Cognitive Daily has a posting related to Human Female Tetrachromancy that refers to some old article on the subject. An interesting and must-read article on the web by Ryan Sutherland in detail explains the rationale as to how four cone receptors may arise due to X-chromosome related procedures. Also It is instructive to note here that if the additional cone that has shifted from Red(Long cone) towards green (the red-shifted) or from Green(The Medium cone) towards the red (green-shifted) has shifted to a considerable extent, then it may assume the role of phantom Yellow and thus lead to some radical re-wiring of the optical system in brain whereby Red(L) and Green(M) do not have to combine to give Yellow that can be considered along with output from Blue (S) cone to give rise to Blue-Yellow opponent process. In this case a simple consideration of output from Blue (S) and Shifted-red/shifted-green (Yellow) would give rise to the Blue-Yellow opponent process. I don’t think such radical shifts are possible or would lead to such radical rewiring, but post-mortem analysis of Tetrachromat women may shed some light. Even if such a shift does occur , it may not lead to any change in the number of hues that could be distinguished, though the colors may appear more colorful and saturated.

Of further interest is the shift from red away from green side towards the ultraviolet. This shift may indeed give rise to ability to perceive Hues differently and to see some infra-red not normally visible to trichromatic humans.

coming back to different dimensions of vision, it is interesting to note that dogs (like most other mammals) have dichromatic vision and utilize the blue-yellow opponent process.

Cats utilize the same trichromatic color mechanisms as humans, but their total perceivable color range is sort of ‘contracted’ i.e. they don’t see some of the human Red and some of the human Blue.

Bees have also trichromatic vision, but apparently their cones lie in UV, Blue and green. Thus they are unable to see human red but able to see beyond Violet (the UV). Maybe the genes coding blue lie on X chromosomes for Bees (instead of the red-green genes of humans and yellow of dogs) and its breakup into two (just like the breakup of mammalian yellow is hypothesized to have resulted in human Red-Green) has resulted in some infra-blue and Ultra-violet cones in the bees.

Further most birds (and some fish and turtles) have tetrachromatic vision with 4 cones : one in UV, one in Blue, one in green/yellow and the other in red. Thus, if humans do want to have a tetrachromatic vision a better way forward would be the split of blue cone in infra-blue and UV cones. That would really give us the capacity to for example view the human-white feathers of some birds as actually ‘colored’/shining’ in UV (as they reflect UV). For more details on comparative chromatic vision information please visit this excellent page on comparative chromatic vision. Also some evolutionary rationale for chromatic vision (and UV in particular) can be found here.

Endgame: Would introduction of a UV cone lead to radical changes in perception of the blue (blue-indigo-violet) end of the spectrum, just like splitting of Yellow into Red and Green led to totally new colors on the original Yellow part of the spectrum?

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