S-cone light adaptation: Effects of moderately intense adapting flashes

Abstract

The course of light adaptation of parafoveal S-cones to a moderately intense steady red field to which a pedestal had been added was studied by psychophysical means. Upon exposure to the red field, the observer adjusted the intensity of a 430 nm probe (flashed with the pedestal) to threshold visibility continuously until equilibrium was reached. With a short wavelength pedestal, S-cone sensitivity decreased initially. About 10 seconds after onset of the adapting stimuli, sensitivity began to increase and reached a maximum about 40-50 seconds later. Then, in the second minute of adaptation, S-cone sensitivity again decreased and stablilized eventually (below maximum sensitivity) in the fourth minute of adaptation. The transient increase in sensitivity during the early stage of adaptation became more pronounced with increased intensity of the short wavelength pedestal. With a long wavelength pedestal present, S-cone sensitivity decreased considerably at onset of the steady field and equilibrium was reached gradually without any sign of a transient increase in sensitivity. The results are discussed in terms of a shift in the balance of short and long wavelength signals reaching a chromatically opponent site.