The initial tOFRs obtained when two competing 1-D radial sine-wave gratings (*3f5f* stimulus) —each consisting of a single annulus (radial thickness, 3°)—rotate in opposite directions: dependence on their relative contrast (Contrast Ratio) and the effect of physical separation. (A): X-Y luminance plot of a sample stimulus configuration in which an inner *5f* annulus is separated from an outer *3f* annulus by a gap of 2°. (B): Diagram indicating the locations of the two competing annuli when separated by each of the four gaps used (−3°, 0°, 2°, 4°); the two annuli were always radially symmetrical with respect to a circle of radius 7.5° (dotted circle); note that when the gap between them was −3° the two annuli exactly overlapped, and when the gap was 0° the two annuli exactly abutted one another; the reverse configuration (inner *3f*, outer *5f*) was also used (not shown). (C,D): Response Ratio, computed using Expression 4 and mean CW-CCW (stimulus-locked) response measures, is plotted as a function of the Contrast Ratio, *3f/5f* for subjects BMS (C) and FAM (D); preliminary analysis had indicated that the data were essentially the same whether the inner annulus was occupied by the *3f* or the *5f* grating, so the two sets of data were pooled; black filled circles, gap=−3°; blue open squares, gap=0°; green closed diamonds, gap=2°; red open circles, gap=4°; continuous curves with matching colors are best-fit Cumulative Gaussian functions; dotted lines with matching colors show the vector sum predictions. Each datum point based on 93–100 samples (BMS) and 93–100 samples (FAM).