The Emerging Mind: The Reith Lectures 2003

called the new pathway. The new pathway in the cortex is doing most of what we usually think of as vision, like recognizing objects, consciously. The old pathway, on the other hand, is involved in locating objects in the visual field, so that you can orient to it, swivel your eyeballs towards it, rotate your head towards it. Thereby directing your high acuity central foveal region of the retina towards the object so then you can deploy the new visual pathway and then proceed to identify what the object is and then generate the appropriate behaviour for that object.

Let me now tell you now about an extraordinary neurological syndrome called Blindsight discovered by Larry Weiscrantz and Alan Cowey at Oxford. It's been known for more than a century that if the visual cortex which is part of the new visual pathway, if that's damaged you become blind. For example if the right visual cortex is damaged you're completely blind on the left side if you look straight everything to the left side of your nose, you're completely blind to.

When examining a patient named GY who had this type of visual deficit, one half of the visual field completely missing, where he was blind, Weizcrantz noticed something really strange. He showed the patient a little spot of light in the Blind region. Weiscrantz asked him 'what do you see'? The patient said 'nothing' and that's what you would expect given that he was blind but now he told the patient 'I know you can't see it but please reach out and touch it' The patient said well that's very strange - he must have thought this is a very eccentric request. I mean, point to this thing which he can't see.

So the patient said, you know I can't, I can't see it how can I point to it? Weiscrantz said well just try anyway, take a guess. The patient then reaches out to touch the object and imagine the researcher's surprise when the patient reaches out and points to it accurately, points to the dot that he cannot consciously perceive. After hundreds of trials it became obvious that he could point accurately on 99% of trials even though he claimed on each trial that he was just guessing. He said he didn't know if he was getting it right or not. From his point of view it might as well have been an experiment on ESP. The staggering implication of this is that the patient was accurately able to point to an object that he denied being able to see. How is this possible? How do you explain his ability to infer the location of an invisible object and point to it accurately?

The answer is obvious. As I said GY has damage to his visual cortex - the new pathway - which is why he is blind. But remember he still has the other old pathway, the other pathway going through his brain stem and superior colliculus as a back-up. So even though the message from the eyes and optic nerves doesn't reach the visual cortex, given that the visual cortex is damaged, they take the parallel route to the superior colliculus which allows him to locate the object in space and the message then gets relayed to higher brain centres in the parietal lobes that guide the hand movement accurately to point to the invisible object! It's as if even though GY the person, the human being is oblivious to what's going on, there's another unconscious zombie trapped in him who can guide the hand movement with uncanny accuracy.

This explanation suggests that only the new pathway is conscious - events in the old pathway, going though the colliculus and guiding the hand movement can occur without you the person being conscious of it! Why? Why should one pathway alone or its computational style perhaps lead to conscious awareness, whereas neurons in a parallel part of the brain, the old pathway can carry out even complex computations without being conscious. Why should any brain event be associated with conscious awareness given the 'existence proof' that the old pathway through the colliculus can do its job perfectly well without being conscious? Why can't the rest of the brain do without consciousness? Why can't it all be blindsight in other words?

We can't answer this question directly yet but as scientists the best we can do is to establish correlations and try and home in on the answer. We can make a list of all brain events that reach consciousness and a list of those brain events that don't. We can then compare the two lists and ask, is there a common denominator in each list that distinguishes it from the other? Is it only certain styles of computation that lead to consciousness? Or perhaps certain anatomical locations that are linked to being conscious? That's a tractable empirical question and once we have tackled that, it might get us closer to answering what the function of consciousness might be, if any, and why it evolved.

Now I should add that the blindsight syndrome in GY seemed so bizarre, when it was first discovered that it was greeted with scepticism and some of my colleagues don't believe that it even exists. Well partly this is because the syndrome is very rare but also partly because it seems to violate common sense. How can you point to something you don't see? But actually that's not a good reason for rejecting it because in a sense we all suffer from blindsight. Now that sounds cryptic so let me explain that.

Imagine you are driving your car and having a lively animated intimate conversation with your friend sitting next to you. Your attention is entirely on the conversation, it's what you're conscious of. But in parallel you are negotiating traffic, avoiding the pavement, avoiding pedestrians, not running red lights and performing all these very complex elaborate computations without being really conscious of any of it unless something strange happens, like you see an actual zebra instead of just a zebra crossing! So in a sense you are not any different from GY all of you here, you have 'blindsight' for driving and negotiating traffic. What we see in GY is simply an especially florid version of blindsight unmasked by disease, but his predicament is not fundamentally different from yours and mine.

Intriguingly you cannot imagine the converse scenario. Paying conscious attention to driving and negotiating traffic while unconsciously having a creative conversation with your friend. This may sound trivial but it is a thought experiment and it is already telling you something valuable, that computations involved in the meaningful use of language require consciousness but those involved in driving, however complicated, don't involve consciousness.

