Poster Presentation "Discrimination of Shifted Centers-of-Motion in a Patient that cannot Perceive Radial Motion "

Purpose. We use psychophysical data from a motion-impaired stroke patient, GZ (Vaina & Goldberg 2002), to challenge the generally accepted view that a precise computation of local and/or global motion information is necessary for recovering the 2D center-of-motion (COM) associated with translational direction in a 3D scene, (Reiger & Lawton 1985; Bruss & Horn 1983). Methods. Motion stimuli were represented as constant density random dot kinematograms presented within a 24 deg. aperture (central 4 deg. removed) for 440±40ms. In two perceptual tasks observers were presented with 30 deg/s radial (expansion/contraction) or circular (CW/CCW) motion-patterns and were required to discriminate (1) the shift in the COM, left or right, relative to a central fixation or (2) opposing motion-patterns (e.g. expansion vs. contraction) defined by a proportion of signal dots embedded in masking motion noise (Motion Pattern Coherence). Results. In normal observers (1) COM thresholds for circular motion-patterns were significantly higher than for radial motion-patterns (p<0.005; t(19)=2.92) while in (2) discrimination thresholds for radial and circular motion-patterns were comparable (~10%). GZ was only able to discriminate moderate shifts (~1 deg) in the COM for radial and circular motion-patterns. In contrast GZ totally failed to discriminate radial motion-patterns even at 100% coherence. However, she could discriminate CW from CCW motion and radial from circular motion-patterns. Conclusion. GZ's performance could be explained by a mechanism that computes a scalar norm from the motion-pattern for use as an error measure to localize the COM (Sundareswaran 1992). In this scheme a sparse sampling of scalar errors across the visual field provides a coarse spatial localization of the COM, independent of the type of radial motion-pattern. Discrimination with finer accuracy can be obtained through computations that use the direction information of the radial motion-patterns.