Mechanisms in the Eye that Mediate the Biological and Therapeutic Effects of Light in Humans

It is useful from both a scientific as well as a clinical perspective to establish the specific mechanisms in the eye which mediate the nonvisual therapeutic and biological effects of light.

Abstract

Research on the therapeutic potency of "bright" light has outpaced the study of the fundamental mechanisms by which light produces its beneficial effects. Originally, it was thought that illuminances equal to or above 2,500 lux were necessary to induce biological change or therapeutic response. Data are presented which show that illuminances as low as 5 lux of monochromatic green light or 100 lux of broad band white light also can produce significant suppression of melatonin in normal human volunteers. To understand how such low levels of illumination can produce strong biological and therapeutic responses, it is necessary to examine the ocular mechanisms which mediate the photic effects. Specifically, in humans it is shown that: (1) gaze behavior relative to a light source can significantly alter the amount of light reaching the retina; (2) the age of the ocular lens can significantly modify the balance of wavelengths reaching the retina; (3) near-ultraviolet radiation traditionally considered outside of the visible spectrum can stimulate visual responses in young adults at least up to the age of 25 years; (4) pupillary dilation modulates the capacity of a light stimulus to suppress melatonin; and (5) the entire retinal field participates in melatonin regulation. Although the specific photoreceptors or photopigments which mediate the biological and therapeutic effects of light in humans presently are not known, basic data are presented on the effects of different wavelengths elucidating melatonin suppression and SAD treatment. In summary, it is useful from both a scientific as well as a clinical perspective to establish the specific mechanisms in the eye which mediate the nonvisual therapeutic and biological effects of light.