Different stimulus conditions were presented equiprobably in a random order. There were 200 test trials in the experiment, with 40 trials per condition. In order to discourage anticipatory responses, 20 ‘catch’ trials were added. On these trials, no reaction signals were presented, and the subjects were instructed not to press the response button. This created a total number of 220 trials, which were presented in two blocks separated by a short resting period. Each block took approximately 4 to 4 1/2 minutes to complete. Prior to the experimental trials, all participants were trained on the task by completing 15 practice trials and 3 catch trials.