Spatial summation of first-order and second-order motion in human vision

This study assessed spatial summation of first-order (luminance-defined) and second-order (contrast-defined) motion. Thresholds were measured for identifying the drift direction of 1 c/deg., luminance-modulated and contrast-modulated dynamic noise drifting at temporal frequencies of 0.5, 2 and 8 Hz. Image size varied from 0.125° to 16°. The effects of increasing image size on thresholds for luminance-modulated noise were also compared to those for luminance-defined gratings. In all cases, performance improved as image size increased. The rate at which performance improved with increasing image size was similar for all stimuli employed although the slopes corresponding to the initial improvement were steeper for first-order compared to second-order motion. The image sizes at which performance for first-order motion asymptote were larger than for second-order motion. In addition, findings showed that the minimum image size required to support reliable identification of the direction of moving stimuli is greater for second-order than first-order motion. Thus, although first-order and second-order motion processing have a number of properties in common, the visual system’s sensitivity to each type of motion as a function of image size is quite different.