Saturday, March 15, 2008

The colour vision of the blind

In my book, A Vision of the Brain, I described a strange syndrome which I named phantom chromatopsia. In all, I have seen four patients suffering from it and have studied two in detail. The syndrome is one in which blind people see colours, usually purple or golden. The colour spreads and fills their entire “field of view”. But they take no pleasure in the sensation. The experience plunges them into a state of deep depression. One patient told me that he often felt suicidal during the chromatopsic episodes.

I accounted for the syndrome by reference to the known organization of the human visual brain. Of the many areas that constitute the visual brain, one – the V4 complex – seemed especially interesting in this context. Described by us many years ago, it is specialized for colour perception and total damage to it leads to the inability to see the world in colour – the syndrome of acquired cerebral achromatopsia. I accounted for phantom chromatopsia by supposing that an abnormal pattern of cellular activity restricted to the V4 complex results in the generation of a colour percept in the absence of an external, coloured, stimulus. The abnormality of the percept – large uniform areas of purple or gold – could be accounted for by the abnormal nature of the internally generated pattern of cellular activity compared to the normal one generated by a coloured object in the field of view.

A very interesting recent result obtained by Dr. Beauchamp and his colleagues at the University of Texas takes this a step further. They found that when they stimulated part of the colour centre through electrodes embedded in the area, the patient reported seeing colours which were not there. And what was the colour? bluish purple!

Still, the correspondence between these new results and the clinical syndrome is not complete, nor would one expect it to be, given that a pathological irritation in the V4 complex is different from a controlled stimulation of only a part of this area. The coloured area projected to the field of view in this new study is limited, whereas the colour invades the whole field of view in the pathological state. Moreover, the subjective colour produced by electrical stimulation was always purple-blue. The authors account for this by supposing that their electrode was stimulating a group of cells specialized for blue – a reasonable interpretation in light of the fact that cells constructing particular colours seem to be grouped together in the V4 complex.

But the main interest of the finding lies in showing that artificial stimulation can result in perceived colour in the absence of a coloured stimulus. This adds further to the evidence that colours are generated in the brain, that the brain does not passively chronicle the colours in the external world but actively constructs them. Isaac Newton saw this long ago when he wrote, “For the Rays, to speak properly, have no Colour. In them there is nothing else than a certain power and disposition to stir up a sensation of this Colour or that” – the power and disposition residing, I believe, within the V4 complex. Edwin Land also put it succinctly – “Colour is always a consequence, never a cause” – meaning that it is the consequence of some activity in the brain (though he did not specify where that activity might occur).

This highly interesting study gives powerful evidence in favour of these suppositions.


MICHAEL said...

Dear Professor Zeki,
Thank you for your insightful comments on our manuscript.
Another interesting point to consider is that electrical stimulation of early brain areas almost always produces a visual percept (typically described as similar to a small flash of light) while electrical stimulation of later brain areas does not usually produce a visual percept. It is not clear if the color center is an exception to this general rule.
Michael Beauchamp

Lennie said...
This comment has been removed by the author.
LinYu Wang said...

One study investigating the neural correlates of face perception also used similar methods....FYI

Abigail M said...

Dear Professor Zeki,

I am curious to know, if (blind people),the phantom chromatopsia patients have any knowledge of the colours that other people exprience?. I was wondering how they could describe or how you could estimate that the colours they perceive are purple and gold.

Professor Zeki said...

Phantom chromatopsia occurs in patients who have become blind because of retinal damage. Previous to that damage, these patients had good vision, including good colour vision. They therefore had experienced colour. Hence when they describe their experience of "golden" or "purple" colours, one is inclined to believe them. There is no means of knowing with certainty that the colours they experience are identical to the ones that a "normal" person experiences. But of course there is no means of being sure that the colour that one person experiences is identical to the colour that another person experiences either. I should have mentioned that, in the examples I have encountered, the phantom episodes are not continuous but occur sporadically. Their appearance causes the patients much "pain" and great distress.

Poornima said...


Once you learn a new word, you end up coming across that word almost everywhere, similarly, after reading your blog, I have come across color purple and especially the combination of purple and gold being used by many poets. Do you think this is just a coincidence, or that there may be more to it? Here is a list of such poems in my blog :
Abigail M

Professor Zeki said...

Difficult to tell. Maybe it is just a coincidence? But it is worth confirming, to see if this is a regular occurence, in which case it is probably not so much of a coincidence. SZ

Giordano Bruno said...

Prof. Zeki,
does the hemisphere type (left/right) of the V4 area involved, have any bearings on the appearance of the color phantoms?
Alfredo Restrepo

Professor Zeki said...

Not to my knowledge. SZ