Many fMRI studies of brain activity could be biased by the effect of large blood vessels, according to an interesting new report: Origins of intersubject variability of BOLD and arterial spin labeling fMRI.
fMRImeasures BOLD, the Blood Oxygenation Level Dependent response. As the name says, BOLD is when a bit of the brain becomes more active, it uses more oxygen, and the oxygenation level of the blood in the area drops - although it then increases to compensate, and it's the increase that most fMRI picks up.
There's a catch though: blood flows. Specifically, it flows from arteries, into tissues - the brain, in this case - and then into veins. Blood leaving the brain tends to end up in the larger veins and, being large, these exert a large effect on BOLD - even though they're some distance from the true site of neural activation.
So, the worry is ...
