Optimal flickering light stimulation for entraining gamma waves in the human brain

Abstract

Although light flickering at 40 Hz reduced Alzheimer’s disease (AD) pathologies in mice by entraining gamma waves, it failed to reduce cerebral amyloid burden in a study on six patients with AD or mild cognitive impairment. We investigated the optimal color, intensity, and frequency of the flickering light stimulus for entraining gamma waves in young adults. We compared the event-related synchronization (ERS) values of entrained gamma waves between four different light colors (white, red, green, and blue) in the first experiment and four different luminance intensities in the second experiment. In both experiments, we compared the ERS values of entrained gamma waves between 10 different flickering frequencies from 32 to 50 Hz. We also examined the severity of six adverse effects in both experiments. We compared the propagation of gamma waves in the visual cortex to other brain regions between different luminance intensities and flickering frequencies. We found that red light entrained gamma waves most effectively, followed by white light. Lights of higher luminance intensities (700 and 400 cd/m2) entrained stronger gamma waves than those of lower luminance intensities (100 and 10 cd/m2). Lights flickering at 34–38 Hz entrained stronger and more widely spread beyond the visual cortex than those flickering at 40–50 Hz. Light of 700 cd/m2 resulted in more moderate-to-severe adverse effects than those of other luminance intensities. In humans, 400 cd/m2 white light flickering at 34–38 Hz was most optimal for gamma entrainment.

Optimal flickering light stimulation for entraining gamma rhythms in older adults

“…In general, longer wavelengths seem to be a better stimulus for gamma entrainment. This was also hinted in our prior study on young adults as red FLS showed a trend of stronger entertainment compared to the white FLS 19 . However, the difference was not significant, and in addition, study on older adults has shown no difference between entrainment of white and red FLS.…”

“…In a previous study on 10 patients with AD, adverse effects of approximately 700 cd/m 2 FLS were also mild 35 . However, in our previous study on younger adults, adverse effects were more common and severe in 700 cd/m 2 FLS compared with those in 400 cd/m 2 FLS 19 . Older adults may be more tolerable to stronger light than younger adults, possibly due to an age-associated increase of miosis and lenticular senescence 36 , 37 .…”

“…In our previous study on younger adults, 700 cd/m 2 FLS entrained stronger and more widely spread gamma rhythm than FLSs with low luminance intensities (400 cd/m 2 , 100 cd/m 2 , or 10 cd/m 2 ), which was the first study on the effect of luminance intensity of FLS on the power and propagation of entrained gamma rhythm simultaneously in younger adults 19 . In line with our previous study, Jones et al found that 377 cd/m 2 FLS entrained stronger gamma rhythm at both occipital and frontal electrodes than 192 cd/m 2 FLS in three healthy volunteers 16 .…”

“…In our previous study, FLS with relatively high luminance (700 cd/m 2 or 400 cd/m 2 ) and longer wavelength (red or white) entrained stronger and more widely spread gamma rhythms than the FLS with weaker (100 cd/m 2 or 10 cd/m 2 ) and shorter wavelength (green or blue), respectively, in younger adults 19 . Furthermore, the lights flickering at 34–38 Hz entrained stronger and more widely spread gamma rhythms than those flickering at other frequencies, including 40 Hz 19 .…”

“…The efficiency of gamma entrainment is influenced by the luminance intensity, color (wavelength), and flickering frequency of FLS 13 – 19 . In our previous study, FLS with relatively high luminance (700 cd/m 2 or 400 cd/m 2 ) and longer wavelength (red or white) entrained stronger and more widely spread gamma rhythms than the FLS with weaker (100 cd/m 2 or 10 cd/m 2 ) and shorter wavelength (green or blue), respectively, in younger adults 19 . Furthermore, the lights flickering at 34–38 Hz entrained stronger and more widely spread gamma rhythms than those flickering at other frequencies, including 40 Hz 19 .…”