Poster Presentation "Processing Relative Object Motion in Complex Patterns of Motion"

Purpose. To quantify relative object motion processing within the visual motion stream. Here we examine the existence of a relative motion mechanism using an object direction discrimination task and we quantify its interaction with pattern motion mechanisms by measuring center of motion (COM) discrimination biases induced by occluding object trajectories. Methods. Motion stimuli were represented by constant density coherent random dot kinematograms presented in a 24 degree aperture for 440±40 ms. In the direction discrimination task, stimuli consisted of a motion-defined object moving in the same or opposite direction as a coherent translating background. Using a central fixation 2TAFC paradigm, subjects were required to discriminate the direction of object motion perturbed perpendicular to the background. In the COM task, stimuli containing radial, circular, or spiral patterns of motion were presented together with a motion-defined object whose trajectory occluded the pattern motion COM. For each pattern motion, subjects were required to identify the COM direction of shift (left/right) relative to the central fixation in a single interval task. The level of interference caused by the occluding object motion was quantified by analyzing the response bias relative to a control task containing object trajectories that did not occlude the COM. Results. Direction discrimination thresholds varied as a function of the relative object motion in the visual scene for moderate dot speeds (15 deg/s). Thresholds for object trajectories that opposed the background were significantly lower (p<0.05) than for trajectories moving in the same direction as the background. Conclusion. The difference in discrimination thresholds between same and opposite object trajectories suggests specialized mechanisms for relative motion processing in the visual system. Together with previously reported pattern motion mechanisms in visual cortex, such relative motion detectors could be used localize moving objects and aid motion-based segregation of the visual scene.