I visited neuroscientist Russell Poldrack’s laboratory at the University of California, Los Angeles, and arranged to get my brain scanned inside its MRI machine. Scanners typically weigh around 12 tons and cost about $2.5 million (not including installation, training and maintenance, which can drive the typical bill up by another $1 million). Right off the bat I realized how unnatural an environment it is inside that coffinesque tube. In fact, I had to bail out of the experiment before it even started. I had suddenly developed claustrophobia, a problem I had never experienced earlier. I’m not alone. Poldrack says that as many as 20 percent of subjects are similarly affected. Because not everyone can remain relatively relaxed while squeezed inside the tube, fMRI studies are afflicted with a selection bias; the subject sample cannot be completely random, so it cannot be said to represent all brains fairly.Via Pure Pedantry. See also here.
A person jammed into the narrow tube also has his or her head locked firmly in place with foam wedges inside the head coil—nicknamed “the cage”—to reduce head motion (which can blur the images) before the experiment begins. The MRI scanner snaps a picture of the brain every two seconds while the subject watches images or makes choices (by pushing buttons on a keypad) presented through goggles featuring tiny screens.
So when you read popular accounts of subjects who had their brains scanned while they were shopping, for example, remember that they were not walking around a Wal-Mart with headgear on. Far from it.
The best experiments, I think, would be precisely those where people could walk around Walmart with headsets on and we could measure some of their brain activity. Sadly, that's not such an easy thing to do. What we do therefore is strap them into scanners or else conduct simulated experiments in the laboratory. While these are very valuable, the point is -- we cannot read too much into them.