After a variable inter-stimulus interval (ISI) of 1100 ms to 1600 ms, target stimuli were presented in the placeholder frames for 100 ms, whereas catch cues were followed at that time point by another cue that started a new trial. Targets were grey circles (radius 1°), interrupted by two opposing gaps. Participants were asked to respond only to the covertly attended stimulus at the cued location, while ignoring the stimulus in the opposite hemifield, by indicating which gap was larger (index versus middle finger of the right hand for larger gap at the bottom versus the top, respectively). On low-difficulty trials, one of the gaps was clearly larger than the other, with gap angles of 90° versus 20°. On high-difficulty trials, the two gaps were more similar, with gap angles of 40° versus 20°, and were thus harder to discriminate. A response time-out was adjusted after every high-difficulty trial to obtain a constant ratio of 75% correct versus 25% error or missed trials thereby ensuring that the task was similarly difficult for all participants. This variable response time-out was used
