magnetoencephalography (MEG)

Magnetoencephalography, or MEG, measures the brain at work in a completely different way from EEG: instead of voltages, it senses the faint magnetic fields that electric currents inside neurons naturally create. Any flowing electric current makes a small magnetic field around it, and MEG picks up that magnetic whisper from outside the head.

Like EEG, MEG is non-invasive and has superb timing, tracking activity millisecond by millisecond. Its advantage is that magnetic fields pass through the skull with much less smearing than electrical signals, so MEG can often pinpoint where activity comes from more precisely than scalp EEG can.

The price is the machine. The brain's magnetic fields are mind-bogglingly tiny — far weaker than ordinary magnetic noise around us — so MEG needs ultra-sensitive sensors inside a large, heavily shielded room. That makes it expensive and fixed in place, more a research and clinical tool than a wearable one.