When local neurons of the brain are excited, the oxygen saturation of blood increases in the corresponding region. Rs-fMRI reflects the change of local brain activity by detecting the change of local oxygen saturation in the brain. ALFF reflects the fluctuation of regional cerebral blood flow signal, which is an index to reflect the strength of spontaneous activity of neurons.15 By calculating the Kendall’s concordance coefficient of the time series between a voxel and its adjacent voxels, ReHo suggests the synchronization of the time series of neural activity among adjacent voxels in the brain region, reflecting the consistency of spontaneous activity of neurons in this brain region.16 ReHo analysis assumes that the hemodynamic characteristics of voxels are similar in functionally in similar clusters and are dynamically synchronized. Therefore, an abnormal ReHo score can suggest a local functional imbalance.17 ReHo has also shown sensitivity to various neuropsychiatric disorders associated with cognitive and emotional changes,18 such as depression and social anxiety.
The concept of DMN was put forward by Raichle et al. in 2001. It is believed that some brain regions maintain a high level of activity in the quiet and awake state (i.e. resting state) despite there being no active task. When stimulated, these brain regions are then inhibited due to decreased functional activity during task execution.19 These regions include the medial prefrontal cortex, cingulate gyrus, precuneus and inferior parietal lobule (including angular gyrus and supramarginal gyrus) which participate in episodic memory extraction, environmental awareness and self-related processing,20 and regulate attention and cognition in a wider range.21-22 Contrary to purposeful thinking activities related to external tasks, the DMN is involved in self-referential internal thinking activities, and tasks that require attention often lead to a reduction in DMN activities.23-24
The precuneus, angular gyrus and supramarginal gyrus are all important components of the DMN. The precuneus is involved in advanced cognitive processes, including visuospatial image processing, episodic memory retrieval, self-related processing and consciousness.25-27 It participates in self-related mental representation at rest and shows greater activity at rest than when computing external tasks.26-28 Both the angular gyrus and supramarginal gyrus are components of the inferior parietal lobule and are involved in visuospatial attention,29 episodic memory,30 mathematical cognition31 and language.32
The results showed that the ALFF value of the precuneus was negatively correlated with the HAMA score. That is, patients with a lesser degree of spontaneous precuneus activity had greater levels of anxiety. The ALFF value of the inferior parietal lobule (angular gyrus and supramarginal gyrus) was positively correlated with the FSAQ score. Patients with lesser spontaneous activity of the inferior parietal lobe had a lower FSAQ score, indicating decreased attention.
These results were consistent with the clinical symptoms of patients with depression, wherein patients with greater attention deficits and anxiety showed decreased spontaneous activity of the precuneus and the inferior parietal lobule (angular gyrus and supramarginal gyrus) in the resting state. Contrary to the spontaneous activity enhancement in the DMN of normal people, the spontaneous activity in the DMN of patients with depression decreases in the resting state. This may weaken the ability of patients with depression to retrieve episodic memory, assess the surrounding environment, conduct self-related processing and self-referential internal thinking activities. These findings can result in clinical symptoms such as depression, anxiety, abnormal increase of self-attention and attention deficit.
In addition, it was also found that the ReHo value of the right angular gyrus was positively correlated with the SDS score and the HAMA score. This means that patients with a stronger synchronization of neuronal activity in the right angular gyrus showed more severe anxiety and self-conscious depression. A growing body of evidence shows that the bilateral angular gyrus is strongly involved in mental activity and the right angular gyrus is related to the disturbance of action consciousness. 33-35 As such, patients with lesions involving the right angular gyrus will have changes in autonomous behavior consciousness, affecting the predictive control of voluntary behavior.36-37 In this study, the abnormal enhancement of neuronal synchronization in the right angular gyrus was positively correlated with the degree of self-evaluated depression, but not with the score of depression scale evaluated by others. This finding was attributed to the change of autonomous behavior consciousness, rendering the self and external evaluations inconsistent.
From previous research studies, it is known that anxiety is the most common concomitant symptom of depression.38 Furthermore, both depression and anxiety are common in breast cancer patients.4 Disordered emotional and cognitive processes are considered to be an important part of the pathophysiology of anxiety disorder.39 In this study, the positive correlation between the synchronization of neuronal activity in the right angular gyrus and the degree of anxiety in patients suggested that the angular gyrus may affect the ability to integrate and cope with negative emotions such as fear and anxiety. Disorders in the angular gyrus would hamper patients' understanding of situational information and self-cognition, contributing to a state of anxiety and depression.
Depressive disorder has a variety of suggested origins. Post-operative patients following breast cancer surgery were chosen to explore the changes in cerebral cortex activity by rs-fMRI. Results suggested that the changes in the cortical activity of multiple functional brain regions, especially components of the DMN, were closely related to the attention deficit in depressive disorder and closely associated with anxiety. We found that the change of neuronal activity synchronization in the right angular gyrus was related to the degree of anxiety in patients after breast cancer surgery, suggesting that the right angular gyrus may be involved in the generation of anxiety. Future studies should examine the changes and role of the right angular gyrus in patients with depression and anxiety.
The values of ALFF and ReHo obtained by rs-fMRI can be used as a quantitative parameter to evaluate the changes of brain activity in patients with depression, which can help to eliminate confounding factors, and achieve a more objective evaluation of depression state in combination with clinical scale scores, in order to help better identify depressive disorder in patients after breast cancer surgery and further improve the quality of life and prognosis of those patients.
The most important limitation is lack of sample size, which may directly lead to no significant difference in the ALFF and ReHo values between two groups. But this study attempted to break the limitation of routine grouping antitheses, the correlation between cerebral cortex activity and clinical scale scores of patients after breast cancer surgery was directly analyzed in the form of non-grouping to avoid setting normal standards for the analysis software, and found that the cerebral cortex activity of patients with depression after breast cancer surgery was significantly related to the clinical scale scores, demonstrating an obvious correlation trend between them.
Future studies will continue to increase the sample size and follow up these patients to conduct a comparative study of changes before and after treatment. Further studies should also examine the cortical activity in the corresponding brain regions after an improvement of clinical symptoms. Taken together, these improvements may be used to further confirm the claim that activity changes of the DMN can be used as an index to evaluate the severity and therapeutic effect in patients suffering from depression.