First the experimenter explained the procedure of the experiment and showed how to use the software. Then participants began practice trials under the instructions of the experimenter. The practice trials were exactly the same as the actual experiment trials except that there was no time pressure. The practice consisted of 40 trials and extra practice was optional if participants considered it necessary. Before each task, there was a red dot on the center of the screen. Participants were required to gaze at the dot and then clicked the left mouse button after which the task interface appeared as shown in figure 3. The mouse icon was hidden to avoid the usage to guide the search process. Once the target was found, the participants were required to click the left mouse button. Then a dialog appeared to let the participants choose the direction of the target’s gap (i.e. left or right) and the location (i.e. upper left, upper right, lower left, or lower right). The task was terminated when the time limit was reached. There was a five minutes break after the experiment lasted fifteen minutes and then the experiment continued. Each participant completed 20 trials in experimental trials.