What Is fMRI?

This blog post is based on the book Brainwashed: The Seductive Appeal of Mindless Neuroscience by Sally Satel and Scott O. Lillenfeld. Please bear with me as this is the first post that I’ve written based on a source viewed on my Kindle.

fMRI is the basis for most of the brain images we see. It stands for Functional Magnetic Magnetic Resonance Imaging. Here is a brief description of how it works. Functional MRI reveals oxygen consumption and regional blood flow in the brain. Blood that is carrying more oxygen has different magnetic properties than blood that has already given up its oxygen to supply neurons. The brain will first be scanned which the participant is resting in the device to establish a baseline level. Then the participant is asked to perform a particular task of interest. The difference between this baseline and the activity when this specific task is being performed is measured and called the BOLD (blood-oxygen level response). The higher the level of oxygenated to deoxygenated blood in a particular area of the brain, the higher the energy consumption in that region. The basic unit of analysis in fMRI is called the voxel, which is a combination of volume and pixel. It is a three dimensional unit.

Subtraction is performed on a voxel by voxel level. Each voxel is then assigned a color depending on the strength of the difference in activation of that individual voxel between the control and experimental conditions. The computer then generates an image highlighting the regions that become more active in one condition relative to the other. By convention, researchers use color gradations to reflect the likelihood that the subtraction was not due to chance. A bright color like yellow might mean that the there is only one chance in a thousand that the differences are due to chance, whereas a darker color like purple might mean that the chances are higher, and that the brain differences were more likely to be attributable to random fluctuations in the data. Finally, the computer filters out background noise and prepares the data to be mapped onto a three-dimensional template of the human brain.

The final brain scan that we see rarely portrays the brain activity of a single person. Instead it almost always represents the averaged results of all participants in the study. Any resemblance between brain scans and photographs is illusory. Photos capture images in real time and space. Functional imaging scans are constructed from information derived from the magnetic properties of blood flowing in the brain. Scans are simply a representation of local activation based on statistical differences in BOLD signals.


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