Abstract

Central acuity is worse for oblique gratings than for vertical or horizontal and this effect is said to extend some 18° into the periphery as well.¹ However, quite a different phenomenon has also been described for peripheral vision, the meridional effect, which is characterized by greater acuity for gratings parallel to visual meridia than for the other orientations.² As these two generalizations are inherently incompatible, we sought to determine which effect prevails. Acuity was determined as a function of grating orientation for the eight primary meridia at six eccentricities in the 10-35° range. Results revealed that either effect may dominate, depending on the particular visual field location, stimulus configuration, and subject. Some individuals show an oblique effect for all eccentricities while others show a transition from the central oblique effect to a meridional effect in the periphery. Furthermore, these two patterns can occur for different meridia in the same individual and may sometimes be altered significantly by varying the luminance of the field immediately surrounding the test grating. Despite this variety of results, the body of our data can be accounted for by supposing that two forces shape the orientational anisotropy of peripheral vision. Just which force is the stronger depends on numerous factors and cannot be summarized yet by a simple generalization.

© 1985 Optical Society of America