Flicker effects of receptor directional sensitivity.

When a Maxwellian-view test field is alternately presented through different small areas in the pupil, a residual flicker is observed even though the luminance of the field is adjusted to compensate for the Stiles-Crawford effect. The stimulus alternation frequency that eliminated this residual flicker was used to estimate the magnitude of modulation in the receptors resulting from the difference in direction of illumination. These results support the concept that directional sensitivity of cones is much more acute than the directional sensitivity represented by conventional measures of the Stiles-Crawford effect.