The procedure is illustrated in Fig. 1. Each stimulus display was presented centrally. Trials began with a 500ms fixation cross subtending 1 of visual angle in size in the horizontal and vertical directions. This was followed by a blank screen whose duration was sampled randomly on each trial from a uniform distribution with a range between 500ms and 1500ms, in steps of 10ms. Afterwards, the two frames were presented for 8ms each, divided temporally by a blank inter-frame interval that ranged in duration between 30ms-86ms in steps of 8ms. These inter-frame intervals were presented on equal numbers of trials within each block, but were presented in a random order. Subjects received a response screen 500ms after the offset of the second frame, within which they had unlimited time to respond. Before beginning the experiment, participants completed sets of 13 practice trials to gain familiarity with each task and to ensure comprehension of instructions. The first 3 practice trials were presented with extended frame presentation durations (200ms each frame) to give the participants a chance to become familiar with the stimuli and the mode of their presentation. The following 10 practice trials were presented with the same frame duration structure as the actual experimental trials. Participants received feedback and were monitored by a researcher while completing these initial tasks to ensure appropriate performance. If a subject’s performance on the practice trials was too poor then an additional round of practice trials would have been performed for said subject, though this was not necessary for any of the tested subjects.

Experimental timeline (A) and example stimulus displays for each task (B,C,D). Each task followed the same timecourse as shown in (A), with only the goal and response of the subject varying across tasks. The green and purple colored stimuli in the “Frame 1 + Frame 2” overlay sections of B, C, and D represent the items from frame 1 and frame 2, respectively. The red circles in (B) and (C) represent the target of the task, to which the participant should move the mouse to click. The red arrow in (C) represents the rotation of the central probe for the averaging response

Participants completed 4 repetitions of blocks consisting of each individual task, for a total of 12 blocks in the experiment. The order of the tasks within each block was semi-random, with a restriction placed that two of the same task could not occur in succession. Within each individual task block, instruction of the current task was given before the first trial, followed by 60 trials of the instructed task. After one block of each task was completed, participants were instructed to take a 2-minute break before beginning the next set of task blocks. Thus, in total there were 240 experimental trials devoted to each task, and a total of 720 critical trials in the experiment.

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