title: Increasing stimulus size impairs first- but not second-order motion perception.
abstract: As stimulus size increases, the direction of high-contrast moving stimuli becomes increasingly difficult to perceive. This counterintuitive effect, termed spatial suppression, is believed to reflect antagonistic center-surround interactions--mechanisms that play key roles in tasks requiring sensitivity to relative motion. It is unknown, however, whether second-order motion also exhibits spatial suppression. To test this hypothesis, we measured direction discrimination thresholds for first- and second-order stimuli of varying sizes. The results revealed increasing thresholds with increasing size for first-order stimuli but demonstrated no spatial suppression of second-order motion. This selective impairment of first-order motion predicts increasing predominance of second-order cues as stimulus size increases. We confirmed this prediction by utilizing compound stimuli that contain first- and second-order information moving in opposite directions. Specifically, we found that for large stimuli, motion perception becomes increasingly determined by the direction of second-order cues. Overall, our findings show a lack of spatial suppression for second-order stimuli, suggesting that the second-order system may have distinct functional roles, roles that do not require high sensitivity to relative motion.
authors: Center for Visual Science and Department of Brain and Cognitive Sciences, University of Rochester, Rochester, NY 14627, USA. dglasser@cvs.rochester.edu
authors: Davis M
authors: DM
authors: Glasser
authors: Duje
authors: D
authors: Tadin
mesh_terms: D004192:Discrimination, Psychological / Q000502:physiology; D006801:Humans; D009039:Motion Perception / Q000502:physiology*; D010468:Perceptual Disorders / Q000503:physiopathology*; D010775:Photic Stimulation; D012684:Sensory Thresholds / Q000502:physiology