Electroencephalography (EEG) sensors — of which there are 5 in Muse 2 and Muse S — measure tiny fluctuations in voltage on the head (directly on the skin). This can be filtered and analyzed to reflect changes in brain state, responses to stimuli, and neuroplastic changes in the brain. The same technology is extensively used in brain research. Other sensors in Muse, including cardiac and movement sensors, monitor blood flow, breathing, and motion in the same way Fitbit and the Apple Watch do (PPG and MEMS). One important distinction is that there are advantages to measuring movement and aspects of cardiovascular physiology on the head because the head reflects aspects of behavior and physiology that the wrist does not.