The FLE was measured with a standard procedure: one bar in the left hemifield moved rightward toward the vertical meridian. During its motion, another bar was flashed at a fixed position, but with a variable timing relative to the stimulus initiation in each trial. This foveopetal stimulus presented in the left hemifield has been reported to drive the largest FLE.
31 There were two possible configurations of the bars in this study. In the first configuration, the moving bar was shown in the lower visual field, and the flashed bar was shown in the upper visual field (
Fig. 1A). In the second configuration, the moving bar appeared in the upper visual field, and the flashed bar appeared in the lower visual field. An orange fixation point was presented at the center of the screen throughout the experiment.
Moving and flashed bars were the same size (5 degrees × 1 degree). The vertical distance between the nearest edges of the bars (bottom edge of the top bar and the top edge of the bottom bar) was 1 degree. The speed of the moving bar was constant at 18 degrees/s. The moving bar moved for 1000 ms and disappeared at the constant position of 4 degrees horizontal distance from the vertical meridian. The flashed bar was presented at a fixed position 8 degrees from the vertical meridian for 1 frame (∼10 ms). The timing of the flash bar was varied within 11 timepoints (−100, −80, −60, −50, −40, −30, −20, −10, 20, 40, and 60) ms (i.e. −10, −8, −6, −5, −4, −3, −2, −1, 2, 4, and 6 frames) relative to the time when the moving bar reached the azimuth (x coordinates) of the flash. Therefore, the flashed bar could be presented for up to 100 ms before the moving bar was physically aligned with the flashed bar, or up to 60 ms after the bar passed the flash position. In each trial, the subject was asked to judge whether the flashed bar was presented at the left (behind) or the right side (ahead) of the moving bar when the “flash” occurred by using a keyboard. The timepoints distribution was skewed toward negative values in order to ensure that the subjects would report an equivalent amount of types of judgements (e.g. “left” and “right”) over probes in the experiment. In one block, each timepoint was tested with 10 repetitions, yielding a total of 110 (11 × 10) trials/block. Three luminance contrasts of the flashed bar were tested: 0.2, 0.6, and 1. The contrast of the bar was defined as the luminance increment of the bar relative to the constant mean grey background (41 cd/m2). Subjects were tested with the left and right eyes separately. Thus, there were a total of 12 conditions (2 stimuli configurations × 3 contrast ratios × 2 eyes). One condition was tested per block, and repeated two times. The order of the conditions was randomized.