Spatial distribution of the inhibition elicited by the offset of a visual cue on manual reaction time to a visual target.

Simple reaction time (RT) to a peripheral visual target (S2) is shortened when a non-informative cue (S1) is flashed at the S2 location 100-150 ms before target onset (early facilitation). Afterwards, RTs to targets appearing at the S1 location are lengthened (inhibition of return). In the present investigation we studied the spatial distribution of the inhibition elicited by the offset of S1. Twelve subjects were asked not to respond to S1 which appeared on a horizontal meridian located 5.5 degrees above the fixation point (FP), but to respond, by pressing a key, to a target (S2) occurring at 5.5 degrees to the left or to the right. S1 could appear at one of 9 locations along this meridian (5.5, 3.5, 1.5, and 0.5 degrees to the left, 0.0 and 0.5, 1.5, 3.5, and 5.5 degrees to the right) and S2 occurred only at the most eccentric positions. Each trial began with the presentation of FP. Five-hundred ms later, S1 appeared and remained on for 700 ms. One hundred or 800 ms after S1 offset, S2 appeared in the same or in the opposite hemifield. We found that the offset of S1 elicits an inhibition (OFF-inhibition) which has the following features: a) it is maximal at cue's position; b) it spreads to other positions in the cued hemifield, and c) it decreases when the time interval between S1 offset and S2 onset increases from 100 to 800 ms.