The Role of Afterimages in Dark Adaptation

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Abstract
When the photosensitive pigments in a patch of retina are bleached, the threshold is elevated for 30 minutes or more, and one is aware of a continuing sensation of light from this region. This positive afterimage was matched by a stabilized image in an adjacent retinal region. The threshold was found to be the same for light falling on the afterimage and for light added to a stabilized image of matching luminance. Thus, the threshold during dark adaptation is the increment threshold against the background of the positive afterimage. It is concluded that the elevation of threshold associated with bleaching is due to a spurious disturbance or "noise" generated in the receptors, and that reduction in the fraction of quanta absorbed is comparatively unimportant. This noise is not normally obvious because afterimages fade like stabilized images. There are indications that fading results from a feed-back process akin to an "automatic brightness control.".

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