The Cognitive Neural Mechanism Of Directed Forgetting For The Emotional Memory

Over the past century, psychological research on forgetting has focused on passive factors that make us forget. This emphasis on passive factors gives us an impression that all the processes underlying forgetting must happen involuntarily. Forgetting is a negative outcome and people don’t want that happen. However, this view ignores an important phenomenon that not all experiences are pleasant. On the one hand, in daily life, we try our best to forget the painful memory to preserve our healthy emotional state and optimistic attitude towards life. On the other hand, intentional forgetting of these painful memories can also make us focus on the present task, and accomplish it more effectively. Researchers have tried to study this phenomenon in the lab and call this phenomenon as motivated forgetting. Anderson and Hanslmayr (2014) give a definition to the motivated forgetting based on previous researches. They hold that’motivated forgetting’ refers to increased forgetting arising from active processes that down-prioritise unwanted experiences in service of creating or sustaining an emotional or cognitive state. In the lab, the directed forgetting paradigm and the memory suppression paradigm---think/no-think task were used to study the motivated forgetting phenomenon. In these two paradigms, the memory cues were given suddenly in the encoding period or in the retrieval period. The results showed that participants’ memory was affected by these memory cues. In the encoding period, the memory cue can stop the encoding process and prevent further rehearsal of these study items. In the retrieval period, the memory cue can interrupt the association from the cues to an associated memory. Both encoding and retrieval stopping would terminate a mnemonic process so that an experience can be excluded from conscious awareness.With the development of neuroimaging technology, researchers begin to focus on the neural mechanisms of intentional forgetting. In most cases, memory we want to forget is negative. However, there is no consistent conclusion on whether people can intentionally forget the negative memory. Experience certainly tells us that there is individual difference in intentional forgetting of the negative memory. What’s the mechanisms of the individual difference in directed forgetting of negative memory? In this study, we firstly combined the structural and the functional images to explore the neural mechanisms of individual difference in directed forgetting of the negative memory. Second, in order to improve the ecological validity of directed forgetting studies, we firstly combined the directed forgetting paradigm and the self-reference task, and tried to explore the neural mechanisms of directed forgetting of the self-reference negative memory. At last, we explored the abnormal neural mechanisms when depressed participants tried to forget the negative memory. We hope these results can give some inspiration for the clinical psychological treatment.In the first study, we combined the structural and functional images to explore the neural mechanisms of individual difference in directed forgetting. We used the amplitude of low-frequency fluctuation (ALFF) as the functional index, and the gray matter density (GMD) as the structural index. In the study, subjects were firstly asked to complete the item-method directed forgetting task. This task included three phases, which is the study period, a distraction task and the recognition memory test. We obtained the behavioral index of the directed forgetting effect for every subject. Then, multiple linear regression analysis was conducted to explore the relationship between the directed forgetting effect and the structure index (GMD) and the functional index (ALFF). The behavioral data showed that though there were significant directed forgetting effect for the neutral and negative stimuli, and the directed forgetting effects of the negative stimuli was smaller than the neutral stimuli. The functional data showed that the directed forgetting ability was positively correlated with the ALFF of the right middle frontal gyrus and the right inferior parietal lobe, while this ability was negatively correlated with the left inferior temporal gyrus. The brain regions which positively correlated with the directed forgetting effect of negative stimuli included the right inferior frontal gryus, the inferior parietal lobe and the superior frontal gyrus. The ALFF of the left angular, the right superior temporal gyrus and the left middle occipital gyrus was negatively correlated with the directed forgetting effect of the negative stimuli. In the experiment 2, the GMD data showed that the regions which were positively correlated with the directed forgetting of the neutral stimuli included the right middle frontal gryus. Whereas, the GMD of left middle temporal gyrus and superior temporal gyrus were negatively correlated with the directed forgetting ability of the neutral stimuli. For the negative stimuli, the GMD of the right inferior frontal gyrus was positively correlated with the directed forgetting effect. On the other hand, the GMD of the left middle temporal gyrus and the parahippocampal gyrus were negatively correlated with the directed forgetting effect for the negative stimuli. Based on these results, it can be found that the GMD and the ALFF of the right middle frontal gyrus and the right superior frontal gyrus were positively correlated with the directed forgetting effect, while the GMD and the ALFF of the temporal regions were negatively correlated with the directed forgetting ability. These results indicate that the GMD and the AlFF of frontal regions would improve the directed forgetting ability, while the GMD and the AlFF of the temporal regions would decrease the directed forgetting ability. These results suggested that if a person is good at the intentional control ability, the directed forgetting ability were improved, whereas, if a person has a good ability at the intentional encoding process, the directed forgetting ability was destroyed. The neural mechanisms of individual difference in directed forgetting of the negative stimuli were associated with a more