Functional MRI is one of the most popular ways of measuring human brain activity. But what is "activity"?
Fundamentally, neural activity is electical potentials and chemical signals. fMRI doesn't measure these directly. Rather, it measures changes in the oxygen content of blood in different parts of the brain.
The more the brain cells are firing, the more oxygen they use up, although oxygenation actually increases as a kind of compensation for the activity and this increase is what gets measured. The oxygenation changes associated with neural firing is called the BOLD response.
Using fMRI you can measure BOLD and end up with some pretty blobs of activation. But what does it mean for a region of the brain to be activated? Just as no man is an island, no brain region can do anything on its own. Every area gets inputs from other areas, and sends outputs as well.
So if an area gets more active, that could mean one or more of three things:
It's sending more outputs
It's getting more inputs
It's doing more "internal" processing within that area - "talking to itself".
Which of these contributes to BOLD? It's known that number 1 - output from the area in question - is not a major contributor to the fMRI signal, but what about 2 and 3?
argues that 80% of the BOLD signal is caused by internal processing, and only 20% is due to input.
They took some rats, and stimulated their whiskers. Using electrodes, they measured blood oxygenation changes in an area called the barrel cortex, which is known to deal with whisker-based sensations (they didn't actually use fMRI, but this would be seen as a BOLD signal if they had.)
But they then added a drug called muscimol to the barrel cortex. Muscimol reduces neuronal firing, but it doesn't affect synaptic input. They show that muscimol strongly reduced the blood oxygenation response, by about 80%. This suggests that 80% of the signal was not caused directly by sensory input to the cortex, but was generated within the cortex.
In many ways this is not surprising: it would be weird if the cortex were just picking up signals and doing nothing with them. However, it's good to be able to put a figure on just how much intra-cortical processing contributes to the fMRI signal. In rats, at any rate.
Harris S, Jones M, Zheng Y, & Berwick J (2010). Does neural input or processing play a greater role in the magnitude of neuroimaging signals? Frontiers in neuroenergetics, 2 PMID: 20740075