Wednesday, July 20, 2011

Blindsight and Consciousness In The Brain

A new paper claims to show the neural activity associated with consciously seeing something:
You might think it would be easy to find the neural correlates of seeing stuff. Just pop someone in the scanner and show them a picture.

However, it's not that simple, because that wouldn't tell you which brain activations were associated with concious awareness as such, as opposed to all of the other things that happen when we see a picture, many of which may be unconscious.

The new paper makes use of a patient, "GY", who has what's known as blindsight, a mysterious phenomenon caused by damage to the primary visual cortex on one side of the brain. In GY's case this was caused by head trauma at age 8. He's now 52, and is unable to see anything on the right side of his visual field. He only sees half the world.

However, he is still able to respond to some kinds of visual stimuli on the right, as if he could see them. But he reports that he doesn't. Blindsight is a rare phenomenon but one that's been extensively studied, because of its obvious scientific and indeed philosophical interest.

In this study the authors used fMRI to try to work out the neural correlates of concious awareness as opposed to unconcious responses. They showed GY a set of horizontal and vertical bars. His task was to say whether the horizontal bars were on top or not.

The stimuli were shown on either the left or the right. The trick was that they set it up such that it was equally easy in either the "good" or the "blind" side of the brain. In order to do that, they had to make the contrast of the bars much less bright on the "good" side.

What happened? As expected, behavioural performace was equal whether the stimuli were on the left or the right. GY got the judgement right about 75% of the time.

However, his brain responded much more strongly to stimuli on the good side - stimuli that were consciously perceived. Activations appeared all over the cerebral cortex in the occipital, parietal and frontal lobes, as you can see in the pic at the top.

The only area more activated by the unconscious stimuli was a tiny blob in the amygdala.

So what does this show? Is it "the neural correlates of conscious awareness", that Holy Grail of neuro-philosophers?

Maybe. It's a clever experimental design, which rules out some alternative explanations. It's hard to argue that the conciously perceived stimuli were just stronger, and hence more likely to affect the brain. They were actually much fainter.

And it's hard to argue that this represents subconscious information processing, or the process of making the decision whether the horizontal bars were top or bottom, because that was also going on in the blind condition and performance was the same.

Yet my concern is that the main route by which visual information gets into the cortex from the eyes, is via V1, the part which was damaged on one side. So in a sense it's no surprise at all that the cortex was more activated in the conscious condition.

Maybe this is the whole point - maybe this study shows us that consciousness is to do with cortical processing. However, when you put it like that, it seems a bit of an anticlimax. I don't think anyone would seriously dispute that. The cortex does almost everything. The interesting debates are about where in the cortex consciousness happens, if indeed it's localized at all, and what kind of processing underlies it.

It's unlikely that all of the activated areas were directly linked to conscious awareness. But we don't know which of them were.

ResearchBlogging.orgPersaud, N., Davidson, M., Maniscalco, B., Mobbs, D., Passingham, R., Cowey, A., & Lau, H. (2011). Awareness-related activity in prefrontal and parietal cortices in blindsight reflects more than superior visual performance NeuroImage DOI: 10.1016/j.neuroimage.2011.06.081

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