Depth targeted opto-neuromodulation of brain regions: From mechanistic insights to disease models.

Depth targeted optical neuromodulation techniques enable spatially precise activation and control of brain circuits, advancing mechanistic insight into neuropsychiatric pathogenesis and informing targeted interventions. Because optical transport and safety constraints vary systematically with depth, effective applications require co-design of light delivery, phototransduction strategy, and anatomical target to ensure adequate photon flux, spatial resolution, and thermal margins. This review synthesizes techniques from superficial cortex to deep nuclei with emphasis on non-genetic optical modulation, one- and multiphoton optogenetics, implanted μLED/fiber systems, and nanomaterial-mediated transduction. Organizing evidence by depth tier, we compare mechanisms, dose-depth relationships, and validation modalities, and map these to disease models in neurodegenerative diseases, depression and epilepsy. We summarize how depth-appropriate protocols modulate circuit dynamics and improve behavioral or physiological endpoints, while delineating limitations arising from scattering, thermal load, off-target effects, and translational constraints. By indexing optical neuromodulation to depth, this review provides a unifying angle to evaluate mechanistic rationality, therapeutic potential, and paths to rigorous, reproducible application in disease research.