REFRACTION, ALIASING, AND THE ABSENCE OF MOTION REVERSALS IN PERIPHERAL-VISION

Reversals in perceived direction of motion of a grating when its spati al frequency exceeds half that of the sampling mosaic provide a potent ial tool for estimating sampling frequency in peripheral retina. We us ed two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high con trast horizontal or vertical sinusoidal luminance gratings presented e ither 20 or 40 deg from the fovea along the horizontal meridian. A fov eal target at a comfortable viewing distance aided fixation and accomm odation. A Maxwellian view optometer with 3 mm artificial pupil was us ed to correct the refraction of the peripheral grating, which was pres ented in a circular patch, 1.8 deg in diameter, in a surround of simil ar colour and mean luminance (47.5 cd . m(-2)). The refractive correct ion at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency wh ich can reliably be detected (7-14 c/deg at 20 deg, 5.5-7.5 c/deg at 4 0 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to ve rtical. Direction discrimination performance shows no consistent rever sals, and depends less on refraction. It falls to chance at frequencie s as low as one-third of the highest that can be detected. Gratings wh ich can be detected but whose direction of motion cannot be discrimina ted appear as irregular speckle patterns whose direction of motion var ies from trial to trial. The absence of motion reversals may reflect i rregularity of sampling, and suggests that reversals are not a simple tool for studying sampling in peripheral vision.