COLOR, CONTRAST SENSITIVITY, AND THE CONE MOSAIC

This paper evaluates the role of various stages in the human visual sy stem in the detection of spatial patterns. Contrast sensitivity measur ements were made for interference fringe stimuli in three directions i n color space with a psycho-physical technique that avoided blurring b y the eye's optics including chromatic aberration. These measurements were compared with the performance of an ideal observer that incorpora ted optical factors, such as photon catch in the cone mosaic, that inf luence the detection of interference fringes. The comparison of human and ideal observer performance showed that neural factors influence th e shape as well as the height of the foveal contrast sensitivity funct ion for all color directions, including those that involve luminance m odulation. Furthermore, when optical factors are taken into account, t he neural visual system has the same contrast sensitivity for isolumin ant stimuli seen by the middle-wavelength-sensitive (M) and long-wavel ength-sensitive (L) cones and isoluminant stimuli seen by the short-wa velength-sensitive (S) cones. Though the cone submosaics that feed the se chromatic mechanisms have very different spatial properties, the la ter neural stages apparently have similar spatial properties. Finally, we review the evidence that cone sampling can produce aliasing distor tion for gratings with spatial frequencies exceeding the resolution li mit. Aliasing can be observed with gratings modulated in any of the th ree directions in color space we used. We discuss mechanisms that prev ent aliasing in most ordinary viewing conditions.