism behind motivated forgetting.As a result,we found that the gray matter volume(GMV)of hippocampus,thalamus,amygdala,ACC and DLPFC were correlated with inhibitory ability,among which hippocampus and DLPFC were significant.Besides,GMV differences between bilateral hippocampus and DLPFC were also positively correlated with the inhibitory ability.Moreover,we found that resting functional connectivity between right hippocampus and right DLPFC also contributed to inhibitory differences,with the higher the connectivity shows,the higher the inhibitory ability was.Finally,we used the MVPA(SVM)machine learning method and found that GMV of right hippocampus could distinguish both the higher inhibitory group with an accuracy of 58.62% and the lower inhibitory group with an accuracy with 60.92%,with a mean accuracy of 60%.However,the GMV of right DLPFC only figured out the lower inhibitory group,with an accuracy with 60.92%.These evidences proved that structural mechanism behind motivated forgetting including two critical brain areas,i.e.,hippocampus and DLPFC.Study 2 aimed to explore the importance of genome differences in affecting individual inhibitory ability and its possible affecting path through brain.Based on the technology of Polygenic Risk Score(PRS),this study analyzed and found a significant negative correlation between ADHD PRS and inhibitory ability.The value of ADHD PRS presents the genomic similarity between healthy subjects and ADHD patients who usually shows cognitive-control deficits.Therefore,this evidence shows that genomic difference could also affect the inhibitory ability of memory,explicitly.Besides,we found that Thalamus and Insula showed close correlations with ADHD PRS,with the Thalamus playing a negative correlation with the ADHD PRS.Moreover,we found that the GMV of Thalamus could mediate the relation between ADHD PRS and inhibitory performance of memory.Given the close connection between thalamus and attention-related functions,these results suggest that the process of motivated forgetting could be affected by attentional mechanisms.In summary,this study found that hippocampus and DLPFC are critical brain areas of motivated forgetting,which could be used as a biomarker in predicting individual inhibitory ability of unwanted memory.Besides,memory suppression may be also affected by genomic differences through attentional mechanisms.