Abstract

Separation discrimination thresholds are ordinarily measured with a pair of targets that are symmetrically placed with respect to the fovea, so that eccentricity increases as separation increases. In these conditions, the separation discrimination threshold (for separations larger than some minimum value) is a constant fraction of the target separation; i.e., localization obeys Weber’s law. However, this is not generally true. If the eccentricity of the features is kept constant, localization thresholds increase with separation at a rate slightly less than predicted by Weber’s law and then become constant, varying solely with eccentricity. [Flattening has also been reported for a bisection task by Levi et al. (Vision Res., in press).] Thus there appear to be two mechanisms involved, one which is sensitive to separation and relatively insensitive to eccentricity [Burbeck, Inv. Ophthalmol. Suppl. (1988)] and one which is primarily sensitive to eccentricity. The separation discrimination thresholds, as ordinarily measured across the fovea, measure sensitivity in the flat portions of the curves where eccentricity is the main factor (because a foveally centered target presents the maximum separation achievable at a given eccentricity). However, separation itself is the primary factor in many other stimulus conditions.

© 1988 Optical Society of America