Polysomnography: understanding this technology's past might guide future developments.

Hans Berger published the first human electroencephalograph (EEG) recording in 1924 [1]. He used a device called the string galvanometer to record brain waves on a light-sensitive plate. The fluctuating potential difference from the scalp oscillated at eight to 13 cycles per second (alpha rhythm) when an individual closed his or her eyes and remained relaxed but awake. Berger noted that when a person fell asleep, the alpha rhythm disappeared. Amazingly, to this day, the alpha rhythm disappearance remains the primary marker for defining sleep onset. Years later, Carl Ludwig invented a kymograph (the ?wave writer?) that used a stylus to record electroencephalographic oscillation on a rotating drum. Later, an alternative approach evolved so that the brain wave recordings were inked onto a roll or fan-folded continuous paper strip moving at a constant speed. Mechanical engineers gradually improved the drive mechanisms for moving paper by using rotating sprockets, pinch rollers, and pressure plates. Gear mechanisms were also incorporated to permit speed changes.