Abstract

At last year’s meeting, we reported a dissociation between detection and orientation discrimination for gratings viewed eccentrically. Specifically, at 7° in the superior visual field, orientation discrimination is limited to ~8 cycles/deg, while simple detection persists to about twice that frequency. We proposed that the discrimination limit represents degradation of the image by aliasing. Coletta and Williams¹ have described a distinctive property of visual aliasing: moving images whose spatial frequency is above the Nyquist limit produce an alias which appears to move in the opposite direction. We asked subjects to discriminate the direction of motion of gratings at frequencies near the discrimination limit. It is very difficult to see motion in these stimuli, but for a small range of frequencies around the discrimination limit there is a clear statistical tendency to perceive motion in the reversed direction. In the absence of any obvious alternative, we regard this as strongly supporting the hypothesis that aliasing occurs in normal parafoveal vision.

© 1987 Optical Society of America