Abstract

The effect of eccentricity on localization tasks such as Vernier acuity or separation discrimination is controversial. While most studies report substantial effects of eccentricity, a few report that for large-scale localization there is no effect of eccentricity for the central few degrees of the visual field (e.g., Ref. ¹). To pursue such reports, we used a separation discrimination task where observers Judged if two luminous squares were closer together or farther apart than a remembered standard. Eccentricity was manipulated independently of separation by varying the vertical position of two horizontally separated patches while controlling observer fixation. For large (18-min of arc square) high-contrast (40:1) stimuli, there was a range of eccentricities that did not affect separation threshold. For example, if the separation was 60 min of arc, the separation threshold did not increase until the eccentricity was increased beyond 120 min of arc. Larger standard separations increased the range of eccentricities that did not affect the separation threshold. In addition, reducing the stimulus size reestablished an eccentricity effect on separation thresholds.

© 1987 Optical Society of America