Human Systems Integration Division @ NASA Ames

One aspect of the retina which influences the information available to the visual system is the geometrical arrangement of the cones. The packing arrangement of the cones in the central fovea has been carefully measured in retinal preparations from monkeys. Psychophysical measurements have been made using laser interferometry in humans. Although these measurements are indirect, they are free from possible histological artifacts. The psychophysical techniques for observing the arrangement are based on the moire patterns which are perceived when very fine gratings are imaged on the retina by laser interferometry. The results of the technique depend on the regularity of the cone array. In the center of the fovea, where the cones can be very regularly packed, the moire pattern is fairly regular and the technique gives both the orientation and the spacing of the cone array. In the center of the fovea, there are very few blue cones, so they will be ignored here and only the red and green ones will be considered. The arrangement of the red or green cones alone is not known. There is no definitive information about the relative number of red and greens in the central fovea, although there is indirect evidence indicating that there are twice as many red cones as green cones. In support of psychophysical investigations of the arrangement of the red and green cones, we have been studying models which generate possible red-green arrangements and then looking at the aliasing patterns that would be generated by fine gratings which stimulated only the red or the green cones.