High acuity vision corrected for chromatic and monochromatic aberrations is associated with color discrimination without red-green or blue-yellow sensations

Abstract

From the earliest measurements of contrast sensitivity for chromatic gratings, it has been reported that isoluminant red-green gratings appear achromatic at high spatial frequencies. To better understand this phenomenon, we used a forced-choice paradigm to measure contrast sensitivity functions for red-green and S-cone isolating sinusoidal chromatic gratings. For all red-green grating experiments, monochromatic aberrations were compensated via adaptive optics. Longitudinal and transverse chromatic aberration were compensated by a specialized Badal system and a Vernier alignment task, respectively. We measured responses to red-green gratings under two conditions. In the first condition, which is comparable to the measurement of conventional chromatic contrast sensitivity, we measured contrast thresholds for detecting the grating pattern from uniform-field distractor targets. In the second condition, for a series of spatial frequencies, subjects were required to pick out a red-green grating from isochromatic grating distractors. Subjects were able to detect red-green gratings with a much higher spatial frequency cut-off (>30 c/deg) than for S-cone isolating gratings. Subjects were able to discriminate colored from isochromatic gratings with a high spatial frequency cutoff of only 10–12 c/deg, which was more similar to that found for S-cone gratings. Thus, subjects were able to detect changing red-green wavelength content at high spatial frequencies without perceiving hue sensations. The results are consistent with the unconventional hypothesis that achromatic and hue sensations are separated at the level of two subpopulations of midget ganglion cells, one serving high acuity black-white vision and a second more sparse mosaic serving hue perception at a lower spatial resolution.