Melanopsin and Rod–Cone Photoreceptors Play Different Roles in Mediating Pupillary Light Responses during Exposure to Continuous Light in Humans

Oct 12, 2012The Journal of neuroscience : the official journal of the Society for Neuroscience

Melanopsin and rod-cone eye sensors have different roles in controlling pupil response to steady light in humans

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Abstract

A study with 24 normally sighted individuals shows that visual photoreceptors are necessary for normal pupillary responses to low light levels.

Rod-cone photoreceptors are essential for tracking fast changes in light intensity during pupillary responses.
Pupillary constriction in sighted individuals decreases steadily for at least 30 minutes under continuous low-light exposure.
Responses to low-irradiance intermittent green light were more than twice as strong compared to continuous green light exposure.
In individuals without rod-cone function, pupillary responses are slower and cannot keep pace with varying light stimuli.
Melanopsin and rod-cone photoreceptors play distinct roles in controlling pupillary light responses.

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In mammals, the pupillary light reflex is mediated by intrinsically photosensitive melanopsin-containing retinal ganglion cells that also receive input from rod-cone photoreceptors. To assess the relative contribution of melanopsin and rod-cone photoreceptors to the pupillary light reflex in humans, we compared pupillary light responses in normally sighted individuals (n = 24) with a blind individual lacking rod-cone function. Here, we show that visual photoreceptors are required for normal pupillary responses to continuous light exposure at low irradiance levels, and for sustained pupillary constriction during exposure to light in the long-wavelength portion of the visual spectrum. In the absence of rod-cone function, pupillomotor responses are slow and sustained, and cannot track intermittent light stimuli, suggesting that rods/cones are required for encoding fast modulations in light intensity. In sighted individuals, pupillary constriction decreased monotonically for at least 30 min during exposure to continuous low-irradiance light, indicating that steady-state pupillary responses are an order of magnitude slower than previously reported. Exposure to low-irradiance intermittent green light (543 nm; 0.1-4 Hz) for 30 min, which was given to activate cone photoreceptors repeatedly, elicited sustained pupillary constriction responses that were more than twice as great compared with exposure to continuous green light. Our findings demonstrate nonredundant roles for rod-cone photoreceptors and melanopsin in mediating pupillary responses to continuous light. Moreover, our results suggest that it might be possible to enhance nonvisual light responses to low-irradiance exposures by using intermittent light to activate cone photoreceptors repeatedly in humans.

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Melanopsin and Rod–Cone Photoreceptors Play Different Roles in Mediating Pupillary Light Responses during Exposure to Continuous Light in Humans

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