Abstract
Purpose. Recent evidence indicates that motion smear can provide useful information for the detection and discrimination of motion (Geisler, 1999; Burr & Ross, 2002). Because the extent of perceived motion smear decreases with the density of stimulus elements (Chen et al., 1995), we used random-dot (RD) targets of different densities to evaluate the contribution of perceived motion smear to direction-of-motion discrimination. Methods. RD stimuli with densities of 1, 2, and 10 dots/sq. deg were presented monocularly for 200 ms at a velocity of 4, 8, or 12 deg/s. To measure direction-of-motion thresholds, stimuli were presented in 7 near-horizontal directions and subjects reported whether the motion of the target included an upward or a downward component. Thresholds were defined as the change in the direction of motion that corresponds to 1 SD on the fitted psychometric function. On separate trials, subjects adjusted the length of a bright horizontal line to match the extent of motion smear that was perceived on each presentation of the RD targets. Results. In agreement with Chen et al. (1995), the extent of perceived motion smear decreased systematically with RD density. For velocities greater than 4 deg/s, direction-of-motion thresholds increased (from approximately 1.0 to 1.8 deg) with the density of the RDs in the stimulus. Across all combinations of velocity and RD density tested, the direction-of-motion thresholds correlated significantly with the extent of perceived motion smear (r = −0.90; p Conclusions. Despite the opportunity for increased summation as RD density increases, our results indicate that direction-of-motion discrimination worsens. This outcome is consistent with the conclusion that direction-of-motion discrimination is facilitated by visible motion smear.
Support: R01 EY05068, P30 EY07551 & Texas ARP award 003652-0185-2001 .