I believe this approach to consciousness will take us a long way toward answering the riddle of what consciousness buys you and why it evolved. My own philosophical position about consciousness accords with the view proposed by the first Reith lecturer, Bertrand Russell, there is no separate 'mind stuff' and 'physical stuff' in the universe, the two are one in the same, the formal term for this is neutral monism.

So we have talked about the messages in the new visual pathway. But now let us turn to the other pathway, the old pathway which goes to the colliculus, mediates blindsight. That projects to the parietal lobe in the sides of the brain. The parietal lobes are concerned with creating a three dimensional representation, a symbolic representation of the special lay- out of the external world so the ability that we call spatial navigation, avoiding bumping into things, dodging a missile that's hurled at you, catching something that's thrown at you all of these abilities depend crucially on the parietal lobes.

Now when the right parietal lobe is damaged, you get a fascinating syndrome, called neglect, in a sense the converse of blindsight. The patient can no longer move his eyes towards the object, which is looming towards him and he can no longer reach out and point to it or grab it. Now bear in mind he isn't blind to events on the left side of the world because if you draw his attention he can see it perfectly clearly and he'll identify it, he'll tell you what it is. So he's not blind. So you can think of neglect, I think the best description of it is, it's an indifference to the left side of the world. That's why you call it neglect.

If he's eating from a plate of food, he'll eat only from the right side of the plate and completely leave the left side of the plate uneaten. Then you draw his attention he will say 'Oh my God, that's a nice avocado' and he'll take it. So when you draw his attention he can see it but otherwise he ignores it. If he is shaving he will only shave from the right side of his face, or if she's a woman only apply make-up on the right side of the face and that looks quite bizarre, as you can imagine.

Now as I said neglect is caused by damage to the right hemisphere. The patient is also usually paralysed on the left side because the right hemisphere of the brain controls the left side of the body. I wondered if it would be possible to 'cure' neglect? Can you treat this patient, making him pay attention to the left side of the world he's ignoring?

So I hit on the idea of using a mirror, as in the case of phantom limbs in my previous lecture. So all I did was I had the patient sitting on a chair and then I stood to the right side of the patient and held a mirror like that so that when the patient rotated his head to the right he would be looking directly into the mirror that I was holding on his right side.

Now the question is, how does he react to this? Obviously he is not neglecting the right side, he's only neglecting the left side of the world so clearly he can see the mirror but he's going to see the reflection of the left side of the world inside the mirror. The question is how is he going to react to that? Well one possibility is he's going to say: 'Hey my God, that is a reflection. There's a whole left side of the world that I have been ignoring, let me pay attention' and turn around and pay attention -- in which case you have cured neglect instantly with a mirror. Or he could say, 'Well look the reflection is on my right side, so the objects are on my left but hey, left doesn't exist in my universe I'm supposed to neglect it so I'll just ignore it' so he ignores the reflection. What happens?

Well what happened actually as often happens in science - neither! OK? I stood on her right, so she is sitting here in her wheelchair I was sitting, standing on her right side, holding up a mirror, she looked inside the mirror, and on her left side was my student standing with a pen, holding a pen. And I asked the patient what do you see? What am I holding? The patient says, oh you're holding a mirror. I said, how do you know? She said well I can see my reflection in and it's cracked on the top -- which is true.

And I said, what do you see in the mirror? She said oh I see your student John, he's holding a pen. I said, OK I would now like you to use your right hand - remember her right hand is not paralysed - to reach out, grab the pen and then write your name on the pad that's on your lap. Now of course you try this on a normal person and actually I have tried it on a normal colleague. You know you just reach, you know you see the reflection in the mirror of the person holding the pen, you turn to the left, grab the pen and write your name on the pad. What did the patient do?

It's absolutely astonishing. The patient looked in the mirror, reaches out into the mirror for the reflection, bang, bang, bang, starts clawing the surface of the mirror, or on some occasions reaches behind the mirror trying to grab the reflection sometime yanking my tie, grabbing my belt buckle, remember I was holding the mirror on the patient's right side. And I said, what are you doing Mrs D, the patient's name?

The patient said, oh I am trying to reaching for the pen. I said, no, no, no I don't mean the reflection, I mean the real pen where is the real pen? The patient says: 'The real pen is inside the mirror, Doctor,' or on another occasion: ' The pen is behind the darn mirror, Doctor.' OK? Now this is absolutely astonishing because we have tried this on a three year old child so the child is sitting here on a chair, you hold the mirror on the right side of the child and you have an assistant holding a candy and you tell the child reach out, reach and grab the candy. The child realizes this is some kind of game, giggles and reaches correctly for the candy on the left side and takes it. Even a chimpanzee can do this, doesn't get confused a mirror image for a real object but the older and wiser Mrs D - seventy years of experience with mirrors - reaches straight into the mirror, bang, bang, bang. Why does this happen? We call it 'mirror agnosia' or ' looking glass