Monday, December 13, 2004


The Manawatu Standard this evening had a story (also on about a man who displayed a "sixth sense" because, while completely blind (in the sense that his visual cortices had been destroyed by a stroke), he showed better than chance probability of guessing emotions in pictures of human faces:

When researchers from the University of Wales showed the man images of shapes such as circles and squares, he could only guess what they were, and had a similar lack of success determining the gender of emotionless male and female faces.

But when presented with angry or happy human faces, his accuracy improved to 59 per cent, significantly better than what would be expected by random chance, with similar results for distinguishing between sad and happy or fearful and happy faces.

Newspaper reports are full of breathless talk of a "sixth sense", but this is nothing of the sort. It's a phenomenon called "blindsight", and it has been known to researchers for almost thirty years. It occurs in some people where there has been damage to the visual cortex, but not the eyes or optic nerve, and leads to an uncanny ability to "guess" some things which are going on in the blinded part of the field of vision. Some blindsighted people can "guess" the location of a moving dot of light, even though they cannot consciously see it; others can "guess" shapes. And the reason for this is perfectly obvious: as noted by Dan Dennett in Consciousness Explained

There are at least ten different pathways between the retina and the rest of the brain, so even if the occipital [visual] cortex is destroyed, there are still plenty of communication channels over which the information from the perfectly normal retinas could reach other brain areas.

In this particular case, information is getting through to the part of the brain which distinguishes emotions in others - so despite the fact that Patient X is blind, he can sometimes register people's emotions solely from visual cues.

The authors of the original paper (Discriminating emotional faces without primary visual cortices involves the right amygdala) are of course completely aware of this - but I guess the journalists reporting on this skimmed the preamble to focus on the "mysterious" results. It seems Patient X is not the only blind person in this story...