¹Syntropic Medical, Clinical, Klosterneuburg, Austria, ²University of Galway, School of Medicine, Galway, Ireland, ³New York University Grossman School of Medicine, Department of Neurology, New York, United States, ⁴UT Southwestern Medical Center, Interventional Psychiatry, Dallas, United States, ⁵University of Sao Paulo Medical school, Interdisciplinary Neuromodulation Service, Sao Paulo, Brazil, ⁶Brigham´s and Women Hospital, Mary Horrigan Connors Center for Women’s Health Research, Boston, United States, ⁷Institute of Science and Technology Austria, Neuroimmunology, Klosterbneuburg, Austria

Background: Flickering light represents a new, fully non-invasive brain stimulation technique. In both humans and animal models, it can induce brain entrainment, defined as synchronization of brain waves to the frequency of an external periodic stimulus. Preclinical studies have shown that 60 Hz light can induce juvenile-like plasticity via microglia-mediated remodelling of the perineuronal nets, underscoring the potential for depression. The neural effect and tolerability of repeated sessions of 60 Hz flickering light in humans, however, have not been described.

Aims & Objectives: This study was aimed to (1) characterize neural entrainment and synchrony induced by repeated 60 Hz light using EEG, and (2) assess safety and tolerability across multiple sessions in healthy human volunteers.

Method: Ethical approval was obtained from the Lower Austria Ethical Commission (GS3-EK-4/908–2024). The effects of 15 daily sessions (30 minutes each, five days/ week, for 3 weeks) of 60 Hz white light were investigated in 14 healthy volunteers. Active stimulation (n=8) was compared to sham, constant light stimulation (n=6). Side effects were self-reported on a daily basis and a Likert-like questionnaire was administered to investigate the tolerability of the stimulation. C-reactive protein (CRP) and cortisol levels in saliva were measured via ELISA. Brain entrainment was tracked using an 8-channel EEG setup during stimulation sessions n. 1, 5, and 15. Data were analyzed with parametric (t-test and ANOVA) or non-parametric (Wilcoxon´s rank sum and Kruskal-Wallis) tests, according to normality. Ordinal questionnaire responses were evaluated using the Wilcoxon rank-sum test. Proportions were compared using Fisher’s exact test.

Results: Participants were young (sham: 25.5 ± 2.59 and active: 28.63 ± 6.37 years of age; p=0.6), balanced across sexes (sham: 50%, active: 62.5% female; p=1), mostly caucasian (sham: 66.7%, active: 100%, p=0.16). Both constant and 60 Hz light stimulation were well tolerated (‘the stimulation was overall tolerable’, average score sham:4.5 ± 0.55, active: 4.8 ± 0.41, p= 0.95). The majority of participants (sham: 83.3%, active: 87.5%, p=1) reported side effects, which were minor (such as sleepiness, headache, and dry eyes) and did not differ statistically across groups (total number of side effects: 20 in sham and 18 in active, p=1). CRP and cortisol levels were unchanged after stimulation and not statistically different across groups, with a trend towards lower cortisol levels in the active group compared to the sham. EEG confirmed that 60 Hz light induced neural entrainment across the occipital, parietal, temporal, and frontal cortex. The entrainment signal remained highly synchronous initially but showed a significant reduction by day 19 (last stimulation session) relative to day 1. This decline, accompanied by an emerging desynchronization, is consistent with progressive neural habituation and may reflect underlying homeostatic plasticity mechanisms.

Discussion & Conclusions: 60 Hz flickering light induces robust neural entrainment and synchrony; the neural habituation observed suggests ongoing plastic changes in the absence of major side effects. Given the role of 60 Hz in cognition, neuroplasticity, and neuropsychiatric disorders, this topic is of interest for potential applications in depression, and feasibility studies in patients are currently ongoing in clinic (Lux study, NCT06922812) and at home (Neon study, NCT07111390).