Slashdot Mirror
This Is Your Brain On Magnets — Or Maybe Not
conspirator23 writes "Jon Hamilton of National Public Radio brings us a story about 'voodoo correlations' in fMRI studies that seek to learn more about emotional states, personality, and social cognition in the human brain. Many of us outside the scientific community have been treated to fascinating images of brain activity and corresponding explanations about how the images reveal which portions of the brain are engaged in certain kinds of thinking. But these images are not actual snapshots; they are visualizations of data generated by repeated scans during experiments. Flaws in the statistical methods used by researchers can result in false images with a variety of inaccuracies. Yet the images produced are so vivid and engaging that even other neuroscientists can be misled by them."
The GP is correct... functional MRI measures blood oxygenation level dependent (BOLD) response - that is, the change in paramagnetism induced by oxygenated hemoglobin. Active neurons require more oxygen than inactive neurons, so oxygenated blood is delivered to them more rapidly (the hemodynamic response). This induces a local shift in magnetic permeability (from paramagnetic to diamagnetic) which can be picked up by the scanner.
Whether the BOLD signal truly correlates well with neural activity is still a matter of contention within the medical community.
I've been lucky enough to work with MR and CT imaging researchers for a while now. One of the benefits of this job is that I've gotten to learn a lot about how these images are acquired and reconstructed. It's not quite as bad as making sausage, but it's a lot more involved than a "snapshot".
For CT, we acquire a bunch of 2D images through you from different angles, then do a lot of number crunching to generate a 3D volume. The problem is that you don't hold still while we're doing it. You can try; you can even hold your breath, but you can't "hold your heart". As your organs move between views, we get motion artifacts -- shape distortion, bright or dark areas, even "things" that aren't really there.
For MR, it's even worse. I can barely tread water in the physics of it, but we're effectively capturing a line at a time in 3D space. (We're actually acquiring data in "k-space", then running it through a Fourier transform to make it spatial.) Not only is it subject to motion artifacts, it's also subject to susceptibility artifacts (distortions because of the magnetic properties of certain materials), flow artifacts (blood moves through vessels between the time that we apply a magnetic pulse and the time that we read back emitted signals), and lots of other things.
fMRI is just adding yet another layer of aggregation and interpretation on top of all this. Sure, it's a "visualization of data generated by repeated scans", but so is every CT or MRI image.
3D imaging, especially MRI, is hideously complicated and indirect. It's almost inconceivable that it could yield results with any physical significance.
...and yet, it does. It's become so routine, so reliable, so well-understood and well-controlled, that doctors and researchers know they can rely on it as a matter of course. They still have to be aware of the errors and distortions that can arise, but that's true of every imaging or monitoring system, all the way down to the stethoscope and the fever thermometer.
Whoever tagged this story 'correlationisnotcausation' is a fucking idiot. You're not as smart as you think you are.