| Individual can be considered to have sub-threshold depression (SD) when he or she has clinically relevant depressive symptoms, without meeting criteria for a full-blown major depressive disorder (MDD). The SD is a significant risk-indicator of MDD because it can be considered as a part of the prodromal phase of MDD. Almost all subjects who developed MDD have initially passed through a period of SD whereas not all sub-threshold depression individuals will finally develop MDD. Research on SD is important because it is important to understand the process how an individual develops MDD and the role of depressive symptoms in the process, what’s more, it may provide a rationale for the development of new interventions that prevent the onset of new cases of MDD.Recent studies showed that depressed samples exhibit cognitive bias when processing information, and biases denote in attention, interpretation, memory, and cognitive control processes. Depression is a disorder of impaired emotion regulation. Cognitive inhibition is a key mechanism in the regulation of emotion. There is emerging evidence that depression is characterized by deficits in the inhibition of mood-congruent material. These deficits could result in prolonged processing of negative, goal-irrelevant aspects of presented information thereby hindering recovery from negative mood and leading to the sustained negative affect that characterizes depressive episodes.In our daily life, individual experience the negative feeling, because negative memory has stored in the memory system, pull way these unwanted experiences in service of creating or sustaining an emotional or cognitive state is important for our mental health. Motivated forgetting play an important role in this process of emotion regulation. Motivated forgetting refers to increased forgetting arising from active processes and this inhibition process can be engaged either during memory encoding or retrieval to limit retention of unwanted memories. Motivated forgetting can be sorted into directed forgetting (which refers to the forgetting in encoding process) and suppression forgetting. Suppression forgetting mainly studied use Think/No-Think (TNT) paradigm, which is a procedure investigating the inhibition in retrieval of memory. Many studies have shown that individuals can successfully limit extracting target memory in TNT task across many stimuli, including word pairs, face-scene pairs, face-word pairs, word-object pairs, and pairs comprising words and nonsense shapes.Depression is accompanied impaired memory and attention, and also a tendency towards ruminations about sadness. Several authors have proposed that depression diminishes cognitive control, making control over negative thoughts and feelings difficult. TNT had been used to investigate some other mental illness (like post-traumatic stress disorder and Anxiety disorders), and found many valuable results. So far, few studies have examined whether diminished memory control accompanies mild or clinical depression with TNT. However, these studies use different paradigms, and not get a consistent conclusion. To some degree, it is unclear what behind the suppression forgetting in SD. We suppose that SD individuals may have a minor deficit in memory control thus may lead to the difficulty in retrieving inhibition process. To investigate the cognitive and neural mechanism of this kind of motivated forgetting and test the assumption, we designed two experiments.In experiment 1, firstly, we screened sub-depression(SD) group and healthy controls(HC) using BDI-II and CES-D. We use the neutral and negative adjective as response while neutral noun words serve as stimulus. The stimulus and response members of each pair had a weak pre-experimental relationship, and were unrelated to words in other pairs. All word are double-word Chinese characters. The word pairs were exposed individually for 5s in the center of a computer screen with the response printed to the right of the stimulus. Test-feedback cycles followed in which subjects were presented with the stimulus member and asked to say the response aloud as quickly as possible. The correct answer was given visually to help subjects to remember word pairs. Test-feedback cycles on all the pairs continued until a minimum of 50% of the pairs were correctly recalled. After initial study, subjects were given the think/no-think phase instructions and take a short practice before scanning. Subjects were told that when stimulus present in red, he or she should prevent the associated memory item from coming to mind, while it is green and respond silently. In test phase subjects were given all learned stimuli were randomly and ask then to recall the target words. We also measure some depression related behavior, like rumination and emotional modification ability. Results show that both two groups show negative control effect under neutral words condition. As for negative words, no significant inhibition effect found in SD group while HC group still find the effect. No significant differences found between two groups. Task fMRI results shows that, sub-threshold depression group show more activation in inhibition control brain network than normal control group under both negative and neutral words condition. Since two group shows no significant difference in suppression effect, sub-threshold individuals may need more cognitive effort to inhibit the retrieval of target words. Psychophysiological Interactions results shows that sub-threshold group had an opposite link between right middle frontal gyrus and hippocampus, which may indicate that the interaction of these two brain region is affected.In experiment 2, we use resting state fMRI to explore the depression interaction between negative control effects and brain. Results shows that both memory system and inhibition control system had a regulation by depression.Based on results of two experiments, we can draw a conclusion that sub-threshold depression individuals’ neural memory suppression control ability may be mild affected. |
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