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I never understood why we have three trichromatic vision and not tetrachomatic vision. We have rods and three types of cones, which look like four channels to

I never understood why we have three trichromatic vision and not tetrachomatic vision. We have rods and three types of cones, which look like four channels to me. In very low light the cones don't fire and we see in black and white. In reasonably bright settings I can imagine that the rods saturate and we only have three useful channels. But there must be a level of light where both cones and rods are within their dynamic range. What am I missing?

lnanek2 13y ago
The wavelength curve the rods are sensitive to spikes in the middle of all the wavelengths curves for all the cones. So I think the mental sensation of gray is pretty accurate, actually, since it is close to being just a rough measurement of all the others.
- alphaBetaGamma 13y ago
  You are saying that the rods' spectral curve response is collinear with the cones. That would do the trick, but I'm a bit surprised as I don't see any reason for evolution to push you in that direction. Do you have a reference for this? Quick googling seems to show that the rods' response curve is not, it seems to be a most sensitive at a wavelengths between the peak of the blue and green cones.
  - icegreentea 13y ago
    I'm not sure if this will satisfy you, but you can give this a shot: http://en.wikipedia.org/wiki/Mesopic_vision http://en.wikipedia.org/wiki/Mesopic_vision http://en.wikipedia.org/wiki/Purkinje_effect http://en.wikipedia.org/wiki/Purkinje_effect
alok-g 13y ago
>> But there must be a level of light where both cones and rods are within their dynamic range. What am I missing? I am not sure if human vision is using one or the other. When both rods and cones are within the dynamic range, possibly response from both get utilized by the brain. I am not sure.
saalweachter 13y ago
Spitballing here: it's not just the sensors, but what you do with them. Is the visual processing for rods handled differently than the visual processing for cones?
- saalweachter 13y ago
  Actually, I feel very good about this line here: "More than one thousand times as sensitive as the cones, they can reportedly be triggered by individual photons under optimal conditions." http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.ht... So while rods have a response curve which is similar to codes, that response curve is shifted to be a thousand times more sensitive than cones. So at any lighting level which activates the cones, your rods are just maxed out, screaming "YES, IT IS VERY BRIGHT", and you don't get any useful information out of them. Hence, no integrated, tetra-chromatic vision.

lnanek2 13y ago

The wavelength curve the rods are sensitive to spikes in the middle of all the wavelengths curves for all the cones. So I think the mental sensation of gray is pretty accurate, actually, since it is close to being just a rough measurement of all the others.

alphaBetaGamma 13y ago
You are saying that the rods' spectral curve response is collinear with the cones. That would do the trick, but I'm a bit surprised as I don't see any reason for evolution to push you in that direction. Do you have a reference for this? Quick googling seems to show that the rods' response curve is not, it seems to be a most sensitive at a wavelengths between the peak of the blue and green cones.
- icegreentea 13y ago
  I'm not sure if this will satisfy you, but you can give this a shot: http://en.wikipedia.org/wiki/Mesopic_vision http://en.wikipedia.org/wiki/Mesopic_vision http://en.wikipedia.org/wiki/Purkinje_effect http://en.wikipedia.org/wiki/Purkinje_effect

alok-g 13y ago

>> But there must be a level of light where both cones and rods are within their dynamic range. What am I missing? I am not sure if human vision is using one or the other. When both rods and cones are within the dynamic range, possibly response from both get utilized by the brain. I am not sure.

saalweachter 13y ago

Spitballing here: it's not just the sensors, but what you do with them. Is the visual processing for rods handled differently than the visual processing for cones?

saalweachter 13y ago
Actually, I feel very good about this line here: "More than one thousand times as sensitive as the cones, they can reportedly be triggered by individual photons under optimal conditions." http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.ht... So while rods have a response curve which is similar to codes, that response curve is shifted to be a thousand times more sensitive than cones. So at any lighting level which activates the cones, your rods are just maxed out, screaming "YES, IT IS VERY BRIGHT", and you don't get any useful information out of them. Hence, no integrated, tetra-chromatic vision.