fMRI uses a signal that localizes changes in AVERAGE neural activity in 3 mm3 regions.
fMRI has quite a few limitations when it comes to brain mapping. The signal we pick up is not clearly one that reflects firing rate of neurons. The signal is, instead, an average of metabolic demand over a 3 mm3 area. Metabolic demand may come from many sources, including cells in the brain called glia, which don’t communicate over long distances like neurons. Metabolic demand is great at axon terminals where the message gets across the synapse. Both excitation and inhibition use energy. Also, we are measuring activity changes over a very long time period for brain functions: 2 to 20 seconds. Still, we can identify significant functions of groups of cells and actions. We can make a map and identify systems related to behavior that we did not know about before.
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