Functional Magnetic Resonance Imaging: Round 2

This is the second installment in a 3-part series (read 1st column here) that discusses some of the mechanisms behind functional magnetic resonance imaging (fMRI) technology. As you may recall, the genesis for this series was reactive…I got mad while sitting on an airplane reading a magazine article about how fMRIs can predict everything from product preferences to political inclination. The article hinted at something I have been noticing with increasing alarm—the confusion about what fMRI can and cannot reveal about information processing in the brain. I decided to write this series hoping that knowledge of the basic science behind fMRI technology could contribute to making more nuanced conclusions about the data it reveals.

Last month, I discussed some of the basic physics behind MRI and described why magnets and radio waves were so important in getting an image. Here I explore how that physics reveals neural activity in the brain. Actually, fMRI does not detect neural activity at all. It only detects changes in blood flow, which may be a source of some of the confusion (more on that in a moment).

To talk about the controversies about what fMRI actually detects (and yes, there are controversies), I will briefly describe the relationship between neural activity and the brain’s hemodynamic properties. I will then move to data that appear to describe the molecular components behind this relationship. Along the way, I will review some basic biochemistry, from glycolysis (remember glycolysis?) to the prostaglandin biosynthetic pathway.

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