Mechanisms of visual sensitivity: backgrounds and early dark adaptation.

There is substantial physiological and psychophysical evidence for an adaptation mechanism whose effect, under many circumstances, is equivalent to placing a neutral density filter in front of the eye. Furthermore, this mechanism is of sufficient strength to predict the generalized Weber's law for increment thresholds on steady backgrounds. However, it was shown that an additional transient mechanism (with a time-course of around 100 msec) is also needed to account for the increment-threshold results. The effect of this mechanism on increment thresholds during early dark adaptation was parametrically examined. Several models for the transient mechanism were considered. The one best able to account for the results consists of a subtractive inhibitory stage operating prior to a saturating nonlinearity.