Acuity for fine-grain motion and for two-dot spacing as a function of retinal eccentricity: Differences in specialization of the central and peripheral retina

The brief presentation in the peripheral field of two closely spaced luminous point stimuli, in rapid sequence, induces the illusion of a single dot moving over an extended path. This fine-grain movement illusion (FGMI) is particularly compelling under conditions of dark adaptation. The strength of the motion percept, assessed by a rating-scale procedure, was found to correlate well, over different flash-flash onset delays, with an objective measure of the illusion requiring discrimination of the direction of the flash-flash sequence. A direction-discrimination measure was used to determine the minimum dot separation that would reliably elicit an FGMI at retinal eccentricities of 5-25 deg. For comparison, measures of static spatial acuity was made based on the minimum angle of resolution of two simultaneous dot flashes, and on the threshold for discriminating the separation of two simultaneous dot flashes with variable initial spacing. The spatial threshold for FGMI was lower than that for each of the static measures at all peripheral eccentricities, and it increased more slowly with eccentricity than the other spatial thresholds, suggesting the involvement of separate visual pathways for generating percepts of motion and percepts of shape or location. The finding that in the periphery the grain for motion detection was finer than that for spatial discrimination constrains a class of motion-perception models that form an initial spatial description of the stimulus and then compute a temporal derivative.