How can electrodes on the scalp record brain activity?

Brain activity involves electrical currents as nerve cells communicate. Their combined effects can produce tiny voltage differences measurable at the scalp. Electroencephalography, or EEG, records these differences using electrodes placed at several locations.

An electrode does not collect a complete message from one neuron underneath it. Signals spread through surrounding tissues, and many sources contribute. A recording channel compares voltages at locations according to a chosen reference or arrangement.

Amplifiers and recording equipment make the small changing signals visible as traces over time. Eye movements, muscle activity and poor electrode contact can also affect the traces. Recording and interpretation therefore require attention to sources beyond the brain.

EEG can help study sleep and support clinical investigations because timing and patterns carry useful information. A wavy line alone does not identify a thought, measure intelligence or locate every source precisely. The instrument records an electrical signal that needs context and expertise.