Now there are several ways of studying the brain but my approach is to look at people who have had some sort of damage to a small part of the brain, or some change in a small part of the brain, and interestingly when you look at these people who have had a small lesion in a specific part of the brain, what you see is not an across-the-board reduction in all their cognitive capacities, not a blunting of their mind. What you see is often a highly selective loss of one specific function with other functions being preserved intact, and this gives you some confidence in asserting that that part of the brain is somehow involved in mediating that function.

You are going to see many examples of this in my lectures but just to give you a flavour for this kind of research, I'm just going to mention two or three of my favourite examples.

One of my favourites is prosopognosia, or face blindness. When there is damage to a structure called the fusiform gyrus in the temporal lobes on both sides of the brain, what you get is a patient who can no longer recognise people's faces. Now the patient isn't blind because he can still read a book and he's not psychotic or obviously mentally disturbed but he can no longer recognise faces by looking at people. Now that syndrome is very well known but there is another syndrome that is quite rare and that's what we call the , and I'll give you an example of this. A patient I saw not long ago who had been in a car accident, had sustained a head injury and was in a coma for about a couple of weeks. Then he came out of this coma and he was quite intact neurologically when I examined him. But he had one profound delusion - he would look at his mother and say "Doctor, this woman looks exactly like my mother but she isn't, she is an imposter".

Now why would this happen? Now the important thing is this patient who I will call David is completely intact in other respects. Now to understand this disorder, you have to first realise that vision is not a simple process. When you open your eyes in the morning, it's all out there in front of you. It's easy to assume that it's effortless and instantaneous but in fact you have this distorted upside down image in your retina exciting the photoreceptors and the messages then go through the optic nerve to the brain and then they are analysed in thirty different visual areas, in the back of your brain. And then you finally after analysing all the individual features, you identify what you're looking at. Is it your mother, is it a snake, is it a pig, what is it? And that process of identification takes place in a place which we call the fusiform gyrus which as we have seen is damaged in patients with face blindness or prosopognosia.

So once the image is recognised, then the message goes to a structure called the which is sometimes called the gateway to the which is the emotional core of your brain, which allows you to gauge the emotional significance of what you are looking at. Is this a predator, is it a lion or a tiger? Is it a prey which I can chase? Is it a mate that I can chase? Or is it my departmental chairman I have to worry about, or is it a stranger who is not important to me, or something utterly trivial like a piece of driftwood? What is it?

Now what's happened in this patient? What we suggest is that maybe what's gone wrong is that the fusiform gyrus and all the visual areas are completely normal in this patient. That's why when he looks at his mother, he says "oh yeah, it looks like my mother", but the wire, to put it crudely, the wire that goes from the amygdala to the limbic system, to the emotional centres, is cut by the accident. So he looks at his mother and he says - "hey, it looks just like my mother, but if it's my mother why is it I don't experience this warm glow of affection (or terror, as the case may be). There's something strange here, this can't possibly be my mother, it's some other strange woman pretending to be my mother". It's the only interpretation that makes sense to his brain given the peculiar disconnection.

Now how do you test an outlandish idea like this? My student Bill Hirstein and I in La Jolla, and Haydn Ellis and Andrew Young here in England, did some very simple experiments measuring galvanic skin response which I'll talk about in detail in my last lecture, and we found - sure enough - there has been this disconnection between vision and emotion as predicted by our theory, just as we had thought. Now what's even more amazing is when David, this patient who when his mother said she's an imposter, an hour later his mother phones him and he picks up the phone and answers the phone and he says "mum, how are you, where are you?" Instantly he recognises her. There is no delusion. An hour later the mother walks into the room and he says "who are you? You look just like my mother but you're an imposter, you're not my mother". Now why does this happen? Well it turns out there's a separate pathway going from the auditory cortex in the superior temporal gyrus to the amygdala, and that pathway perhaps is not cut by the accident. That's why when he listens to his mother on the phone, he says "oh my god, this is my mum, where are you?" But when he sees her, the delusion kicks in immediately and he says "who are you"?

Now, there are many other twists to this story which I'm going to tell you about in my last lecture on neuropsychiatry, but I thought I'd just mention it briefly today because it's a lovely example of the sort of thing we do, of cognitive neuroscience in action.

Now we have talked about visual response to visual images, your emotional response to visual images. Obviously this response is vital for your survival but the existence of connections between visual brain centres and the limbic system or emotional core of the brain also raises another interesting question, and that is what is art? How does the brain response to beauty? Given that these connections are between vision and emotion, and art involves an aesthetic emotional response to visual images, surely these connections must be involved, and this is a topic I'll take up my lecture in Birmingham.

Now we've been talking about all these intricate connections in the brain, in the limbic system, in the visual centres, in the amygdala. An obvious question is the question of nature versus nurture. In other words, are these connections laid down by the genome in the foetus, or are they acquired in early infancy as the infant interacts with the world, the so-called nature/nurture debate. This takes me to the next syndrome I'd like to talk to you about and that is .

Everyone here knows what a phantom limb is. A patient has an arm amputated because there's a tumour, malignant tumour on the arm or there's been a car accident and the arm has to be amputated, but the patient continues to vividly feel the presence of that arm. Some of you here, this being England, would now about Lord Nelson who vividly felt a phantom arm. I'll tell you about an experiment we did on these patients. So we have a patient with a phantom left arm. His arm had been amputated above the left elbow so I had him sitting in my office blindfolded and I took a Q tip and touched different parts of the body and asked him what do you feel? I touched his shoulder and he said oh you're touching my shoulder. I touched his belly and he said oh you're touching my belly. I touched his chest and he said you're touching my chest - not surprising. But the amazing thing is when I touched his face, the left side of his face - remember his left arm is amputated so he has a phantom on the left side - when I touched his cheek he said oh my god doctor, you're touching my left thumb, my missing phantom thumb and he seemed as surprised as I was. Then I touched him on the upper lip and he said oh my god you're touching my phantom index finger, and then on his lower jaw and he said you're touching my phantom pinkie, my little finger.