Optimal flickering light stimulation for entraining gamma rhythms in older adults

Abstract

With aging, optimal parameters of flickering light stimulation (FLS) for gamma entrainment may change in the eyes and brain. We investigated the optimal FLS parameters for gamma entrainment in 35 cognitively normal old adults by comparing event-related synchronization (ERS) and spectral Granger causality (sGC) of entrained gamma rhythms between different luminance intensities, colors, and flickering frequencies of FLSs. ERS entrained by 700 cd/m2 FLS and 32 Hz or 34 Hz FLSs was stronger than that entrained by 400 cd/m2 at Pz (p < 0.01) and 38 Hz or 40 Hz FLSs, respectively, at both Pz (p < 0.05) and Fz (p < 0.01). Parieto-occipital-to-frontotemporal connectivities of gamma rhythm entrained by 700 cd/m2 FLS and 32 Hz or 34 Hz FLSs were also stronger than those entrained by 400 cd/m2 at Pz (p < 0.01) and 38 Hz or 40 Hz FLSs, respectively (p < 0.001). ERS and parieto-occipital-to-frontotemporal connectivities of entrained gamma rhythms did not show significant difference between white and red lights. Adverse effects were comparable between different parameters. In older adults, 700 cd/m2 FLS at 32 Hz or 34 Hz can entrain a strong gamma rhythm in the whole brain with tolerable adverse effects.

Audiovisual gamma stimulation for the treatment of neurodegeneration

“…Using electroencephalography (EEG) to record the brain response, multiple groups have tested different visual and auditory stimulation paradigms to achieve the highest gamma activity or most widespread effects. Regarding visual 40 Hz stimulation, brighter lights induced the strongest gamma brain activity in cognitively normal cohorts [18,19]. Different light wavelengths (i.e., colors) were tested, with some groups reporting enhanced gamma brain activity with red light [18], or enhanced connectivity with violet light as compared to white light [20].…”

“…found that red light at 34–35 Hz induced stronger and more widespread cortical activity beyond the visual cortex as compared to 40–50 Hz in young adults [18]. Additionally, this group found that regardless of the color, light flickering at 32 and 34 Hz induced stronger gamma brain activity than higher flickering frequencies (i.e., 36, 38, and 40 Hz) in older adults [19].…”

Cognitive and Neuropathophysiological Outcomes of Gamma-tACS in Dementia: A Systematic Review

“…Even sensory stimulation studies are casting doubts about the most effective stimulation frequency to entrain gamma oscillations. For instance, Lee et al (2021) showed that lights flickering at 34 to 38 Hz entrained gamma oscillations stronger than flickering at 40 to 50 Hz in young adults, whereas 32 or 34 Hz light flickering induces stronger gamma entrainment than 38 or 40 Hz flickering in older adults (Park et al, 2022). Further, Khachatryan et al (2022) found that the inclusion of a cognitive task during the GENUS session exerts positive effects on the strength and extent of gamma oscillations, promoting the propagation of gamma entrainment to deep brain areas.…”

An update on the use of gamma (multi)sensory stimulation for Alzheimer’s disease treatment

“…Recently, Lee et al (2021) and Park et al (2022) investigated the optimal color, intensity, and frequency of gamma light stimulus for entraining gamma activity in young and older adults, respectively. Particularly, adverse events, event-related synchronization (ERS) values (assessed through EEG), and the propagation of gamma oscillations were evaluated using different light colors, luminance intensities, and flickering frequencies from 32 to 50 Hz.…”

“…Findings from older adults were slightly different. Specifically, 700 cd/m 2 intensity and 32/34 Hz frequency were the optimal parameters of the flickering light stimulus to entrain gamma rhythm in parieto-occipital cortex and, by propagation, in fronto-temporal cortex with tolerable adverse effects ( Park et al, 2022 ). These findings demonstrate that the optimal stimulation parameters to effectively induce gamma oscillations vary based on the characteristics of the entrained brain (e.g., age, neurodegeneration).…”