Magnetoencephalography (MEG) is the measurement and localisation of the weak magnetic fields generated by neural activity in the brain. As regions of the brain become activated during specific cognitive tasks, an electrical current is generated due to synchronous neural firing. As all electrical currents generate an associated magnetic field, this can be measured outside the head using suitably sensitive equipment.
These neural magnetic fields are extremely weak – 10-12 Tesla – which means that sensitive detectors known as SQUIDs (Superconducting Quantum Interference Devices) must be used. SQUIDs need to be bathed in liquid helium so that they are at their superconducting operating temperature of -269 degrees C.
Magnetic noise from the environment is also a problem. Cars moving past the CUBRIC building generate a larger magnetic field than that from the brain, as does the nearby lift, and even people walking up the metal staircase in the building generates a measurable noise signal. In order to reduce this electromagnetic noise, the MEG system must be housed in a completely shielded room, which consists of 3 metal layers around the walls, ceiling and floors of the scan room. In addition, dedicated SQUIDS in the centre of the MEG system record the environmental noise and subtract it from those SQUIDs measuring signals from the brain.