The primary visual pathway in humans is regulated according to long-term light exposure through the action of a nonclassical photopigment.

BACKGROUND: The mammalian eye shows marked adaptations to time of day. Some of these modifications are not acute responses to short-term light exposure but rely upon assessments of the photic environment made over several hours. In the past, all attempts at a mechanistic understanding have assumed that these adaptations originate with light detection by one or other of the classical photoreceptor cells (rods or cones). However, previous work has demonstrated that the mammalian eye contains non-rod, non-cone photoreceptors. This study aimed to determine whether such photoreceptors contribute to retinal adaptation.
RESULTS: In the human retina, second-order processing of signals originating in cones takes significantly longer at night than during the day. Long-term light exposure at night is capable of reversing this effect. Here, we employed the cone ERG as a tool to examine the properties of the irradiance measurement pathway driving this reversal. Our findings indicate that this pathway (1) integrates irradiance measures over time periods ranging from at least 15 to 120 min; (2) responds to relatively bright light, having a dynamic range almost entirely outside the sensitivity of rods; (3) acts on the cone pathway primarily through a local retinal mechanism; and (4) detects light via an opsin:vitamin A photopigment (lambda(max) approximately 483 nm).
CONCLUSIONS: A photopigment with a spectral sensitivity profile quite different from those of the classical rod and cone opsins but matching the standard profile of an opsin:vitamin A-based pigment drives adaptations of the human primary cone visual pathway according to time of day.