Melanopsin and rod–cone photoreceptors play different roles in mediating pupillary light responses during exposure to continuous light in humans

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Summary

Rod-cone photoreceptors and melanopsin play distinct, non-redundant roles in the pupillary light reflex: rods/cones are essential for fast, transient responses and low-irradiance sensitivity, while melanopsin drives slow, sustained constriction. Notably, intermittent light stimulation of cones can more than double the sustained pupillary constriction compared to continuous light, suggesting that pulsed lighting protocols may be a practical strategy to enhance non-visual (circadian) light responses at low light levels.

Key Findings

Intermittent green light (543 nm, 0.1–4 Hz, 30 min) elicited sustained pupillary constriction more than twice as great as continuous green light at the same irradiance.
In the absence of rod-cone function (blind individual), pupillomotor responses were slow and sustained and could not track intermittent light stimuli, confirming rods/cones are required for encoding fast modulations in light intensity.
In sighted individuals, pupillary constriction decreased monotonically for at least 30 minutes during continuous low-irradiance light, indicating steady-state pupillary responses are an order of magnitude slower than previously reported.
Rod-cone photoreceptors are required for normal pupillary responses at low irradiance levels and for sustained constriction during long-wavelength (red) light exposure.