The first thing you’ll notice about MEG machines is that they live in carefully shielded sealed rooms. The second you’ll notice is that they look like giant hair dryers.
MEG works by picking up the magnetic field generated by the brain. You might remember from high school physics that every current produces a magnetic field, meaning neurons produce a magnetic field every time they are active.
However, because even groups of active neurons only have a small current, the associated magnetic field is very weak.
So weak, in fact, that a car passing in a nearby street, and even the earth itself, produces a much stronger magnetic field. This is why the shielded room is needed.
Even with the shielding, to pick up such a weak magnetic field MEG uses ultra sensitive Superconducting Quantum Interference Devices or SQUIDs.
Because they rely on superconductors, they need to be supercooled to work. To achieve this, they’re bathed in a pool of liquid helium held in a large container above the head – this is why MEG machines look like giant hairdryers.
An advantage of MEG over EEG is that unlike electrical fields, magnetic fields travel virtually unaltered across the skull, meaning it’s possible to more accurately measure where activity comes from.
An advantage over fMRI is that MEG can measure brain changes on a millisecond by millisecond basis, meaning it’s much quicker than the typical fMRI 1 second time frame.
Also, MEG is measuring brain activity directly, rather than inferring it from changes in blood flow as fMRI and PET do.
However, as MEG only measures activity on the surface of the brain and can’t distinguish activity from small areas, it doesn’t give the full depth or spatial resolution of fMRI.
As the Wired article notes, MEG is going to become an increasingly important player in our quest to understand psychology and neuroscience, so keep a lookout for an increasing number of important findings based on MEG scans.
Link to Wired article ‘MEG Scanners Are Mega Powerful’.