Electroencephalography

Berger’s recordings were an amplifier problem before they were a neuroscience result. Scalp potentials are tens of microvolts, sitting on top of electrode drift, mains interference, and muscle activity orders of magnitude larger. That he saw the alpha rhythm at all in the 1920s is largely a story about differential amplification and careful electrode contact.

What it established

That the brain is always electrically active, in rhythms, and that those rhythms change with state — eyes open or closed, awake or asleep, seizing or not. Before this, cortical electrophysiology was about evoked responses to stimuli. EEG made spontaneous activity a subject.

Its clinical life followed immediately: epilepsy diagnosis, sleep staging, and depth-of- anaesthesia monitoring all rest on reading these rhythms, and all of them predate any mechanistic account of what generates them.

The trade it makes

EEG has excellent time resolution and poor spatial resolution, and the reason is physical rather than technological. The skull is a low-conductivity layer between source and sensor, so potentials spread and superpose; what reaches an electrode is a blurred sum over a large volume of synchronously active cortex. Solving backwards from scalp voltages to sources is ill-posed — many source configurations produce the same measurement.

That limitation is the mirror image of fMRI, which has the spatial resolution and lacks the timing, and it is why the two are so often used together.

Because the signal requires synchrony across many neurons to be visible at all, EEG is inherently a population measurement: activity that is not coordinated cancels before it reaches the scalp.