Tetartanopia

A type of blue-yellow Dichromacy that exists only theoretically as part of the Opponent Process Theory. The blue-yellow opponent channel is a balance between the excitation of the S-Cones (representing blue) and the sum of excitations of the L- and M-Cones (together representing yellow). While Tritanopia (Blue Blind) exists by deletion of the S-Cone, Tetartanopia (Yellow Blind) would require deletion of both the L- and M- Cones. While this is possible, and is known as Blue Cone Monochromacy, this also necessarily destroys the red-green opponent channel, leaving the individual as a Monochromat, not a Tetertanope. In fact, there is not even a theoretical mechanism on the photoreceptor level that could result in Tetartanopia, though theoretical mechanisms may exist further down the Phototransduction Pathway (Neurological CVD).

If hypothetically human Color Vision consisted of 4 cones, where the 4th cone is introduced with Peak Sensitivity at 545nm, and replaced the L- and M-Cones in the blue-yellow Opponent Channel “calculation”, the resulting vision would be nearly identical to the current Standard Observer. However, if this 4th Cone were to be deleted, then the individual would experience Tetartanopia.

This adaptation for humans to use ‘replace’ a yellow cone with the sum of the L- and M-Cones is one reason that it took so long for Color Theory to resolve the seemingly contradictory theories of Trichromatic Color Vision and Opponent Process Theory.

Tetartanomaly is theoretically possible via several mechanisms.