Adaptability of the visual system is inversely related to its sensitivity

Prolonged viewing of a high-contrast pattern (the adapter) makes similar patterns harder to detect. This threshold-elevation effect was measured as a function of the contrast of the adapter, with use of sinusoidal grating patterns. Increases in the spatial frequency, temporal frequency, or eccentricity of the stimuli had two major consequences. First, the minimum contrast required for detection of the pattern rose; and second, the function that related threshold elevation to adapting contrast became steeper. It is suggested that this increase in the slope of the function reflects the increased gain of these mechanisms, which might occur as compensation for their relatively poor sensitivity. Changing the subject's ability to detect the adapting pattern by means of masks of an orthogonal orientation had markedly different effects on the threshold-elevation-versus-adapting-contrast function. Under these conditions the slope was decreased. The results support the idea that the human visual system tries to compensate for differences in sensitivity by having different gains be associated with different mechanisms.