Human orientation discrimination: changes with eccentricity in normal and amblyopic vision.

The authors measured orientation discrimination of a single line as a function of eccentricity, line length, and standard orientation. Orientation discrimination improved with increasing line length at all eccentricities. The shortest length at which orientation discrimination was optimal increased with eccentricity. When a line length was used for which discrimination was optimal at all eccentricities, it was found that orientation discrimination performance changed little with increasing eccentricity. Under the same conditions, the oblique effect in orientation discrimination decreased with increasing eccentricity. Similar results were also obtained in both eyes of strabismic amblyopes. The difference between the just noticeable differences in orientation of the amblyopic and nonamblyopic eye decreased with increasing line length for central vision. This interocular difference also decreased with increasing eccentricity. The absence of the oblique effect in orientation discrimination at large eccentricities support the hypothesis that the area 17 S-cell orientation anisotropy underlies the oblique effect in orientation discrimination of long lines since this sensorial anisotropy is limited to the central visual field.