Electroencephalograph (EEG)

The EEG measures electrical activity of the brain using pairs of electrodes placed at different (internationally specified) points on the scalp. It is used by doctors for diagnosis and research.

It seemed that the EEG would provide the key to understanding how the brain functions, but it proved very difficult to interpret these brain waves, or to deduce from where in the brain they originated.

Magnetoencephalograph (MEG)

The MEG, however, can measure

the oscillating millisecond fluxes of the brain in real time. Furthermore, unlike the EEG, granted enough mathematical sophistication and computing power, you get a good idea of the location of the electromagnetic source in the brain.

And it can be used to

record magnetic and electrical fields within the brain simultaneously, tracking impulses moving (a distance of) a few millimetres at up to 200 miles per hour.

In real time, that is 'in perhaps 10 milliseconds'. And 'usually accurate to within one or two millimetres in pre-surgical mapping'.

And in this way enabling responses to be tracked within the brain.