Poster Presentation "Psychophysical evidence for a relative object motion mechanism in humans"

Purpose. To quantify the perceptual properties of a relative motion mechanism in which object trajectory discrimination is differentially affected by the presence of a moving background. First, in a series of experimental conditions we identify a global relative motion mechanism whose properties can be perceptually differentiated from more local direction mechanisms. Second, to gain additional insight into the nature of the underlying computational mechanisms, we examine several motion specific properties in greater detail (i.e. velocity tuning, and spatial summation). Methods. Motion stimuli were represented as constant density random dot kinematograms presented within a 24 degree aperture for 440±40 ms. In the basic trajectory discrimination task, stimuli consisted of a motion-defined object moving in the same or opposite direction as a translating background. Using a central fixation 2TAFC paradigm, subjects discriminated the direction of object motion perturbed perpendicular to an imaginary horizontal or vertical line centered on the screen. Spatial summation and velocity tuning were examined by modifying the visual extent, degree of motion noise, and relative velocities between the object and background motions. Results. Trajectory discrimination thresholds for object motions that opposed the background were significantly lower (p<0.05) than for objects moving in the same direction as the background. As the relative trajectory differences between the object and background increased through 90 deg., discrimination thresholds consistently decreased across subjects. A coarse sampling of discriminability across a range of dot and object speeds (9-22 deg/s) showed no significant variation in threshold performance. Conclusion. The difference in discrimination thresholds between same and opposite object trajectories coupled with the observed speed invariance and a lack of non-motion based visual cues suggests the presence of a global relative motion mechanism within the visual motion pathway that differentially segregates object trajectories from the background. Such mechanisms could play a role in motion-based object localization and visual scene segregation.