A new neuroimaging study from China has found that an eight-week course of bright light therapy helped reduce depressive symptoms in individuals with subthreshold depression. The treatment also altered dynamic functional connectivity in several brain regions associated with mood regulation. The study was published in the Journal of Affective Disorders.
Subthreshold depression refers to the presence of depressive symptoms that are clinically relevant but do not meet the full diagnostic criteria for major depressive disorder. Individuals with subthreshold depression may experience persistent sadness, fatigue, sleep disturbances, or concentration problems, but with fewer symptoms or a shorter duration than required for a formal diagnosis.
Despite being “subthreshold,” the condition can impair daily functioning and reduce quality of life. It is also linked to an increased risk of developing major depression in the future. Subthreshold depression is common—especially among adolescents, older adults, and individuals with chronic illnesses—and it often goes undiagnosed and untreated because the symptoms are perceived as mild or situational. However, research shows that even mild depressive symptoms can negatively affect social relationships, job performance, and physical health.
Study author Guixian Tang and his colleagues aimed to examine the effects of bright light therapy on both depressive symptoms and brain function in individuals with subthreshold depression. Bright light therapy involves exposure to artificial light that mimics natural sunlight and is commonly used to regulate circadian rhythms and improve mood. While it is best known as a treatment for seasonal affective disorder, it has also shown benefits for non-seasonal depression, sleep disorders, and circadian rhythm disruptions. The authors hypothesized that it might also be effective for subthreshold depression.
The study involved 95 students from Jinan University in China. Participants were between 18 and 28 years old, ethnically Han Chinese, and right-handed. They were included in the study if they had a mild, nonseasonal depressive symptom profile, with scores meeting specific thresholds on two established depression measures.
Participants were randomly assigned to one of two groups. One group received bright light therapy: daily 30-minute exposure to a 5000 lux light box before noon for eight weeks. The placebo group used an identical-looking device that emitted only a dim light (less than 5 lux).
Before and after the treatment period, participants completed standardized assessments of depression (the Hamilton Depression Rating Scale and the Center for Epidemiologic Studies Depression Scale) and anxiety (the Hamilton Anxiety Scale). They also underwent resting-state functional magnetic resonance imaging (rs-fMRI) scans, with a focus on dynamic functional connectivity in the cingulate cortex—a brain region involved in emotional regulation.
The results showed that participants who received bright light therapy had significantly greater reductions in depressive symptoms than those in the placebo group. Neuroimaging data revealed that bright light therapy led to increased dynamic functional connectivity between:
- The right supracallosal anterior cingulate cortex and the right temporal pole,
- The left middle cingulate cortex and the right insula,
- The left supracallosal anterior cingulate cortex and the pons.
Conversely, the therapy decreased dynamic connectivity between the right supracallosal anterior cingulate cortex and the right middle frontal gyrus.
Importantly, increases in connectivity between the right supracallosal anterior cingulate cortex and the right temporal pole were positively associated with reductions in depressive symptoms, suggesting a link between brain network changes and mood improvement.
“BLT [bright light therapy] alleviates depressive symptoms and changes the CC dFC [cingulate cortex dynamic functional connectivity] variability in StD [subthreshold depression], and pre-treatment dFC variability of the CC could be used as a biomarker for improved BLT treatment in StD. Furthermore, dFC changes with specific neurotransmitter systems after BLT may underline the antidepressant mechanisms of BLT.”, study authors concluded.
The study sheds light on the effects of bright light treatment on subthreshold depression. However, the study was conducted on a small group of university students. Results on larger groups and groups that include older individuals might differ.
The paper “Effects of bright light therapy on cingulate cortex dynamic functional connectivity and neurotransmitter activity in young adults with subthreshold depression” was authored by Guixian Tang, Pan Chen, Guanmao Chen, Zibin Yang, Wenhao Ma, Hong Yan, Ting Su, Yuan Zhang, Shu Zhang, Zhangzhang Qi, Wenjie Fang, Lijun Jiang, Qian Tao, and Ying Wang.
