title: Discrimination contours for the perception of head-centered velocity.
abstract: There is little direct psychophysical evidence that the visual system contains mechanisms tuned to head-centered velocity when observers make a smooth pursuit eye movement. Much of the evidence is implicit, relying on measurements of bias (e.g., matching and nulling). We therefore measured discrimination contours in a space dimensioned by pursuit target motion and relative motion between target and background. Within this space, lines of constant head-centered motion are parallel to the main negative diagonal, so judgments dominated by mechanisms that combine individual components should produce contours with a similar orientation. Conversely, contours oriented parallel to the cardinal axes of the space indicate judgments based on individual components. The results provided evidence for mechanisms tuned to head-centered velocity-discrimination ellipses were significantly oriented away from the cardinal axes, toward the main negative diagonal. However, ellipse orientation was considerably less steep than predicted by a pure combination of components. This suggests that observers used a mixture of two strategies across trials, one based on individual components and another based on their sum. We provide a model that simulates this type of behavior and is able to reproduce the ellipse orientations we found.
authors.affiliation: School of Psychology, Cardiff University, Cardiff, UK.
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Champion
authors.forename: Tom C A
authors.initials: TC
authors.lastname: Freeman
mesh_terms: D004192:Discrimination, Psychological / Q000502:physiology*; D005133:Eye Movements / Q000502:physiology*; D005556:Form Perception / Q000502:physiology*; D006801:Humans; D008959:Models, Neurological; D009039:Motion Perception; D009949:Orientation / Q000502:physiology; D011698:Pursuit, Smooth / Q000502:physiology*; D012027:Reflex, Vestibulo-Ocular / Q000502:physiology; D013028:Space Perception / Q000502:physiology*