The sample of human EEG with in resting state. Left - EEG traces (horizontal - time in seconds; vertical - amplitudes, scale 100uV). Right - power spectra of shown signals (vertical lines - 10 and 20Hz, scale is linear). 80-90% of people have prominent sinusoidal-like waves with frequencies in 8-12Hz range - alpha rhythm. Others (like this) lack this type of activity.