Abstract
The massive motion that saccades impose on the retinal image routinely goes unseen. Here we advance a novel account of this perceptual omission, based on object motion perception studied during fixation. We show that the boundaries of object motion visibility emulate lawful kinematics of saccades—the tight relation between their amplitude and peak velocity known as the ‘main sequence’. In a series of experiments, we investigated the visibility of the motion of a full-contrast Gabor stimulus. On each trial, the stimulus appeared at one screen location, rapidly moved to a new location (at near-saccadic speed), and then disappeared again. Increasing stimulus speed qualitatively changed the percept from continuous to apparent motion, a transition that we quantified by having observers report a small curvature added to the motion trajectory (only visible for continuous motion). For each movement amplitude, the resulting speed threshold increased systematically with the movement amplitude, closely matching the main-sequence relationship for saccades. This result suggests that the amplitude of a stimulus’ motion, in combination with its velocity, controls perceptual omission of motion. To investigate if amplitude information—rather than some form of motion integration—was instrumental for the amplitude-dependence of omission, we varied the duration for which the stimulus was visible before and after its movement (0, 12.5, 50, or 200 ms). When the stimulus started to move as soon as it appeared on the screen and disappeared as soon as it reached its final position (0 ms condition), motion visibility was directly proportional to movement speed, and independent of movement amplitude. Providing amplitude information—with as little as 12.5 ms of pre- and post-movement stimulus duration—fully restored the initial pattern of results that visibility was directly proportional to the main sequence. We argue that the kinematics of saccades entail systematic sensory consequences that the visual system exploits to effectuate perceptual omission.