Abstract: Psychological stress refers to the process in which the body produces a series of physiological, psychological and behavioral responses to cope with internal and external challenges and maintain the homeostasis of the body when individuals are confronted with social psychological challenges or threats. Although the functions of stress response systems can maintain stability to a certain extent, studies have found that the functions of these systems may also be affected by factors such as previous stress experiences, thus showing individual differences in stress regulation. Therefore, researchers put forward the concept of background stress, which refers to the frequent or/and long-term stressors experienced by individuals in the social life environment, to explore the impact of these accumulated stress experiences on the individual's stress response system and physical and mental health. A large number of studies focused on the influence of background stress on the function of the hypothalamic-pituitary-adrenal axis (HPA axis) and found that higher level of background stress is related to a higher level of HPA axis activity. The brain is a core organ in stress processing. The brain defines stressors, which determine what is threatening to an individual. At the same time, the brain orchestrates physiological and psychological mechanisms to respond to stressors. In addition, the brain and the experience of stress have a wide range of two-way effects, with the brain changing both structurally and functionally as a result of the experience. Studies have found that individuals’ stress responses are mainly regulated by the brain region of the limbic system like hippocampus and the structure and function of these brain region are affected by background stress thus exhibiting a high degree of plasticity. Although studies have shown that high background stress leads to changes in the activity of the HPA axis, and the activity of the HPA axis is highly regulated by the brain regions related to the limbic system, it is still unknown whether the effect of background stress on the HPA axis can be reflected in the stress related neural activities, especially in the spontaneous neural activity. To this end, the daily life stress was quantified as an index of background stress. And the spontaneous neural activity in resting state was collected, and the regional homogeneity (ReHo) was quantified as an index. Results revealed that a higher level of background stress was associated with higher level of ReHo in right hippocampus (extending to thalamus and brainstem). Considering the function of these brain regions in stress regulation, we speculate that the higher level of ReHo in right hippocampus caused by the high level of background stress may reflect a stronger negative feedback regulation of hippocampus on the activity of the HPA axis. Furthermore, the higher level of ReHo in the right thalamus and brainstem may reflect a heightened alertness to the external threat information. Taken together, the present study suggests that higher level of background stress leads to a continuous active state of the spontaneous neural activity in the limbic brain region. These results not only help us to understand the effects of background stress on the function of the stress response system through evidence of neural mechanisms in the brain, but also help to understand the biological mechanisms by which background stress "penetrates the skin" to affect health outcomes.

Key words: background stress, regional homogeneity (ReHo), hippocampus

摘要： 本研究试图探究背景应激对应激相关大脑自发神经活动的影响。使用日常应激水平作为背景应激的量化指标。同时，使用静息态fMRI技术，采集了个体在静息态下的大脑自发神经活动，并使用局部一致性（ReHo）作为指标。结果发现，背景应激越高的个体，右侧海马（扩展到丘脑和脑干）静息态下局部一致性水平越高。结果提示，经历了高背景应激的个体会表现出边缘系统脑区的自发神经活动的持续活跃状态。

关键词: 背景应激, 局部一致性（ReHo）, 海马