Here at Neuroskeptic we see a lot of dizzyingly bad (and sometimes even good) neuroscience, but did you know that brain scanners can literally send your head into a spin? A new paper explains why, with implications for all MRI researchers.
MRI scanners rely on extremely powerful magnetic fields. This is why you can't take metal objects into the scanner room, as they'd be pulled into it. Yet the fields can also exert other kinds of effects on the body.
I'd always been told that static, unchanging magnetic fields are biologically inert. But moving through the field too quickly can cause side effects. When an object moves through a magnetic field, induction happens - electrical currents are produced.
In the case of the human body, these small currents can activate nerve cells. Depending on which cells they hit this can cause you to feel dizzy, see flashes of light, experience tingling sensations, and so on. Or so I thought.
However, a new paper from Dale Roberts et al of Johns Hopkins shows that just being in a powerful magnetic field can cause dizziness and vertigo - with no movement required. They noticed that lying still in or near an MRI scanner causes nystagmus, abnormal horizontal eye movements, and that the amount of eye movement is directly correlated with the angle at which the head is positioned relative to the field.
The nystagmus was caused by an automatic reflex in response to effects in the vestibular ("balance") system of the ear. Roberts et al realized that the static magnetic field causes electrical currents that activate vestibular cells, even when the head is perfectly still. It happens because there's a natural flow of electrically charged ions into these cells in a part of the ear called the semicircular canal. The magnetic field interacts with this ion current, in what's called a Lorentz force.
The semicircular canals normally allow us to sense when our head is moving. Our eyes automatically compensate for head movement to keep us looking in the same direction. The MRI magnet fooled the ear into thinking the head was rotating, and the eyes produced nystagmus as a result.
Two patients who had suffered damage to their semicircular canals were immune to the effect.
This has important implications for functional MRI studies of brain function. Many people are interesting in measuring eye movements during MRI scans. This finding suggests that these movements may be unusual, compared to normal eye movements outside the scanner. Worst, the vestibular stimulation could alter brain activity:
Vestibular stimulation induced by the magnetic field in healthy subjects simply lying in the bore could activate many brain areas related to vision, eye movements, and the perception of the position and motion of the body.
Roberts, D., Marcelli, V., Gillen, J., Carey, J., Della Santina, C., & Zee, D. (2011). MRI Magnetic Field Stimulates Rotational Sensors of the Brain Current Biology DOI: 10.1016/j.cub.2011.08.029