The effect of orientation on the visual resolution of gratings.

1. Visual resolving power is known to be poorer for objects oriented obliquely as compared with horizontal and vertical orientations. Experiments were designed to evaluate the optical and neurophysiological factors involved.2. Gratings with a sinusoidal light distribution were generated on the face of an oscilloscope. Spatial frequency and contrast could be varied while keeping the mean luminance of the grating constant.3. Using a homatropinized eye with an artificial pupil and carefully corrected refraction, high resolution in the vertical and horizontal meridians as compared with the oblique meridians was found for gratings ranging in spatial frequency from 1 to 35 c/deg.4. It is concluded from the similar behaviour of low and high frequency gratings that neither focus errors nor optical aniseikonia can account for these findings.5. Additional proof that optical factors cannot significantly account for these preferred directions of resolution was obtained by forming interference fringes directly on the retina using a neon-helium laser as a coherent light source.6. Similar orientational changes in resolution were found by by-passing the dioptrics with interference fringes. It is concluded that the effect is due to some orientational inequality in the visual nervous system.