Physiologie Neuronaler Systeme

Dept. of Physiology, Med. School, University of Vienna

4) The tri-chromatic color vision of some primates - including us
- is the exception, not the rule.

Some million years ago the gene controlling the synthesis
of long wavelength photopigment has undergone certain mutations in some primates including our ancestors.
A duplication of the origunal long wavelength gene resulted in 2 genes. They are now controlling the production of opsin molecules with slightly different amino acid sequences leading to spectral maxima for "RED" or "GREEN".

Together with the other "old" short wavelength partner from the original dual system this lead to a trichromatic state. It extended the abilities of these primates for distinguishing hues in the longer (yellow - red) wavelengths as they occur on fruits and leaves.

Recent evidence suggests that the shift towards trichromacy occured three times independently among different primate groups.

South African monkey conquering a colorful tourist car.

Photo by Renate Pflug

This secondary splitting
of the longer wavelength into
G+R - pathways has led to the

Modern subsystem of color vision.

3 Cone Pigments in

3 Cone Photo- receptor-Types

Absorption Curves of photopigments

Two-Level Antagonistic Processing
(R / G) and [(R+G)=Y] / B

in Retina

This leads to the spectral band of hues we are familiar with
(compare the simulated dichromatic state)

We do not yet understand clearly however how the basic information is actually processed
within the retina and the brain to produce thousands of colors.

Can we identify the types of human cones
carrying the different cone pigments ?

Well, we can now distinguish the "Blue" sensitive cones including those of humans (see Other Research Topics) by several criteria.
However there has been very limited success so far to distinguish all three types, namely "Red" from "Green" sensitive cones.
Below is one such attempt from our lab. It was only a partial success und thus remained unpublished - as many things in science.

The image below may not look too impressive indeed. It is a sample from peripheral human retina from an eye
that had to be enucleated for a tumour. The isolated retina was first incubated in a solution with Procion Yellow,
a stain known to be preferentially uptaken by Blue cones. Then it was stimulated with long wavelength light under the assumption
that this would preferentially activate the Red cones.
This activation was then expected to be visualized by NitroBlueTetrazolium incubation as above.

The preparation is not ideal and many cones cannot be distinguished but on the scheme below is what one may identify:

There are 3 cones that are not labeled with NBT but show bright Procion Yellow fluorescence (arrows).

This affinity indicates that they may be the Blue cones in this sample.

The other cones however show various intensities of NBT labeling density. It is not likely that the few heavily stained ones (labeled black in the schematic representation) really belong to a specific subtype, eg. red cones.Their proportion is much less than what one has to expect from psychophysical studies (30-70%).
Besides the problems of maintaining retinal tissue for prolonged incubation times the spectral absorption curves of G- and R- cones are probably too close together and overlap to get signifcant separation of subpopulations.

Recently more promising methods have been be developed (using adaptive optics and digital subtractive densitometry)
to separate B-, G- and R-cones in the all-cone mosaic around the foveola. These images suggest that patterns and ratios of R- and G- cones may indeed vary between local areas as well as between individuals.
See some fascinating samples at the pages of Austin Roorda.

Next page: 4) Evolutionary traits of former photoreceptor diversity in certain mammals