Abstract
Introduction. S cone excitation is lower in the fovea because short wavelength light is filtered by the foveal macular pigment. Despite reduced S cone excitation, white fields appear uniform — there is no yellow spot in the center of the visual field. This could be due to filling-in across space of the macular shadow, or it could due to local compensation of chromatic signals for white. By measuring white rings in a dark surround, we eliminated the effect of filling-in across eccentricity. Methods. In the first experiment, rings of various diameters were repeatedly, briefly, flashed on a dark surround while subjects fixated the center. Subjects adjusted the chromaticity of the rings until they appeared colorless. Luminance was kept constant. In a second experiment to rule out simple Weber adaptation, we measured achromatic settings for very dim rings. In addition, to estimate the density of macular pigment across the retina for each subject, we used minimum motion photometry. Results. The estimated macular pigment density was on the average 2 times greater in the fovea than in the periphery. However, despite greater filtering in the fovea, S cone light required for neutral white was approximately the same at all eccentricities, different at most by a factor of 1.2. This was true even for very dim rings which were unlikely to cause Weber adaptation of S cones. Conclusions. The perception of white is locally neurally compensated for reduced foveal S-cone excitation due to macular pigment. Since the compensation is found even at low intensities it is apparently not due to Weber adaptation of the cones.