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 syndrome" in honour of Alice who actually walked into the mirror thinking it was a real world. Why does it happen?

Well I think what happens is this patient's brain is saying, speaking metaphorically, look that's a mirror, I know it's a mirror, that's a reflection, therefore the object is on my left but left doesn't exist in my universe. Therefore the object must be inside the mirror, however absurd it seems to all of you chaps, and therefore I'll reach into the mirror, bang, bang, bang. All of that abstract knowledge about the laws of optics and mirrors is now distorted to accommodate this strange new sensory world that the patient finds herself trapped in.

Now I'll turn to another disorder which is also caused by damage to the right parietal and that is even more extraordinary. It's called denial or . Remember most of these patients with right parietal damage also have some damage to the internal capsule so they are completely paralyzed on the left side of the body. It's what you mean by a stroke, this complete paralysis of the left side of the body, and most of them complain about this as indeed they should. They say when am I going to get better, my arm doesn't work. But a subset of them, a small percentage of them will vehemently deny that their left arm is paralyzed, and these are patients who don't have any neglect. They'll say doctor, it's moving fine. Why does this happen?

It's not clear but it is only seen when the right parietal is damaged, rarely seen when the left parietal is damaged and that gives out a clue. It tells you that the denial syndrome has something to do with hemispheric specialization. The manner in which the two cerebral hemispheres deal with the external world, especially the manner in which they deal with discrepancies in sensory input and discrepancies in beliefs. Specifically I would like to suggest when confronted with a discrepancy, the left hemisphere's coping style is to smooth over the discrepancy, pretend it doesn't exist and forge ahead. The right hemisphere's coping style is the exact opposite. It's highly sensitive to discrepancies so I call it the anomaly detector.

Now imagine a patient with a right hemisphere stroke left side paralyzed. The patient is sending a command to move the arm, he is getting a visual signal saying it is not moving so there is a discrepancy. His right hemisphere is damaged, his left hemisphere goes about its job of denial and confabulation smoothing over the discrepancy and saying, all is fine, don't worry. On the other hand, if the left hemisphere is damaged and the right side is paralyzed the right hemisphere is functioning fine, notices the discrepancy between the motor command and the lack of visual feedback and says, my god you are paralyzed. This was an outlandish idea but it's now been tested with brain imaging experiments and shown to be essentially correct.

Now this syndrome is quite bizarre - a person denying that he or she is paralyzed - but what we found about seven or eight years ago something even more amazing. Some of these patients will deny that another patient is paralyzed so the patient is sitting here. We saw the patient, another patient sitting in a wheelchair - I'll call him patient B - and I've told patient B move your arm. Patient B of course is paralyzed, doesn't move. And then I ask my patient - is that patient moving his arm? And the patient says yes of course he is moving his arm. He is engaging in denial of other people's disabilities.