Dependence of the magnitude of the Stiles-Crawford effect on retinal location.

1. The directional sensitivity (Stiles-Crawford effect) of retinal cones is supposed to be associated with their shape, but only extrafoveal cones have a cone-like shape; cones in the central fovea are elongated and look like rods.2. To determine whether the directional sensitivity of cones depends on their shape, the Stiles-Crawford effect was measured both in the central fovea and in the parafovea of the human eye.2. To ensure that the cone population tested was homogeneous, a small brief test flash, brought into the eye through the centre of the pupil, was placed at threshold by varying the intensity of a large adapting field. The directional sensitivity of the cones was determined by finding the efficiency of light to act as an adapting background as a function of position of entry in the pupil.4. Central foveal cones have a less pronounced directional sensitivity than parafoveal cones and this lends support to the conclusion that the Stiles-Crawford effect is connected with the shape of the retinal receptors.