Photosensitivity of an aging brain.

Red light is considered less phototoxic than blue light and is widely used in both research and photobiomodulation therapy. The difference in the response to brain exposure to light between young and old mammals is currently unknown. We found that brain exposure to blue light caused local damage in the cerebral cortex in both young and old mice. Brain exposure to red light did not have any noticeable effect on young mice. However, it caused a marked reduction in electroencephalogram power, damaged fiber bundles throughout the brain, and brought about a coma-like state in old mice. The effect of red light on electroencephalogram power was dose-dependent and particularly strong in the theta range. When delivered at a lower intensity but over a longer period, red light produced a similar reduction in electroencephalogram power and brain damage as those seen in the mice treated with higher irradiation over a shorter period. These results indicate that the impact of light on electroencephalogram and brain tissue strongly depends not only on the light wavelength, duration and intensity of the exposure, but also on the age of the animal and type of tissue exposed to light.