Binocular recognition summation in the peripheral visual field: contrast and orientation dependence.

Spatial frequency thresholds for recognition were measured for binocular and monocular viewing conditions at two contrast levels (95% and 7%). Measurements were obtained at the fovea and at four different eccentric retinal locations. Each eccentric retinal location was 8 degrees from the fovea, one on the horizontal axis (180 degrees ), and the other three in the superior field on retinal axes of 90 degrees, 45 degrees and 135 degrees. For the superior and horizontal retinal locations, the orientations of the gratings tested were horizontal (180 degrees ) and vertical (90 degrees ). For the retinal points on the oblique axes, the orientations of the gratings were 45 degrees and 135 degrees. Measurements were also obtained at the fovea for all four different grating orientations at both levels of contrast. Recognition threshold was defined as the highest spatial frequency at which luminance gratings were perceived vertically. At the fovea, binocular summation ratios (binocular spatial frequency/monocular spatial frequency) showed no significant differences for gratings of either contrast levels or for any orientation (p>0.05). In the superior periphery, significantly higher summation ratios were shown by low contrast vertical gratings (p<0.05), and in the horizontal periphery by low contrast horizontal gratings (p<0.05). On the oblique axis, low contrast gratings that were parallel to the oblique meridian showed higher summation ratios compared to those at right angles to it. High contrast gratings, at least at 8 degrees eccentricity, did not show this effect. Data suggest that meridional organisation of the retina (e.g. vertical gratings seen maximally in the superior field) occurs for resolution and that it is evidenced closer to the fovea than previously shown.