Dark-adaptation: a new hypothesis.

Abstract Bright lights cause an elevation of the visual threshold which persists after the light is turned off. It is suggested that this is caused by the receptors becoming noisy, rather than by weakening of their responses. Apparently the presence of bleached photopigment causes the receptors to show effects similar to those induced by light, even when they are in total darkness. The action of bleached pigment adds to that of light, so that the separate contributions from each cannot be distinguished at the receptor's output. The “spurious light”, or noise, may arise by spontaneous change of the photosensitive pigment, thus providing a possible unifying explanation at a molecular level.

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