Re: Can the view of different colours influence changes in the brain?

Dear Valentina,

Since you are interested about the neuronal effects of viewing colors, you 
probably want to know about how the brain perceive colors. One thing that 
should be clear is that vision requires light and that the latter is 
physical while the other is a perceptual phenomena. In the case of colors, 
one should make the difference between the physical properties of light 
(intensity, wavelenght) and the perceptual colors that we see. You 
probably heard that 3 types of color receptors (cones) exists in the human 
retina and that they are sensitive to blue, green and red colors which in 
turn can be mixed to produce all visible colors. That is only partially 
true. First, we can not state that receptors are « tuned » to different 
colors (perceptual) but to different light wavelenghts (physical). 
Secondly, one should know that cones are actually more or less sensitive 
to a particular RANGE of wavelenghts and that considerable overlaps exists 
between the sensitivity ranges of the different types of cones. 

You probably once see this old experiment : a person have spots of blue, 
green and red light and project them on a white wall. What we see is that 
where spots overlaps, new colors are created and where the 3 spots 
overlaps, it appear white. That is call « additive » color mix. If color 
perception was striclty a question of retina and wavelenght, it would be 
therefore impossible to see something white without having the 3 types of 
wavelenghts. That is not the case. Try the following experiment : 
illuminate a room with only a red light and look at something white: it 
will appear as... white. Thats what we called « color constancy ». 
Intuitively, we can think that an object is perceived as white when it 
reflect the maximum wavelenghts present in the actual environnement.  That 
is, one can see white by actually having its cones stimulated by only one 
part of the spectrum (in our example, the long wavelenght typically 
associated to red) and a strict wavelenght color theory does not hold in 
these circumstances. In a series of elegant demonstrations, E. Land have 
shown that the perceived color of an object can not simply be predicted by 
the wavelenghts reflected by this object without considering the total 
amount of light that is reflected to the eye from the whole visual field 
(i.e. surrounding objects). In other words, the color perceived in a part 
of the visual scene is highly dependent on the remaining visual field. 
This have helped to understand that the colors we perceived are products 
of the brain ; not of the world outside.

The work of E. Land have stimulated many other studies. By these, studies 
of  S. Zeki have shown, using exactly the same appartus used by Land, that 
individual cells in the brain show patterns of activity that may account 
for color constancy. Indeed, these cells (present in a region of the brain 
rich in color-selective neurons named area V4) are activated when a given 
color is displayed, regardeless of the spectral components of light that 
it reflect. That is probably a major difference between retina’s receptors 
(which may be considered as strict wavelenght analysors) and color-
selective brain cells, for which the activity seems to be correlated with 
our actual perception. The way by which these color-selective cells get 
such properties, either in developpement or in the adult brain, is not 
known. As you can see, what is happenning in the brain when we see color 
remains very mysterious. 

I’m afraid that it is very difficult for me to resume the numerous 
experiements that have been done about color and the brain. Therefore, I 
strongly encourage you to read this book :

Zeki, S. A vision of the brain. Blackwell Scientific Publications. (1993)

Finally, feel free to e-mail me directly if you have questions 
(eric.tardif@iphysiol.unil.ch)

Best regards,
Eric (Lausanne University)