Abstract
Purpose :
Two hypotheses have been proposed with regard to the double-duty role of midget ganglion cells in hue and achromatic sensations. The more conventional one is that signals originating from every midget ganglion cell are separated at higher levels to extract components responsible for chromatic and achromatic sensations predicting similar high frequency contrast sensitivity cutoffs for both types of percepts. The alternative hypothesis is that a majority of midget ganglion cells only give rise to black-and-white percepts although they are capable of detecting isoluminant color boundaries. Whereas, a second smaller submosaic of midgets is responsible for hue perception, predicting lower frequency contrast sensitivity cutoffs for hue perception compared to detection.
Methods :
To differentiate these hypotheses, we used a forced-choice paradigm to measure contrast sensitivity functions for red-green and S-cone isolating sinusoidal chromatic gratings. 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. 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.
Results :
Consistent with conventional measures of detection threshold, subjects were able to detect red-green gratings with a much higher spatial frequency cut-off (28-30c/deg) than for S-cone isolating gratings. However, 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.
Conclusions :
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.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.