Despite the widespread use of functional magnetic resonance imaging (fMRI) to investigate the neural basis of everything from digestion to facial recognition, scientists have never definitively shown that fMRI measurements directly reflect neural activity.1 But in late July, researchers at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, presented the newest concrete information concerning exactly what fMRI measures.2 The results, achieved through use of a specially designed magnet, suggest that researchers' interpretations have generally been valid, though there are some caveats.

Strictly speaking, fMRI machines measure the blood's oxygen level through a technique called blood oxygenation level-dependent (BOLD) contrast. The BOLD fMRI signal is supposedly related to blood changes, which are supposedly related to local metabolic changes, which are supposedly related to changes at the neuronal population activation level. These activation changes are presumably associated with changes in neural information processing. "Almost all of those links...

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