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.