There are a few techniques:
Patch clamp recording: you are literally sticking a needle into a single neuron and measuring it’s activity
EEG/MEG: using electrodes to measure activations of many thousands or millions of cells
Patch clamp recording: you are literally sticking a needle into a single neuron and measuring it’s activity
EEG/MEG: using electrodes to measure activations of many thousands or millions of cells
It sounds like they have very sensitive electrodes that are either very close to individual neurons, and then they shift those around, or they have a lot of those sensitive electrodes and compute the location of signal sources in 3D space. The latter would be a lot more economical.