Motivated Forgetting Based On Think/No-think Paradigm:Behavior And Brain Mechanism

Motivated forgetting refers to a process during which individuals have the motivation to forget their unhappy life experiences and this type of forgetting is totally different from passive forgetting. For instance, individuals who have survived from traumatic events such as a huge earthquake but lost his/her families must have a strong motivation to forget his/her memory about that disaster. With his/her continuously will to stop extracting those memories, he/she will eventually forget the unfortunate past. There are two fundamental experimental paradigms to study motivated forgetting:DF (directed forgetting) paradigm and think/no-think, TNT paradigm. DF paradigm refers to inhibitory control about unwanted memories entering into consciousness by stopping encoding; TNT paradigm is used to suppress awareness of an unwanted memory at retrieval. Though there are two types of motivated forgetting:encoding stopping and retrieval stopping, people usually choose to exclude their unwanted memories from awareness to preserve their emotional state and positive self-image, which is more involved in the process of retrieval stopping. Therefore, the whole research only implemented TNT paradigm to explore the behavior and neural mechanism of motivated forgetting. The main research goal is to reveal the dynamic process of motivated forgetting using TNT paradigm, which consists of three progressive parts:1) the behavioral research of memory suppression; 2) the electrophysiological basis of memory suppression; 3) the dynamical process of memory suppression revealed by simultaneous EEG-fMRI.In recent years, a plenty of TNT behavioral studies have consistently indicated that memory suppression could produce negative control effect, which reflects impaired memory for "Suppress" items below that of Baseline items arising from people’s effort to stop retrieval, thus verify the feasibility of the implementation of motivated forgetting in a laboratory context. In the first part of the research, we have recruited 24 participants to complete our modified TNT experiment. According to our behavior results, we have observed significant negative control effect, but no positive control effect, which means enhanced memory for "Respond" items above that of Baseline items, was acquired in this experiment. The obtained negative control effect indicated that we were able to successfully obtain the process of motivated forgetting by implementing TNT paradigm.On the basis of the abovementioned behavior results in the first part, we further employed Electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) to comprehend the electrophysiological and blood oxygenation level dependant process of memory suppression respectively. In the second part of our research, we have chosen four ERP components, which included P2(180-240ms), N2 (350-450ms, the parietal positivity, PP (450-600ms) and the frontal slow wave, FSW (700-900ms) based on previous reported findings. Then we have computed the averaged amplitude of each component for every subject. We only found significant difference on N2 according to our one way ANOVA analysis, with Recall amplitude< Baseline amplitude< Suppress amplitude.We further split the participants into two groups on the basis of their size of negative control effect:the high forget group and the low forget group. We then found that individuals who have better pre-test memory performance would experience more memory intrusive, so they tried harder to suppress unwanted memories, which were confirmed by the observed bigger amplitude of N2 on these participants, but they finally forgot less "Suppress" items. On the individual level, after our regression of gender effect, we found a positive correlation between the amplitude of P2 and individuals’forgetting performances, which suggest P2 might play an important role in the memory suppression process.The results from discussed above verified our successfully implementing TNT paradigm to obtain the memory suppression process to a big extend. However, we did not know the exact time and place of neural activity of memory suppression’s occurrence based on previous researches. To address the dynamical process of memory suppression, we employed single trial parametric modulation based on the technique of coupled EEG-fMRI to analysis the process of memory suppression at a high time-space resolution in the third part of the research.Firstly, our fMRI evidences showed that suppressing awareness of an unwelcome memory is achieved by inhibitory control processes mediated by right middle frontal gyrus and posterior parietal cortex. These mechanisms interact with brain regions such as hippocampus that represent experiences in memory, disrupting traces that suppprt retention, which is in accordance with previous brain imaging findngs.’More importantly, the four abovementioned ERP components were treated as separate components to model their successive, independent parametric modulations of the fMRI in order to gain new insights into the time course and the specific memory suppression characteristics of their BOLD correlates. Our results indicated that the suppression of unwanted memories was achieved by four separateprocesses:1) the first phase was an initiation of inhibition reaction; 2) the second phase was response conflicts because of the intrusive memories; 3) the third phase was adjustments from brain dorsal attention network; 4) the final phase was the deactivation of brain regions for storing memory traces.Focused on memory suppression, this research project provided a good insight of behavior and brain mechanisms for motivated forgetting. More importantly, our research was the first to integrate EEG and fMRI study of memory suppression, which further proposed the dynamical process of motivated forgetting. Finally, because the ability of memory inhibition can represent individual’s mental health, future research on the neural mechanisms of motivated forgetting will have broad clinical applications to mental diseases such as post-traumatic stress disorder and psychological amnesia.