We examined the limitations imposed by neural factors on spatial contrast sensitivity for both isochromatic and isoluminant gratings. We used two strategies to isolate these neural factors. First, we eliminated the e~f~ct of blurring by the dioptrics of the eye by using interference fringes., Second, w~correc.ted our data for addl~lOnal sensitivity losses up to and including the site of photon absorption by applymg an Ideal-observer analysis described by Geisler [J. Opt. Soc. Am. AI, 775 (1984)]. Our measu~ementsind~cate t~at the.neural~isu~syst~m modifies the shape of the contrast-sensitivity functions for both isochromatic and isoluminant~tImuh at hlg.h spatial frequencies. If we assume that the high-spati~-~reque~cyperforman~e of the neural visual~ystem IS determined by a low-pass spatial filter followed by additive noise, then the VISUal system has a spatIal.bandwidth 1.8 times lower for isoluminant red-green than for isochromatic stimuli. On the other hand, we fmd no difference in bandwidth or sensitivity of the neural visual system for isoluminant red-green and S-coneisolated stimuli.

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This paper evaluates the role of various stages in the human visual system in the detection of spatial patterns. Contrast sensitivity measurements were made for interference fringe stimuli in three directions in color space with a psychophysical technique that avoided blurring by the eye's optics including chromatic aberration. These measurements were compared with the performance of an ideal observer that incorporated optical factors, such as photon catch in the cone mosaic, that influence the detection of interference fringes. The comparison of human and ideal observer performance showed that neural factors influence the shape as well as the height of the foveal contrast sensitivity function for all color directions, including those that involve luminance modulation. Furthermore, when optical factors are taken into account, the neural visual system has the same contrast sensitivity for isoluminant stimuli seen by the middle-wavelength-sensitive (M) and longwavelength-sensitive (L) cones and isoluminant stimuli seen by the short:wavelength-sensitive (S) cones. Though the cone submosaics that feed these chromatic mechanisms have very different spatial properties, the later neural stages apparently have similar spatial properties. Finally, we review the evidence that cone sampling can produce aliasing distortion for gratings with spatial frequencies exceeding the resolution limit. Aliasing can be observed with gratings modulated in any of the three directions in color space we used. We discuss mechanisms that prevent aliasing in most ordinary viewing conditions.There has been considerable recent interest in understanding how color and luminance information are processed in the visual system. It is well known that the mechanisms sensitive to chromatic modulation have inferior resolution to those sensitive to luminance modulation (1), but the reasons why this is so are not yet clear. This paper examines the role of the first few stages of the visual system in detecting spatial patterns that vary in chromaticity or luminance. Wilson Geisler (2) has developed an elegant computational approach to this problem based on the theory of ideal observers. We describe an ideal observer similar to Geisler's and use it to examine the effect on contrast sensitivity of optical factors such as those associated with the cone mosaic. We then compare ideal observer performance with new measurements of human contrast sensitivity for patterns modulated in different color directions. This comparison reveals the properties of the neural mechanisms that underlie human performance. This paper summarizes work described by Sekiguchi et al. (3,4) to which readers are referred for details of the psychophysical experiments and computational model. Stimuli for Measuring Ideal and Human Contrast SensitivityWe computed the contrast sensitivity of the ideal observer and measured the contrast sensitivity of human observers with the same stimulus set. For the ideal observer, analytic descriptions of the stimuli were used as input to a computa...

We developed a new apparatus and psychophysical technique to extend isoluminant contrast-sensitivity measurements to high spatial frequencies. The apparatus consists of two identical laser interferometers that are designed to produce phase-locked two-color interference fringes on the retina without the influence of diffraction and most aberrations in the eye. However, even with interferometry, transverse chromatic aberration of the eye can produce a wavelength-dependent phase shift in the interference fringes, which can be exaggerated by head movements. To reduce the effect of head movements, isoluminant red and green interference fringes of equal spatial frequency and orientation were drifted slowly in opposite directions to guarantee a purely isochromatic (in phase) and a purely isoluminant (out of phase) stimulus during each cycle of stimulus presentation. With this technique we found that observers could resolve red and green stripes at spatial frequencies higher than 20 cycles per degree (c/deg) (20-27 c/deg), substantially higher than has previously been reported. This places a lower bound on the sampling density of neurons that mediate color vision. At all spatial frequencies, even those above the isoluminant resolution limit, a relative phase of the red and the green components could be found that obliterated the appearance of luminance modulation at the fringe frequency. Above the resolution limit, red-green-isoluminant interference fringes are seen as spatial noise, which may be chromatic aliasing caused by spatial sampling at some stage in the chromatic pathway.

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