Purpose: To investigate the chromatic characteristics and intensity-response function of light-induced reflex lacrimation and its correlation with the melanopsin-driven postillumination pupil response (PIPR).
Methods: Eleven visually normal participants completed the experiment. Lacrimation was measured in one eye by placing a calibrated filter paper strip in the conjunctival sac over a 1 minute-interval (Schirmer's test) during which participants received either no light stimulation (baseline trial) or one flash of blue or red light stimuli presented binocularly with a Ganzfeld stimulator, while the pupil response was recorded simultaneously from the fellow eye by using an eye tracker. Light stimulation trials were presented in alternating fashion at seven incremental intensity steps (0.1, 1, 3.16, 10, 31.6, 100, and 400 cd/m2). Postillumination pupil response was defined as the mean pupil constriction from 10 to 30 seconds post illumination.
Results: The amount of lacrimation in response to 10 to 400 cd/m2 blue light was significantly greater than baseline and increased monotonically with increasing light intensity. Red light did not induce significant reflex lacrimation until the brightest stimulation at 400 cd/m2. There was a positive linear correlation between PIPR and lacrimation in response to blue light (r = 0.74, P < 0.001) but not to red light (r = 0.13, P = 0.25).
Conclusions: The chromatic characteristics and intensity-response of light-induced lacrimation are highly consistent with the features of melanopsin phototransduction. This finding is the first in vivo evidence in humans, supporting the hypothesis that light-induced reflex lacrimation is mediated primarily by melanopsin photoactivity, and provides new insight into the putative mechanisms of photophobia.