Abstract
Purpose :
Due to random X-inactivation and polymorphism in the spectral sensitivity of the L and M cones, some females have four distinct cone classes in their retinae. However, psychophysical studies of females with the gene for four cones have found the vast majority of subjects do not exhibit four-dimensional color vision (Jordan et al., JoV, 10, p1, 2010), even when the three middle-to-long wavelength photopigments are well-separated in spectral peak. We propose that the absence of functional tetrachromacy in this population is a result of the circuitry in the outer retina and a dearth of visual experience that would train the visual system to distinguish an additional color axis.
Methods :
To test this hypothesis, we created a receptive field response and detection algorithm based on the theory of color appearance put forward by Schmidt et al. (JOSA A, 31, pA195, 2014). According to this theory, all conscious vision is mediated by the midget ganglion cell system: achromatic vision is mediated by L vs M receptive fields, while hue perception is mediated by double opponent receptive fields formed when S-cone signals are combined with L vs M signals in the outer retina. The resulting model was presented with millions of colored edges and correlations between the response of each receptive field were calculated in order to ascertain how many distinct outputs the brain could distinguish. Our model differs from previous attempts, which relied on individual cone responses rather than the outputs of midget ganglion cells.
Results :
We found that any 4th cone added to the model, no matter how well spaced between the M and L cone photopigments, produced a response that was highly correlated to the existing receptive fields of trichromats. Receptive fields with the 4th cone as a center were consistently classified as achromatic L vs M in the absence of S-cone input, or Blue vs Yellow when S-cone signals were included.
Conclusions :
These results show that it is unlikely the brain would be able to differentiate the signal produced by receptive fields with a fourth cone without consistent exposure to scenes in which their activity is decorrelated from those that mediate the red-green, blue-yellow, and black-white percepts of normal trichromatic vision. As a result, functional tetrachromacy is exceedingly rare in the general population despite the significant number of females born with four well-separated cone photopigments.
This is a 2021 ARVO Annual Meeting abstract.