Conditioning stimuli of differing luminances, which appear equally bright because of the effects of previous light adaptation, may produce essentially equivalent on-responses, as evaluated by the temporal changes in the threshold of a superimposed test flash. The magnitude of these responses is directly related not to the actual luminance of the conditioning stimulus in each case, but to its brightness. Over a fairly wide range of conditions of light adaptation and brightness, constant brightness is accompanied by constant changes in log visual sensitivity, as assessed both by an increment threshold and the conditioning-stimulus-test-flash-threshold techniques.
Only at intermediate sensitivity levels is there any evidence for a simple “filter factor” model for the light-adaptation mechanism. The visual response at the extremes of the sensitivity range appears to approach physiological limits which render a simple model of the effects of light adaptation untenable.
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