Brain Connectivity, Or Head Movement?

By Neuroskeptic | July 27, 2011 7:25 am

“It’s pretty painless. Basically you just need to lie there and make sure you don’t move your head”.


This is what I say to all the girls… who are taking part in my fMRI studies. Head movement is a big problem in fMRI. If your head moves, your brain moves and all fMRI analysis assumes that the brain is perfectly still. Although head movement correction is now a standard part of any analysis software, it’s not perfect.

It may be a particular problem in functional connectivity studies, which attempt to measure the degree to which different parts of the brain are “talking” to each other, in terms of correlated neural activity over time. These are extremely popular nowadays. It’s even been claimed that this data may help us understand consciousness itself (although we’ve heard that before).

A new paper offers some important words of caution. It shows that head motion affects estimates of functional connectivity. The more motion, the weaker the measured connectivity in long-range networks, while shorter range connections were stronger. Also, men tended to move more than women.

The effect was small – head movement can’t explain more than a small fraction of the variability in connectivity.


The authors looked at 1,000 scans from healthy volunteers. They just had to lie in the scanner at rest. They looked at functional connectivity, using standard “motion correction” methods, and correlated it with head movement (which you can measure very accurately from the MRI images themselves.) Men tended to move more than women. Could this explain why women tend to have higher functional connectivity?

Disconcertingly, head movement was associated with low long range / high short range connections, which is exactly what’s been proposed to happen in autism (although in fairness, not all the evidence for this comes from fMRI).

This clearly doesn’t prove that the autism studies are all dodgy, but it’s an issue. People with autism, and people with almost any mental or physical disorder, on average tend to move more than healthy controls.

One caveat. Could it be that brain activity causes head movement, rather than the reverse? The authors don’t consider this. Head movement must come from the brain, of course. Probably from the motor cortex. The fact that motor cortex functional connectivity was positively associated with movement does suggest a possible link.

However, this paper still ought to make anyone who’s using functional connectivity worry – at least a little.

Head motion is a particularly insidious confound. It is insidious because it biases between-group studies often in the direction of the hypothesized difference….even though there is considerable variation that is not due to head motion, in any given instance, a between-group difference could be entirely due to motion.

ResearchBlogging.orgVan Dijk, K., Sabuncu, M., & Buckner, R. (2011). The Influence of Head Motion on Intrinsic Functional Connectivity MRI NeuroImage DOI: 10.1016/j.neuroimage.2011.07.044

  • http://www.blogger.com/profile/06815277098386812048 Jayarava

    movement comes from the brain, probably from the motor cortex…

    and the motor cortex is presumably responding to signals from elsewhere in the brain, that suggest movement is our best option.

    Is it genuinely possible to see isolated activity in something like the brain which is constantly talking to itself about what is going on internally and externally?

  • Anonymous

    Stupid question – could a plastic ' halo' be used to keep the patient's head from moving? Never had an MRI nor an enclosed CT, just curious.

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    In most MRI scans you use little foam cushions to wedge next to people's heads to try to prevent movement. They did this in this study too. However the cushions are soft so you can still move a little. Hard plastic would be uncomfortable. And if people are too uncomfortable they'll tend to move more…

  • http://www.blogger.com/profile/05660407099521700995 petrossa

    I had several normal MRI's. I suffer from head tremor. The more i try to keep it still the worse it trembles. They usually stuff the headcasket full to keep me somewhat still. Worn out 3 cervical disks already, one prosthesis now, second one on the horizon.

    It always made me doubt the accuracy of fMRI. Glad to see my doubts are confirmed.

  • Anonymous

    Why don't they just put a guillotine next to the MRI scanner? Cut off subjects heads and then place them in a vise in the scanner? That would work.

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Good idea – but you'd need to make a special MRI-safe, metal-free guillotine.

  • http://www.blogger.com/profile/15225859145004971487 Jon Brock

    I don't quite understand why movement would lead to the specific pattern of reduced long-distance “connectivity” but increased local “connectivity”. Do they attempt an explanation?

    From the abstract, it seems that motion is related to changes in the Default Mode Network and Motor Cortex, which would be consistent with your alternative explanation.

    But either way, it's a bit of a worry!

  • omg

    I would never go near that can I'm not a salmon. You could tell the ladies a radioactive facial gives a radiant glow.. leaves the skin supple like 10 years younger when the head is still as possible – dead still. For the guys I agree with the guillotine.

  • http://www.blogger.com/profile/06448577864182126540 Stuart Brown

    I had a PET scan years back, and they had a cushion of soft gel which you placed your head in. Upon the addition of another chemical it turned absolutely rigid, making any head movement (or comfort, for that matter) virtually impossible. Do they not use that, or a similar system?

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Stuart: I think the reason they do that for PET is that PET scans are a) very long and b) there is no way to do motion correction, because you don't get a picture of the head.

    So any movement would render the data unusable.

    Maybe we should think about doing that for MRI. Because motion correction is not perfect.

  • http://practicalfmri.blogspot.com practiCalfMRI

    I need to read the paper so I'm going to restrict my comments to a general perspective. The motion correction we use today isn't very discerning. It will treat thermal drifts as motion, for example. What's more, with large receive arrays of small coils, head motion appears to the coils in a fundamentally different fashion than for, say, a birdcage coil. All of this means that there are likely many tools in common use that are inappropriate, and could easily lead to accidental “connectivity.” I can't say more because (a) I'm three beers and a red wine past my bed time, and (b) this is a subject that requires an extensive thesis, not a precis. The bottom line: I'm not at all surprised if there are spurious effects from motion correction. (It also means I'm possibly not totally wasting my time trying to divine better methods to do this stuff!)

    I'll read the paper and perhaps post more thoughtful comments in the coming days, pub visits permitting…

  • Oldmonkey

    Apologies for my stupid question but I have been trying really hard to understand what is the question exactly and the conclusion as well.

    If someone could be generous enough to respond I would highly appreciate it.

    Thanks in advance.

  • Oldmonkey

    A never mind, I read the references where it was written “The Influence of Head Motion on Intrinsic Functional Connectivity MRI NeuroImage”. I was puzzled at the title of this article. Hence.

  • http://practicalfmri.blogspot.com practiCalfMRI

    Read the paper, nothing more to add, really. Perhaps fMRIers will start paying more attention to the stream of methods they apply (blindly)? I have a dream….

  • http://practicalfmri.blogspot.com practiCalfMRI

    Actually I do have something to add… It will be interesting to explore these gender differences further. I wonder if differences in blood pressure or the mechanical effects of the cardiac cycle could be factors. We know that women have lower hematocrit than men on average, for example. One wonders if other physiologic parameters will differ systematically and whether much of the residual, unexplained motion is inside the skull, thus difficult (or impossible) to fix with affine transforms that assume the whole head is a block.

    Two further points, related. First, they did the global signal normalization. While that should only probably affect the ability to interpret inverse correlations, it would be interesting to see a parallel analysis using the Birn/Chang resp and cardiac corrections instead. Second, having independent resp and cardiac measures (as needed for the Birn/Chang corrections), one wonders if a lot more variance can be explained by physiology. (All of which reminds me that I need to look into an MR-compatible blood pressure monitor again!) Imho, all people doing rs-fMRI should be acquiring separate resp and HR data. It's not hard, or expensive.

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Neuroskeptic

No brain. No gain.

About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.

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