Visual Light- and Dark-Adaptation

Processes That Mediate Adaptation

The pupil opens wide at night and thereby lets in up to 10 times more light, but this is only a small part of the entire adaptive range. (This effect is greater in species with slit pupils.) Adaptation in humans is instead controlled primarily within the eye, by the retina. The retina is duplex, possessing both a photopic system dependent on cone receptors that operates in daylight and a scotopic system dependent on rod receptors that operates in moonlight and starlight, when the photopic system is insensitive. In twilight, the intermediate or mesopic range, both receptor classes are active. In daylight, the rods saturate—their responses become constant, independent of the visual stimulus—and vision becomes entirely photopic because the cones are protected from saturation by light adaptation. Light adaptation in bright sunlight is photochemical—many light-sensitive visual pigment molecules in the cones are bleached, becoming transparent, and, with fewer active pigment molecules to pick up light, saturation is avoided. At lower light levels bleaching does not occur, and adaptation in both rods and cones is neural, in part reflecting modifications of synaptic transmission.

Adaptation and Visual Processing

The transition from photopic to scotopic vision is not just a matter of changing receptors, but also of a fundamental change in the nature of visual processing. In sunlight, the photopic system does not attempt to record every photon—there are so many that this would be pointless—but rather it encodes contrast, the intensity of light at one location relative to that from an adjacent location. Thus surfaces of objects are encoded as darker (negative contrast) or lighter (positive) than their surrounds. Encoding contrast implies discounting the ambient illumination, so a sunny sky looks only a little darker when the clouds roll in, even though the ambient light level may decrease a thousand times. In starlight, there are few ambient photons, and the ability of the fully dark-adapted scotopic system to detect individual photons becomes critical. Negative contrast no longer exists as the surroundings are dark; processing is devoted instead to small increments in light reflected from dim surfaces. In moonlight, contrasts begin to be encoded as well as increments.

The time course of dark adaptation reveals both the changeover from photopic to scotopic vision and a progressive improvement of sensitivity within each system. The solid symbols in Figure 1 plot psychophysical (behavioral) measurements of visual thresholds obtained after prior exposure of the eye to a bright adaptation light, which was extinguished at time zero. Each threshold indicates the intensity of a briefly flashed test light just needed for an observer to report seeing the test in the dark. Test flashes whose intensities are below the threshold are invisible; those above it are visible. As dark adaptation progresses, the plot reveals [Page 1086]a descending curve, as decreasing amounts of light are needed to just see the test flash, which corresponds to an increase in visual sensitivity of 4.6 logarithmic units (40,000 times). The curve has two limbs: (A) in the first 2 min, detection thresholds drop rapidly and then plateau; later on, (B) thresholds again drop, reaching a final level after about 30 min. These progressive drops in threshold correspond to increases in visual sensitivity in each system, the changeover from cones to rods or rod-cone break occurring at 7 min in Figure 1. Evidence that the break indeed corresponds to a change of system is provided by a shift (the Purkinje shift) in the peak spectral sensitivity of the eye from 555 nm (the photopic maximum) before the break to 500 nm, the maximum sensitivity of the rod photo-sensitive pigment, rhodopsin, after the break. Test lights whose intensities are just above the detection threshold appear colored if flashed before the break, indicating cone function, and colorless after it, indicating rod function. The thresholds in Figure 1 were obtained with a middle-wavelength test flash delivered to the periphery—a retinal area outside the fovea that contains both rods and cones. When dark adaptation is measured with long-wavelength tests invisible to the rods, or with test flashes of any color presented to the rod-free fovea, there is only a photopic limb, as indicated by the dashed line (D). When dark adaptation is measured in a rod monochromat, a rare individual without cones, the beginning of the sco-topic recovery curve (C) is uncovered. The two limbs of the dark adaptation curve shown in Figure 1 can each be shifted vertically by altering the color or brightness of the preadaptation light and by varying the wavelength, size, retinal location, and duration of the test flash, all factors that govern visual sensitivity. If the two limbs do appear, their time courses will be as shown, indicating that the recovery curve for each receptor class is independent of the state of adaptation of the other.

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