A displaced Stiles-Crawford effect associated with an eccentric pupil.

Receptors tend to be oriented toward a point near the center of the pupil in the normal eye. We report psychophysical studies of receptor orientation and directional sensitivity in a subject whose right pupil is displaced nearly 3 mm nasally as a result of injury sustained 25 years ago. The Stiles-Crawford effect was measured for foveal cones and for parafoveal cones and rods. Greatest sensitivity was found in all cases at a point close to the center of the natural pupil, indicating that the receptors in this eye are trained toward the abnormally situated pupil. At large angles of incidence, foveal cones exhibited a clear asymptote of sensitivity 0.83 log units below the sensitivity for axially incident light. Parafoveal cones were more directionally sensitive, with a suggestion of an asymptote for oblique incidence about 1.2 log units below the sensitivity for axial incidence. Rods showed a sensitivity pattern decentered like that of the cones, with a greatest observed sensitivity loss of 0.28 log units. Best acuity for cones was observed for entrance pupils close to the optical axis of the eye, remote from the pupillary region for best sensitivity.