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Medicine Science

This Is Your Brain On Magnets — Or Maybe Not 59

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."
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This Is Your Brain On Magnets — Or Maybe Not

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  • by AtomicDevice ( 926814 ) on Wednesday July 08, 2009 @11:17AM (#28623767)
    I will say, it would be easy to make wild claims about what areas of the brain "do" things just by looking at a scan and showing a pretty picture.

    That said, consider these things:
    While non-peer-reviewed publications often publish exciting results, the scientific community typically does not accept brain regions without corroboration from many different studies with different stimuli, often including monkey studies where real electrodes and not just low-res fMRI can be used
    It is difficult to get the numbers of subject that would be considered standard in other studies for fMRI studies. First off you actually need subjects who will do the assigned task, then you need them to do it perfectly still, for anywhere between 20 minutes to several hours (usually in no more than 1-hour segments). So the likelihood that just one study could prove something is quite small.
    In many (perhaps most) studies, all the subjects brains are averaged together for data analysis, there are several different ways of doing this, none of them particularly accurate. This again calls attention to the need for multiple studies

    It's also important to actually know what you're looking at when you see pictures of "brain activity", usually you are looking at the averaged activity of many subjects, after it has been run through (most likely) some form of general linear model or event-related analysis. Both of these methods estimate and fit a hemodynamic response function (the pattern of brain response to a stimulus), and what you're actually looking at is the fit or perhaps t-values (roughly fit/std. deviation) for each voxel.

    Also note, that for almost any study, I could pick some random brain areas that are "lighting up" and claim a response, but they would almost certainly be shot down with more subjects, another study, etc.

    bottom line, responsible investigators can make good sense out of fMRI data, but doing one experiment and claiming you "found the love [or insert whatever emotion/though] center is irresponsible and should be correlated with other studies and hopefully monkey studies as well.
  • Re:Really Useful? (Score:5, Informative)

    by Metasquares ( 555685 ) <slashdot@me t a s q u a r e> on Wednesday July 08, 2009 @11:29AM (#28623975) Homepage

    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.

  • by jeffb (2.718) ( 1189693 ) on Wednesday July 08, 2009 @12:12PM (#28624723)

    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.

We all agree on the necessity of compromise. We just can't agree on when it's necessary to compromise. -- Larry Wall