The visual resolution of Landolt-C optotypes in human subjects depends on their orientation: the 'gap-down' effect.

The visual resolution was measured in 184 healthy subjects using luminance contrast defined Landolt optotypes at eight different gap locations. The investigation used monocular detection for both eyes up to acuity thresholds. Acuity estimates were obtained under forced choice conditions and the threshold defined by a sensitive criterion. The results show that the percentage of correct responses was lowest when the gap of the Landolt-C was at 6 o'clock. There were no statistically significant differences between left vs. right or orthogonal vs. diagonal gap detection. Since anisotropic axes in orientation perception are not retinotopically mapped, we suggest the involvement of higher cortical mechanisms.