BOLD Signals Do Not Always Reflect Neural Activity

Each year, thousands of publications present functional magnetic resonance imaging (fMRI) data that suggest that a particular brain region is active during a particular cognitive task.  Casual readers of such papers might forget that this technique does not actually measure neural activity, but rather blood oxygenation level-dependent (BOLD) contrasts.  Synaptic transmission requires large energy expenditures, and increased energy metabolism has been hypothesized to act directly on blood vessels to increase blood flow and alter BOLD signals.  This week, however, Devor et al.  Report that this hypothesis is not always correct.  As expected, stimulating the forepaw of rats increased blood oxygenation, vessel diameter, glucose uptake, spiking, and synaptic release in the contralateral primary somatosensory cortex.  In the ipsilateral cortex, however, neural activity and glucose uptake increased, but blood oxygenation and blood flow did not.  These results indicate that blood flow is not directly tied to metabolism, and BOLD signals do not always reflect neural activity.

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