If the fovea contains no S cones why don humans have functional dichromancy in foveal vision?
The visual system is indeed dichromatic for very small stimuli. It is called small-field tritanopia. The key is that most of the fovea does contain S cones. It is only the very central area, less than 0.25 degrees that has no S cones. To observe small-field tritanopia the stimulus must be entirely contained within that small central region of the fovea. Even brief eye movements to areas that will expose the stimulus to S-cones will destroy the effect due to perceptual filling-in (just like the blind spot).
