THE PROCESSING OF TEMPORAL-MODULATION AT DIFFERENT LEVELS OF RETINAL ILLUMINANCE

How does our temporal vision change as the mean illuminance reduces? W e have examined the processing of near-threshold temporal information for a range of illuminance values (2850 - 0.15 phot td). At high illum inance, the modulation transfer function can be shown to be mediated v ia three underlying temporal filters that vary in sensitivity with spa tial frequency, As the mean illuminance decreases these channels appea r to change their sensitivity, Even at the lowest (scotopic) illuminan ce levels we were able to find evidence for at least two channels medi ating detection threshold, There are also changes in the tuning proper ties of these channels such that the processing of high temporal frequ encies is differentially compromised, resulting in a reduction in the flicker fusion limit of each channel, and a shift in the peak of the b and-pass channel. The slope of the fall-off in sensitivity at high tem poral frequencies is unaffected by test spatial frequency at each illu minance level, suggesting its limiting factor is one that is insensiti ve to spatial frequency, We propose that the changes in the tuning of the temporal filters occur because of an early (e.g. photoreceptor) ch ange in the response dynamics, or by interactions between photorecepto rs, rather than changes at or beyond the level of the channel response .