You are searching for the neural correlates of consciousness. The challenge is not simply to find the hot spots in the brain that are active during certain mental activities, but actually to explain how neurons and synapses and chemical circuits generate conscious experiences. This is more than mapping correlations between brain activities and what people say they're thinking about, which we can already see to some degree with fMRI scans. Isn't the real challenge to explain how specific neural correlates are wired together to generate conscious experiences?

Koch: The challenge is to see why this activity that's in your visual brain gives rise to a picture of a cup of coffee in front of me, and other neural activity in the olfactory part of my brain gives rise to the smell of freshly brewed coffee. In both cases, neurons are firing. They look the same. They release a bunch of chemicals like sodium and potassium. Somewhere else, a similar neural activity gives rise to pain. But in another part of the brain, the cerebellum, similar neural activity doesn't give rise to anything. The cerebellum, which is like a little brain at the back of the cortex, contains three-quarters of all the cells in your brain, roughly 69 billion. If you lose the cerebellum, you won't be a ballet dancer or a rock climber anymore. You'll walk with a funny gait. You'll talk very strangely. But your consciousness is only mildly impaired, if at all. So that implies that some types of neural activity are privileged. Certain parts of the brain seem to have a much closer association with consciousness than others. We have to understand what type of activity, what type of circuit, gives rise to conscious sensation.

I know a lot of neuroscientists get very excited about fMRI, but it sounds like the current state of technology is actually a fairly blunt instrument for trying to figure out what's going on in the brain.

Koch: It's a wonderful instrument because it allows us to peer inside your brain safely, but it's also very blunt. Remember, each cubic millimeter of the brain has a hundred thousand cells. So if I take the volume of brain that would be the size of a grain of rice, it contains about a half a million different nerve cells, and maybe seven miles of cable. It's very, very complicated. Yet in fMRI, all you can see clearly is this one box as a single point. But we know this one point contains half a million neurons, some of which may fire, but most of which don't fire. Some might fire less, some might fire more or fire in a very complicated pattern. But fMRI can't see any of that. It can just see the part of the brain that's active or the part that's suppressed. So fMRI is a wonderful tool, but it is very, very crude.

We are only now beginning to realize the complexity of it all. Over the last ten years, we realized there might be a thousand different cell types. They all have highly specialized jobs to do. They look different. They have different chemicals. They use different neurotransmitters. They have different genes encoding for them. They are wired up in a very distinct manner. So it's a little bit like having 86 billion tiles, but they come in sets of a thousand different colors and shapes. They all fit in very specific ways. And now, the big challenge is trying to unravel all of that and see the amazingly specific way that tile number 55 only talks to tile 255 and 972. And it inhibits tile number 17 and 31, and it doesn't talk to any of the other ones. That's what it's beginning to look like. So how does anxiety or sadness or the color blue emerge out of all of that?