Research progress on the neural mechanism of light therapy improving cognitive function in depression: from retinal projections to neuroplasticity.

Cognitive impairment is a core and debilitating symptom of depression that severely impacts long-term patient prognosis and functional recovery. As a non-invasive physical intervention method, Light Therapy (LT) holds considerable therapeutic promise in improving the cognitive function of patients with depression. This review synthesizes clinical and preclinical evidence to delineate the complete mechanistic pathway of LT from the reception of peripheral light signals to the remodeling of central brain functions, that is, the regulatory pathway "from retinal projections to neuroplasticity." We first summarize clinical efficacy and its parameter dependence. We then detail the retinofugal projections via intrinsically photosensitive retinal ganglion cells (ipRGCs) to key brain regions such as the suprachiasmatic nucleus (SCN), the limbic system (e.g., amygdala, raphe nucleus) and the thalamus, and their role as the "neural highway" for light signal transmission. Finally, we elucidate how these pathways converge to induce neuroplastic changes such as hippocampal neurogenesis, prefrontal synaptic remodeling, up-regulation of neurotrophic factors, neural oscillation entrainment and inhibition of neuroinflammation. By proposing an integrative "peripheral input-central processing-functional output" model, this review provides a comprehensive framework for understanding LT's long-range brain regulation and a foundation for its precise clinical application.