Local nonlinearity in S-cones and their estimated light-collecting apertures

When an high frequency grating of high retinal contrast is presented intermittently by modulating its contrast at constant average luminance, observers experience uniform field flicker, even if the grating is too fine to be resolved. For long and middle wavelength cones, this contrast-modulation flicker can be seen for fringe periods as small as the diameter of a cone [MacLeod & He (1993). Nature, 361, 256–258], implying no substantial neural spatial integration prior to the nonlinear site. We now report that the short-wavelength cone system, despite its greater spatial integration than the other cone systems, can generate contrast-modulation flicker at spatial frequencies as high as 50 cycles/deg, a value comparable with that of the other cone systems in the same retinal area. Spatial resolution at the nonlinear site is in all cases apparently limited by the size of the cones. Likewise, little temporal filtering (in the range up to 18 Hz) precedes the S-cone nonlinearity. This suggest that the reduced S-cone system sensitivity for rapid flicker is due to postreceptoral limitations.