Speed discrimination for equiluminant and low-contrast luminant gratings deteriorates with eye movements

Purpose. Previous findings indicate that motion discrimination worsens when the subject's eyes move faster, and in the same direction as the target. The present study tested whether speed judgments for equiluminant and low-contrast luminant gratings would be affected similarly. Methods. The stimulus was a vertically oriented, 0.5 c/deg sinusoidal grating (4.5 x 5 deg) composed of either equiluminant red-green bars (1.5 log units > threshold), high contrast-luminant bars (1.5 log units > threshold) or low-contrast luminant bars (0.5 log units > threshold). A blur screen was used to filter out high spatial frequencies. Superimposed across the center of the grating was a horizontal strip (1 x 5 deg), with a small, luminant spot in the center that served as the pursuit target; the pursuit spot always moved in the same direction as the grating. Grating speeds, ranging from 0.75 to 3 deg/s, and pursuit-spot speeds, ranging from 0 to 3 deg/s, were factorially combined for the three types of gratings. Subjects viewed two successively presented gratings, while they simultaneously pursued the moving spot. Subjects judged which grating was faster. On each trial, the speed of the test grating was changed by a delta determined from two interleaved staircases. Horizontal movements for the right eye were recorded with an eyetracker. Results. Speed discrimination for the high-contrast gratings declined as a function of eye speed; however, judgments for equiluminant and low-contrast luminant gratings were affected far more negatively by higher eye speeds. Conclusions. The results cannot be explained solely by changes in the speed of the retinal image. The interaction suggest; that motion information for equiluminant and low-contrast gratings are degraded in a similar manner.