ABSTRACT
Simple reaction time (RT) to a peripheral visual target is shortened when a non-informative cue is flashed at the target location 100-150 ms before target onset (early facilitation). With longer intervals, RT to targets appearing at cue hemifield is lengthened (inhibition of return). In the present study, we investigated these effects inverting the stimulus contrast in relation to background to see how these effects are related to the onset and/or to the offset of a cue darker or brighter than background. Ten subjects were asked not to respond to a non-informative cue (S1) appearing on a computer screen 6§ to the right or to the left of the center of a fixation cross (FP), but to respond, by pressing a microswitch, to a target (S2) occurring at 4§ from the FP in the same hemifield as S1 or in the opposite hemifield. There were two different types of sessions. In one, S1 and S2 were bright against a dark background and in the other, S1 and S2 were dark against a bright background. In each session there were two types of trials. In OFF trials, each trial began with the presentation of FP. Five hundred ms later, S1 appeared and remained on for 700 ms. S2 appeared 100 or 800 ms after the offset of S1. In ON trials, S1 onset occurred 1200 ms after the beginning of the trial and remained on until the end of trial. S2 appeared 100 or 800 ms after S1 onset. When S2 onset followed S1 onset by 100 ms, RT to S2 occuring in the same hemifield did not differ from RT when S1 and S2 were in opposite hemifields. In contrast, after an S1-S2 interval of 800 ms, S1 onset elicited an inhibition of its hemifield. This inhibition was similar to that observed 100 ms or 800 ms after S1 offset. The same results were obtained if the cue and target were brighter or darker than the background, showing that the ONSET and OFFSET inhibitions are related not to cue luminance increase and decrease, respectively, but to the appearance and the disappearance of a salient stimulus in a homogeneous background. Moreover, these results suggest that ON and OFF channels of the visual system have similar effects on the orienting of attention