Elevation of Vernier thresholds during image motion depends on target configuration

Previously we showed that thresholds for abutting Vernier targets are unaffected by motion, as long as the targets are processed by the same spatial-frequency channel at each velocity and remain equally detectable [Invest. Ophthalmol. Visual Sci. (Suppl.) 37, S734 (1996)]. In this study we compared Vernier thresholds for stationary and moving abutting and nonabutting targets (gaps = 0, 18, and 36 arc min) for velocities of 0-16 deg/s. The Vernier targets were spatially filtered vertical lines (peak spatial frequency = 3.3 or 6.6 c/deg), presented at contrast levels of two, four, and eight times the detection threshold of each component line. Unlike the results for abutting targets, Vernier thresholds for nonabutting targets worsen with velocity as well as gap size. The results for abutting Vernier targets are consistent with the hypothesis that thresholds are mediated by oriented spatial filters, whose responses increase proportionally with the stimulus contrast. The velocity-dependent thresholds found for nonabutting Vernier targets can be explained on the basis of local-sign comparisons if the comparison process is assumed to include a small amount of temporal noise.