Subjects need to be free of magnetic materials, like coins, watches and other metal items. Underwired bras can be a problem, and even dyes in clothing or hairdye can cause interference.
To track head movements volunteers need to have 4 small HPI (Head Position Indicator) coils attached to their head: 2 on the forehead and 2 behind the ears. In addition we normally attach 5 electrodes to the face as well to measure eye movements and blinks.
The location of the coils needs to be recorded with a 3D digitiser. We also measure 3 landmarks and a number of additional points on the head to indicate headshape and enable easier matching to an MRI structural scan. The digitiser has a pen-like sensor that will record the location of its tip when the button is pressed.
The glasses in the picture have an additional sensor attached to them, to register movements of the head and correct for that. They are only used for the 3D recording process.