>> Measuring Brain Activity: fMRI

	Interview with Mark D'Esposito, University of California, Berkeley From Studying The Mind, VHS © 2003, W. W. Norton

How does fMRI measure brain activity?

It's been known for over a hundred years that neural activity and blood flow are tightly correlated. In other words, when you increase neural activity there's a subsequent increase in blood flow. It's also been known that there is probably some measurable correlation between behavior, blood flow and neural activity. However, it's important to remember that functional MRI is a measure of blood flow and not a direct measure of neural activity. So it's an indirect measure of neural activity, but we believe that it directly relates.

Only about ten years ago it was discovered that you can measure blood flow non-invasively using MRI, magnetic resonance imaging. And magnetic resonance imaging had been around for at least ten years prior to that, but it was used to get pictures of the brain in a static state, to look at the structure of the brain to identify anatomy and pathology, but not to study the brain in a dynamic state. MRI was a major advancement because it led to a method that was noninvasive—you didn't have to inject anything—and the great discovery was that changes in blood flow or blood oxygenation could be detected by scanning very rapidly.

The basic idea is fairly simple: oxygen is carried by hemoglobin, which exists in either an oxygenated state or a deoxygenated state. Hemoglobin, because of its iron content, has magnetic qualities—it acts like a little magnet. And it was discovered that by scanning very rapidly you could detect changes in oxygenation. So if you have a subject do some type of cognitive task, there will be an increase in neural activity and an increase of blood flow to that area of the brain. That increase in blood flow will lead to an increase in the amount of oxygen that's being delivered to the tissue, which will go from showing relatively equal amounts of oxygenated and deoxygenated hemoglobin to showing a lot of oxygenated hemoglobin. What the MRI scanner detects is that change in oxygenation. So the inference that we draw when we see an area with increased blood flow is that it is an area of increased neural activity. And for all the experiments that have been done over the last ten years, that assumption holds up pretty well.