LOWER THRESHOLD OF MOTION FOR ONE-DIMENSIONAL AND 2-DIMENSIONAL PATTERNS IN CENTRAL AND PERIPHERAL-VISION

Lower motion thresholds for discriminating opposing motion directions were compared for one dimensional (grating) and two dimensional (plaid) stimuli in central and peripheral vision. The results were consistent with a two-stage model of motion sensitivity in which threshold-limiting noise occurs at both stages, and the speed as well as the direction of the resultant motion is determined by intersection-of-constraints (IOC) from the component motions. The results do not support a purely geometric interpretation of the IOC model, in which thresholds for plaid stimuli are related to thresholds of component gratings by a geometric factor. Neither do the data favour explanations in which local luminance features (i.e. blobs) are detected and their velocity determined. Monte-Carlo simulations of the two-stage process predict thresholds across variations in component direction, contrast, and visual field eccentricity. Lower motion thresholds for gratings and plaids both follow a saturating function of contrast; the fit between grating and plaid data is improved when the plaid contrast is expressed in terms of the contrast of its components. Although less contrast saturation found in the periphery, in relative terms, plaid and grating motion thresholds were similar in central and peripheral vision, implying cortical magnifications are similar for mechanisms which process grating and plaid motion.