How small is the brain's signal at the scalp compared with the noise around it? · Grey Matter

Scalp EEG is measured in tens of microvolts, and it already reflects several square centimetres of cortex firing in step; a single neuron contributes nothing measurable at that distance.

How small is the brain's signal at the scalp compared with the noise around it?

Scalp EEG is measured in tens of microvolts, and it already reflects several square centimetres of cortex firing in step; a single neuron contributes nothing measurable at that distance. Blinks, eye movements, jaw and neck muscles and the heart produce signals of similar or larger size, and mains wiring adds interference at 50 or 60 Hz, so much of EEG practice is about recognising and removing those artefacts. The noise does not shrink as fast as the brain's signal does with distance, which is why recording closer to the source improves signal more than any better amplifier.