How we recognise faces

More than a third of the brain in a human is devoted just to decoding what we see, and although brain scans can show us the parts of the brain that switch on when we look at certain things, they can't tell us what tasks those active areas are actually carrying out. But, very rarely, an opportunity arises for scientists to literally get inside the human mind. Chris Smith heard how this happened recently to neuroscientist Nancy Kanwisher…

Nancy - I’ve been studying a region on the bottom of the back of the right hemisphere for about 20 years - it’s called the fusiform face area. It’s a little region that’s about the size of a penny and it responds quite strongly when you look at faces and we’ve known that for quite a while. The question is: what does it do with faces and does it also play a little bit of role in the perception of other things that aren't faces?

What I wanted to know is when that region of the brain is stimulated and a person is looking at something that isn’t a face, does it affect their perception of that other thing? We can’t do things to people’s brains. That’s not ethical, so what we have to do when we study humans is wait for clinical opportunities where a neurosurgeon is going to do something for clinical reasons.

This patient had intractable epilepsy and you can’t live a normal life that way. In these situations, what neurosurgeons do is first map out the brain, find out the focus of the epilepsy seizures, and take it out surgically. So the neurosurgeons placed tiny electrodes all over the surface of his brain, including a lot of electrodes right in an area I happen to be interested in. It was an opportunity to find out what happens when the patient looks at different things and when those electrodes are stimulated electrically.

Chris - Right. So you’re both eavesdropping on what’s going on in those areas natively and you can also then reverse the equation and put electricity, and therefore stimulus, into those areas and ask well, if I change the activity in that area, what does it do to the patient’s experience?

Nancy - Exactly. So we gave the neurosurgeons a bunch of pictures of faces, and words, and objects, and different kinds of things, both in colour and in grayscale. We said “please show these images to the patient and we think we’ll be able to find out what exactly each electrode responds to.” And, sure enough, there were ten electrodes right next to each other that responded nearly exclusively to faces.

Chris - What happened when you then stimulated those areas, so you know I’m recording from this particular area underneath at the back of the brain, and I’m seeing it becoming very excitable when this person looks at faces? But when you then put energy into that area, what does the patient see?

Nancy - What the person saw was he saw a face on top of whatever he was looking at. So, when he was looking at a box, he saw a face on top of the box. When he was looking at this orange soccer ball, he saw a face on top of the soccer ball. He reported that the face looked like an anime character; it wasn’t a person familiar to him. Now that may be because when you electrically stimulate the brain, you are essentially activating tens of thousands of neurons in that region. It may be that when you do that, what that codes for is a very general presence of a face, not a particular face which might require activating a subset of those neurons.

Chris - What is your interpretation: the fact that you see the face on top of the object you were looking at, and the object itself is not changed in your perception when you do this?

Nancy - My interpretation is that region is only involved in face perception. Not even a little bit in the perception of objects. So this is the strongest possible evidence that the kinds of minds we have are these very specialised minds that have special purpose machinery for solving very particular problems.

Chris - When the visual system is looking at things, can one sort of summarise by saying well, if you look at something and there is a face somewhere in it, then the right bit of the brain extracts the face shape, tell your consciousness ah, that must be a face by activating this area you’ve been looking at, so it’s like a trip switch? When you’re area you're interested in goes active that says the the brain yep, that’s a face?

Nancy - Exactly.

Chris - Does your findings suggest then that if you had done the opposite of stimulating that area and you deactivated it, that this person wouldn’t be able to recognise a fact for what it was; they would just see a shape like any other shape?

Nancy - Exactly. I think what would happen is the patient would probably know that the face was a face, but probably would not know which face it was. Actually, we didn’t have time or the neurosurgeons to test that in this patient, but my guess is that when the patient was looking at a face, if we had asked him which person is that while stimulating that region he would probably be disrupted with that function. He’s probably know it was a face, but not know which face either if you deactivated the region or if you stimulate a lot of neurons in that region. Both of those things are kinds of disruption that interfere with processing in that region.