REGIONS LIGHT UP with extra oxygen when a subject moves fingers (yellow) or toes (red). Researchers have now verified that this MRI signal reflects active neurons, not just enhanced blood flow. Image: SEONG-GI KIM and KAMIL UGURBIL University of Minnesota
We've all seen the images: a grainy picture of the brain's contours with one or two areas lit up, supposedly indicating the regions that are active while the subject carries out a specific task. First developed about a decade ago, functional magnetic resonance imaging (fMRI) has become the leading research tool for mapping brain activity. The technique works by detecting the levels of oxygen in the blood, point by point, throughout the brain. Until now, however, there has been no proof that those oxygen levels truly correspond to neurons getting busy. Researchers at the Max Planck Institute for Biological Cybernetics in T¿bingen, Germany, have now supplied that proof, and in addition they have shown that the fMRI signal largely comes about when neurons are receiving input and depends less on whether they are sending out signals.
The T¿bingen group, led by Nikos K. Logothetis, monitored the electrical activity of neurons directly through implanted electrodes while simultaneously taking fMRI scans. That is no easy task: MRI uses pulses of radio waves and a very strong, changing magnetic field, both of which interfere severely with nearby circuitry. The group built special devices to sense and compensate for some of the interference; computer processing filtered out what remained.
This article was originally published with the title Magnetic Revelations.