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Is color information sent from the eye to the brain frequency-modulated, or are different colors transmitted by different axons?

I know that each ganglion cell is connected to multiple photoreceptors. How do ganglion cells tell the brain which photoreceptors were exactly activated and by what color?

Do ganglion cells correspond to particular colors?

Can one ganglion cell transmit different signals to the brain?

How does the brain know which color the light has, if signals of different receptors are mixed in the ganglion?

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I believe that some preprocessing takes place in the retinal ganglion cells (see also en.wikipedia.org/wiki/Color_vision#Color_in_the_human_brain), so the brain does (probably) not have any information on which types of cones that are activated. I dont know how the colour information is transmitted from ganglion cells to brain though. –  fileunderwater Dec 18 '13 at 15:24
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possible duplicate of What gives things their colour? –  The Last Word Jun 6 '14 at 7:16
    
@The Last Word no, that answer deals with the eye. –  Anixx Jun 6 '14 at 9:22
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@fileunderwater I heard somewhere that without preprocessing the optical nerve would have a diameter of several centimeters. :-) –  inf3rno Mar 18 at 16:39

1 Answer 1

Short answer

Color vision is mediated by spike rate modulation (frequency modulation if you like), as well as ganglion-cell specific stimulation.


Background

Ganglion cells exist that transmit opponent colors. The color opponency model predicts that we cannot perceive a greenish-red or yellowish blue, which is quite right. We can, however, see mixtures of the two channels, such as red and yellow (orange), and red and blue (purple).

Then opponent model predicts the existence of ganglion cells that are sensitive to red/green opponency and blue/yellow opponency (in trichromats like humans):

enter image description here

Indeed, it has been shown that these ganglion cells exist, i.e. cells that increase their firing in response to short-wavelength-sensitive (S+) and decrease their firing to middle-wavelength-sensitive (M-) and vice versa (Yin et al., 2009). Hence, it is not so that the frequency of light is represented in firing rate per se, but the frequencies of light are translated in spike rates, because spike rate is the only means in which neurons can transmit graded responses. In other words, every type of graded information is translated into spike rate eventually, including pressure differences, acoustic loudness etc.

Hence, as ganglion cells are sensitive to opponent pairs of colors, they do respond to particular colors. However, a "purple ganglion cell" does not exist, it is the combination of red and blue activations that lets us see colors due to synthetic processes in higher neural centers.

One ganglion cell can send different signals to the brain as it is basically an analogue converter of the red/green and yellow/blue opponent colors.

These inputs are sent to the thalamus (lateral geniculate nucleus) and higher visual cortices, where color-selective cells pick up the signals and integrate them for further processing. Here color mixtures are made to present the full gamut of colors as we know it:

color spectrum
Source: Dot-Color


Reference

Yin et al. J Neurosci 2009;29:2706-24


Further reading

Advantage of opponent color?

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