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.
Van 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