How many pathways determine the speed of grating detection?

Reaction time (RT) for detection of gratings of different spatial frequencies (SF, 0.87-13.87 c/d) and contrast (2-32 times above the detection threshold) was measured. It was found that at low stimulus contrast, 2 and 4 times above the detection threshold, the dependence of mean RT on SF is an "S"-type function with the two plateaus 50-60 ms apart. The standard deviation (SD) increases significantly for SFs within the range 2.60-6.93 c/deg and this coincides with the transition of mean RT from one plateau to another. At higher contrast the dependence of RT on the SF is a monotonically increasing function without any plateau and SD does not demonstrate any extrema within the entire SF-range. The results suggest that RT is determined by two mechanisms (transient and sustained) at low contrast, and by one (transient) mechanism at higher contrasts.