The Directed Forgetting For Emotionally Negative Information: Evidence From FMRI

In general, forgetting is regarded as a memory failure. Rather than treating forgetting as a memory disease, Ribot has mentioned its adaptive function. If we can not only remember the useful information, but also forget the outdated or irrelevant information, our life will keep relative balance. Intentional forgetting is a memory mechanism that people can consciously forget something useless which may interfere with current memory. The item-method directed forgetting paradigm is the major way to investigate the intentional forgetting. In the item-method, the study items are solely cued as to-be-remembered(TBR) or to-be-forgotten(TBF) on a trial-by-trial basis. For the item-method, selective rehearsal and attentional inhibition are proposed to account for the directed forgetting effect. The current study aimed to further explore the mental mechanisms underlying directed forgetting.The potential for the directed forgetting mechanism to disrupt emotional memories has been a hot subject. Intentional forgetting of emotional information is that people are intentionally to forget emotional memories, such as painful memories. A number of studies have suggested that emotion can enhance memory. Importantly, if emotion can enhance memory, might emotional memories be harder to forget? With this in mind, researchers start to explore the intentional forgetting of emotional memories. But there are inconsistencies between these studies. Some studies suggested that a directed forgetting effect can arise for emotional information, whereas others indicated that emotional information is resistant to forgetting. Considering these inconsistencies, we explored the mechanisms underlying intentional forgetting for emotionally negative and neutral words by using task-related functional magnetic resonance imaging(fMRI)(The first experiment). Behavioral results showed that the directed forgetting effect is obvious for emotionally negative and neutral words, but the directed forgetting effect is smaller for negative words in comparison with neutral words. At the neural level, the initial viewing of negative words caused enhanced activations in inferior frontal gyrus and superior parietal lobule when contrasted with neutral words. Increased activations in right inferior frontal gyrus and right superior parietal lobule have been implicated in attentional orienting, so the enhanced activities in these regions may reflect the capture of attention by negative content, which may have disrupted subsequent processing of the TBF instructions. In contrast with TBR instructions, TBF instructions for both negative and neutral words were associated with enhanced activations in frontal and parietal cortex. The frontal and parietal cortex were involved in inhibitory control, so the directed forgetting was associated with the active inhibitory control. In addition to frontal and parietal cortex, fusiform gyrus and precuneus also have been found to be activated. Since precuneus is associated with internal imagery and fusiform gyrus is linked to visual processing, the TBF instruction may initiate the process of visualization of TBF items in order to ensure which item was asked to be forgotten. Next, by comparing intentional forgetting and incidental forgetting, we found intentional forgetting is associated with inhibitory control, whereas incidental forgetting results from a failure of encoding.Researchers usually employ task-related electroencephalogram(EEG) and fMRI to investigate the neural correlates of intentional forgetting for emotional memories, but resting-state fMRI is rarely used. Resting-state fMRI is easy in operation and simple in application, so increased emphasis has been placed on it. Resting-state is referred to the process that subjects should keep their eyes closed, keep as still as possible and avoid engaging in any intentional brain activities. Resting-state fMRI reflects the baseline brain activity, and this state is closely associated with various of cognitive tasks. Based on these, we investigated the neural substrates of directed forgetting for negative words by using resting-state fMRI(The second experiment). By computing the correlation coefficient between ReHo/fALFF and the directed forgetting performance(TBR-TBF), we found that left inferior temporal gyrus, left middle temporal gyrus and left superior frontal gyrus were positively correlated with directed forgetting performance, whereas left posterior cingulate and left precuneus were negatively associated with directed forgetting performance. Left temporal gyrus and superior frontal gyrus are linked with memory encoding, posterior cingulate and precuneus are involved in mind wandering, so subjects put more cognitive resources into current cognitive tasks instead of mind wandering. Namely, the current results indicate that the directed forgetting of negative words involves in the selective rehearsal mechanism.