Phototransduction: Adaptation in cones

The ability of our eye to function over the enormous range of light intensities that we experience throughout the cycle of night and day is due in part to our possession of a duplex system of rod and cone photoreceptors, but primarily to the ability of each of these systems to adapt its performance to the ambient light intensity. The photopic (cone) system operates from moonlight to sunlight levels, covering more than a million-fold range of intensities. In contrast to the case in the scotopic (rod) system, where much of the adaptation occurs postreceptorally, the enormous range of adaptation in the photopic system is accomplished within the outer segments of the cone photoreceptors themselves. Moreover, crucially, the cones do not saturate in steady light, no matter how intense that light is.