EEG and MEG

Recording the brain's electrical rhythms through the scalp, from Hans Berger's alpha waves in the 1920s to event-related potentials, sleep studies, epilepsy diagnosis and wearable magnetoencephalography.

Hans Berger's discovery

In 1924 the German psychiatrist Hans Berger recorded electrical activity from the human brain through the scalp — the first human electroencephalogram (EEG) — publishing in 1929. He described rhythmic waves at about 10 per second when people relaxed with eyes closed, which disappeared when they opened their eyes: the alpha rhythm (sometimes called the Berger rhythm). Many scientists were sceptical until Edgar Adrian confirmed his findings in 1934.

Where the signal comes from

EEG mainly reflects the summed synaptic currents of large numbers of pyramidal neurons in the cortex, aligned in parallel and active in synchrony. Single neurons or deep structures contribute little. Electrodes are placed in standard positions (the international 10–20 system), with modern caps using 32 to 256 electrodes.

Brain rhythms

EEG activity is described by frequency bands: delta (below 4 Hz; deep sleep), theta (4–8 Hz; drowsiness, memory tasks), alpha (8–12 Hz; relaxed wakefulness, eyes closed), beta (13–30 Hz; active thinking, movement) and gamma (above about 30 Hz; associated with attention and binding of information).

Event-related potentials

By averaging EEG across many repetitions of a stimulus, researchers extract event-related potentials (ERPs) — reliable voltage deflections at particular delays. The P300, a positive wave about 300 milliseconds after a rare or meaningful stimulus, is linked to attention and has been used in brain-computer interfaces. The N400 responds to unexpected word meanings ("I take my coffee with cream and dog").

Uses

EEG is essential for diagnosing epilepsy, studying sleep (polysomnography), monitoring anaesthesia depth and coma, and research on cognition. It's cheap, portable, silent and has millisecond timing — but poor spatial resolution, because the skull smears the signal.

MEG

Magnetoencephalography (MEG), first recorded by David Cohen in the late 1960s and greatly improved with sensitive SQUID detectors, measures the extremely weak magnetic fields produced by the same neural currents — about a billion times weaker than Earth's magnetic field. Magnetic fields pass through the skull without distortion, so MEG localises sources better than EEG. Traditional MEG needs liquid-helium cooling and a magnetically shielded room; newer optically pumped magnetometers (OPMs) allow wearable MEG helmets, even for people moving or for children.