E4 Photoreception: Cones, Rods and Visual Transduction

Topic

Each retinal photoreceptor contains a visual pigment—a molecule capable of absorbing a photon of light and changing shape upon doing so. This shape change in the visual pigment triggers a phototransduction cascade within the photoreceptor; unlike other sensory receptors that open ion channels upon receiving a stimulus, this cascade results in the closing of ion channels that remain open in the dark, allowing a steady influx known as the "dark current."

The more photons a photoreceptor's visual pigment absorbs at a given moment, the more ion channels the phototransduction cascade closes, and the more the dark current entering the photoreceptor is reduced. This reduction in dark current alters the voltage inside the photoreceptor, generating a receptor potential whose magnitude depends directly on the number of photons absorbed by the visual pigment: more absorbed photons result in a greater reduction of the dark current and a larger receptor potential.

A photoreceptor's sensitivity—the number of photons required to produce a receptor potential of a specific magnitude—is not fixed. In a brightly lit environment, the photoreceptor reduces its sensitivity through light adaptation to avoid saturation from the high volume of incoming photons, thereby maintaining the ability to distinguish intensity differences within that bright setting. Conversely, when the environment suddenly darkens, the photoreceptor gradually increases its sensitivity through dark adaptation—a process taking several minutes—to once again generate a detectable receptor potential from the sparse photons available in the dim environment.

Among retinal photoreceptors, cones—specifically the S, M, and L cones distinguished in models of color vision deficiency—each possess unique spectral sensitivity and are concentrated primarily in the fovea, the region of highest visual acuity. Rod cells, in contrast, share a single spectral sensitivity; thus, they do not allow for comparisons between different spectral sensitivities and do not contribute to color discrimination. However, they are far more sensitive than any cone to small numbers of photons and are concentrated outside the fovea—across the rest of the retina—which explains why, in very low-light conditions, one sees better by looking slightly to the side of a dim object rather than directly at it.

Based on the number of photons required to generate a detectable receptor potential in a photoreceptor—determined by its sensitivity at that moment—this model calculates the detection threshold: the minimum amount of light a photoreceptor can detect. Contrast sensitivity is calculated based on how the receptor potential changes when the photon count varies slightly across different points of an image; this is the same capacity that is impaired by cataracts, which scatter light before it reaches the photoreceptors. Finally, color perception is calculated by comparing the distinct spectral sensitivities of the various cones—a process described by the model of dyschromatopsias for cases where this comparison fails.

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