Displacement limits, directional anisotropy and direction versus form discrimination in random-dot cinematograms

Apparent motion mediated by the short-range process was studied. Random-dot cinematograms were used to determine the maximum displacement (dmax) for motion-direction and pattern discrimination. dmax is the maximum spatial separation between a correlated region (target) in the two successively presented random-dot images, when observers can correctly identify the direction of motion or the shape of the target. We have found that: (1) dmax for motion-direction discrimination increases with the square root of the display area; (2) dmax is invariant with eccentricities of 4 deg arc; (3) For rectangular targets and brief presentations, dmax increases with increases of that target-dimension which is parallel to the orientation of the movement. These findings indicate that short-range movement perception is an orientation specific global process. Furthermore, motion-direction discrimination and pattern discrimination of random-dot cinematograms may be mediated by different levels of processes. However, even for pattern discrimination dmax in visual angle increases with increased target area.