Two-dot vernier discrimination within 2.0 degrees of the foveal center

Vernier acuity (delta v) as a function of two-dot separation (s) was measured at five retinal locations between the foveal center and 2.0 deg of eccentricity. We compare these results with average cone spacing at each of the corresponding retinal eccentricities and find that the angular dot separations at which the delta v versus s function intersects the angular cone spacing at each eccentricity remain nearly constant. Further, we define a vernier Weber fraction delta v/s and find that the average Weber fraction increases by nearly a factor of 2 from the fovea to 2.0 deg of retinal eccentricity. We suggest that both receptor factors and postreceptor factors contribute to limits of vernier acuity within 2 deg of the foveal center.