Abstract
Purpose: When a sine-wave grating presented at the central retina moves by 90°, the perceived direction of motion depends on the duration of a blank average-luminance inter-stimulus interval (ISI). At short ISIs, the perceived motion direction is reversed. This is predicted from the biphasic temporal response of first-order motion detectors. At longer ISIs, the perceived direction becomes veridical, suggesting the operation of a feature-tracking mechanism. Here, we measured the effect of ISI on the perceived direction of moving gratings at different retinal illuminances and eccentricities. Procedure: Subjects judged the perceived direction of high contrast (30 x threshold) moving 0.25 cpd gratings. Results: Under photopic conditions, at 20° eccentricity in the upper visual field, motion reversal was stronger with short ISIs, and perception did not become veridical at longer ISIs. In the lower visual field, performance was more similar to that in the central retina. Under mesopic and scotopic conditions, motion reversal disappeared in central vision, but strong motion reversal was still observed in the periphery. When the stimulus covered both central and peripheral visual fields, no motion reversal was seen. Conclusions: In photopic vision, both first-order and feature-tracking mechanisms operate in the central and lower visual fields, while the first-order motion mechanism dominates strongly in the upper visual field. This suggests the feature-tracking mechanism is attention-based. As retinal illuminance decreases, the relative contribution of the feature-tracking mechanism in central retina becomes larger but perception at the periphery depends on a biphasic first-order motion mechanism. When both central and peripheral visual fields are stimulated simultaneously, the feature-tracking mechanism in the central retina determines the perceived direction of motion.