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Chapter 1 endnote 37 & 39, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

[note 37] ...a phrase like "increased brain activity" is a simplification. Scientifically speaking, brain imaging (specifically functional magnetic resonance imaging, or fMRI) measures changes in magnetic fields, which come from changes in blood flow, which are themselves linked to changes in neural activity. [I] speak of increases and decreases in “activity” as a convenient shorthand.

[note 39] fMRI [can] peer harmlessly into the heads of living people who are experiencing emotion or perceiving emotion in others, recording the changes in magnetic signals related to firing neurons.

Functional magnetic resonance imaging (fMRI) takes place in a special room with a very large, very powerful magnet (whose strength is measured in units called the "tesla" after Nikola Tesla). To begin a brain scanning session with fMRI, you are stripped of all metal objects, laid on your back with a magnetic coil around your head, and slid into the middle of the magnet (called the "bore" of the magnet). Some people find it a bit claustrophobic, but most people manage just fine.

Regular MRI works like a camera to take a snapshot of your brain’s anatomy, but fMRI is more like a video, detecting and measuring changes in neural activity as you think, feel, or perceive. More accurately, fMRI measures small magnetic fluctuations that are related to blood flow changes that occur with neural activity. The magnetic signals come from the concentration of hemoglobin in red blood cells that are not carrying oxygen molecules (i.e., deoxyhemoglobin). Deoxyhemglobin in your brain’s veins acts like a little magnet that can disrupt the big magnetic field of the scanning magnet (because iron in the hemoglobin molecules is exposed); the amount of oxygenated blood in your brain’s arteries dilutes the magnetic effect and it is this relative amount of deoxyhemoglobin that allows the magnet to record relative blood flow and track neural firing.[1] It’s simpler just to say “neural activity.”

So, fMRI measures the magnetic fields associated with blood flow changes in the brain. Neurons fire, oxygen is used, blood flows to bring more oxygen, and magnetic fields subtly shift and are magnified and measured as people experience emotions or perceive others’ emotions. Then we take those fluctuations, process the hell out of it to determine the signal that is embedded in a whole lot of noise, and generate pretty pictures of brains with colored blobs on them.

It’s important to realize that the maps of colored blobs you look at are not maps of neural activity. They are not even maps of magnetic signal. They are maps of statistics that indicate the locations of significant change in magnetic signal relative to some baseline or control condition. Scientists choose the colors and decide how stringent or lenient they want to be when deciding “statistical significance,” usually according to standard scientific rules of thumb or practices. Thus, brains with colored blobs on them are curated representations of brain activity with a heavy dose of interpretation at many stages along the way.

Notes on the Notes

  1. Raichle, Marcus E. 2009. "A brief history of human brain mapping." Trends in Neurosciences, 32 (2): 118-126.