Abstract
During saccadic eye movements, objects routinely move across the retina at high speeds but we hardly ever see motion. Here, we present data suggesting that the upper threshold for conscious retinal motion perception may in fact be set specifically to eliminate velocities created by saccades. Using a high-speed projection system with sub-millisecond temporal resolution, we determined velocity thresholds for objects that moved across the retina at the speed of saccades. Upon fixation at the center of the screen, a vertical Gabor stimulus (spatial frequency: 1 cycle per degree of visual angle, dva) appeared either left or right, quickly ramping up to full contrast. The stimulus then moved rapidly in a slightly arched trajectory towards the other side of fixation. Observers performed a curvature discrimination task, judging whether the stimulus passed above or below fixation. We varied the movement amplitude between 4 and 12 dva, in steps of 2 dva. In Experiment 1, the stimulus' horizontal motion followed the velocity profile typical for a saccade of the given amplitude (van Opstal & van Gisbergen, 1987). In Experiment 2, its motion was constant (peak velocity for that saccade amplitude). Across trials, we slowed down or accelerated this velocity profile by factors between 0.25 to 2 (resulting in velocities between 48 and 708 dva/s). For each movement amplitude, there was a threshold speed where the percept changed from continuous motion (high performance) to apparent motion (curvature could no longer be judged). Importantly, this velocity threshold was not fixed, but depended on the movement amplitude. Indeed, the relation between velocity threshold and movement amplitude mirrored the main sequence—the relation between saccade amplitudes and their peak velocity. Conscious perception thus settles on a fine compromise that ensures sensitivity to objects moving at high-speeds while eliminating motion consistent with saccadic eye movements
Meeting abstract presented at VSS 2017