Detection of light and flicker at low luminance levels in the human peripheral visual system. I. Psychophysical experiments.

The interaction between summation and adaptation mechanisms near the absolute threshold of vision is studied. In this paper, results are presented of measurements of both light detection and flicker detection for circular flashes with a diameter of 5.7-480 min of arc, a flash duration of 5-1000 msec, a period of 0-8000 msec, and at eccentricities of 7 and 40 deg in the temporal retina. It is confirmed that the estimates of the summation times obtained from the light-detection-threshold energy as a function of the period and from the light-detection threshold as a function of the flash duration are similar; these estimates depend on the stimulus size and eccentricity. It is suggested that two summation mechanisms can be distinguished, one for the preprocessing and one for the detection mechanism. The summation time of the first mechanism is estimated to be about 100 msec; that of the latter, about 500 msec. At 40-deg eccentricity, decreasing the period of presentation leads initially to a rise in the light-detection threshold, pointing to adaptational phenomena. The flicker threshold is determined by the energy per flash (for brief flashes); when the flash duration exceeds 100 msec, the intensity of the flash is the important parameter. The flicker threshold depends strongly and in an irregular way on the stimulus size because of lateral inhibition effects.