wide spread frontal network.In study 2, we firstly combined the directed forgetting paradigm and the self reference task to improve the validity of the directed forgetting studies.In the experiment 3, we used the classical item-method directed forgetting task to explore the directed forgetting of the self reference emotional memory. The only difference from the classical directed forgetting paradigm is that the participants need to make a self-reference judgment when learning the study items. When they finished the study period, there was a distraction task. At last, there was a recognition task. The participants need to make a "remember (R)" or "Knowing (K)" response after the recognition task. The results of the recognition task showed that there was a directed forgetting effect for the self-reference information, while no directed forgetting effect was found for the other-reference information. The results suggested that self-reference process improve the discrimination of all these materials. Participants could use more inhibition mechanism to inhibit the TBF items, whilemore cognitive resources could be used to encode all the TBR items. Nevertheless, there was no interaction between the memory cue and the emotion valence of the materials. Maybe the reason is that we used the recognition as memory test. So in the experiment 4, we used the recall as the memory test. The experiment procedure is similar to experiment 3, the difference is that participants need to complete the recall memory test. The results showed that the self-reference negative and neutral stimuli can be directed forgetted. However, the self-reference positive stimuli can’t be directed forgetting. In the other-reference condition, the positive stimuli can be directed forgetting, however, the negative and neutral stimuli can’t be directed forgetting. The results suggested that self-reference influenced differently on emotional memories. The self-enhancement motivation may be the reason for this effect. In order to have a positive self-image, participants try to forget the negative self-referential items, and keep the positive self-reference items. To further explore the neural mechanisms of the negative self-referential stimuli. In experiment 4, we combined the item directed forgetting paradigm and the self reference task. The behavioral data showed that the directed forgetting effect was obtained for the neutral and negative stimuli. However, the directed forgetting effect of the negative stimuli was smaller than the neutral stimuli. fMRI results showed that, compared with the negative other-referential information, forgetting of the negative self-referential information were associated with a more widespread activation, including the orbital frontal gyrus (BA47), the inferior frontal gyrus (BA45, BA44), and the middle frontal gyrus. Our results suggest that forgetting of the self-referential information seems to be a more demanding and difficult process.Previous studies also used the directed forgetting paradigm and the memory suppression paradigm to explore the directed forgetting ability for the depressed participants. There was no consistent result on this question. And there was no study focused on the neural mechanisms of directed forgetting of negative memory for depressed participants. In the experiment 6a, we firstly used the item-method directed forgetting paradigm to explore the abnormal activations when depressed participants tried to forget the negative stimuli. The participants were asked to complete the classic item-method directed forgetting task during this study. The behavioral results showed that depressed participants can’t forget the negative stimuli. The standard univariate analysis based on the General Linear Model (GLM) showed that compared to the healthy control group, depressed participants have increased activations in the Inferior Frontal Gyrus (IFG), the Superior Frontal Gyrus (SFG), the Inferior Parietal Gyrus (IPG) and the Inferior Temporal Gyrus (ITG). These findings indicate that abnormalities in neural functions, especially in the IFG, SFG,IPG and the ITG, may underline the difficulties for depressed patients in directed forgetting of negative words. Although depressed participants recruited more frontal brain resources to forget negative words, stronger activation of the attentional and the working memory networks resulted from the negative bias made this attempt failure. In experiment 6b, the Support Vector Machine (SVM) was used to perform multivariate pattern classification based on brain activation during the intention to forget in the same depressed and the healthy control individuals. The results showed that the pattern of brain activity in the directed forgetting of negative words condition allowed correct group classification with an overall accuracy of 75%(P=0.012). The brain regions critical for this discrimination were just those abnormal activated brains during depressed participants tried to forget the negative words. These results indicated that abnormal neural circuitry when depressed individuals tried to forget the negative words might provide some neurobiological markers for depression.In general, this study firstly combined the structural and the functional imaging method to explore the neural mechanisms of individual difference in directed forgetting. Then, this study combined the directed forgetting paradigm and the self-reference task to explore the cognitive and neutral mechanisms when participants try to forget the self referential information. In addition, we also firstly explored the abnormal neural activations when depressed participants tried to directly forget the negative memories. To examine whether the pattern of brain activity in the directed forgetting of negative words condition allowed correct group classification, we also performed multivariate pattern classification based on brain activations during intentional forgetting in the depressed and the healthy individuals. From the basic research point of view, these studies can provide some evidence for the neutral mechanisms of directed forgetting, and add some evidence to the theory of forgetting. On the other hand, these studies can give some inspiration to the clinical therapy of depression.