[Temporal summation and reaction time for grid detection]

We studied the stimulus-duration effect on the reaction time (RT) to gratings with different spatial frequencies. At each duration value, the product of the contrast and the duration, i.e. the contrast "energy" was kept constant. We found that at near-threshold "energy" levels the RT was constant for up to 15 ms at lower spatial frequencies and up to 30 ms at higher spatial frequencies. At higher "energy" levels, this critical interval was the same (up to 15 ms) for both lower and higher spatial frequencies. The effect of the duration of gradual onset of the stimulus on RT was also studied. An increase of onset duration (up to 60 ms) at low spatial frequencies substantially delayed reaction time for the contrasts under study. Conversely, at high spatial frequencies, this effect was present only for gratings with a high contrast. These results suggest that reaction time is determined by two types of mechanisms (transient and sustained) at a near threshold contrast, and by one type (transient) mechanism at higher contrasts.