alpha rhythm

The alpha rhythm is a steady electrical pulsing of the brain that beats roughly 8 to 12 times per second — about as fast as you can flutter your fingers. You can think of it as the cortex (the brain's wrinkled outer layer) humming an idle tune when it is awake but relaxed and not busy with a demanding task. It shows up most strongly when a person sits quietly with their eyes closed, and it tends to fade or shrink the moment they open their eyes, pay close attention, or start working hard on a problem.

This rhythm is one of the easiest brain signals to spot. When the German doctor Hans Berger first recorded human brain waves in the 1920s using electrodes on the scalp (a method called electroencephalography, or EEG, which reads the faint voltage from billions of neurons firing together), the alpha rhythm was the clear, regular wave that jumped out at him. It is strongest over the back of the head, above the visual areas, which is why simply opening or closing the eyes flips it on and off so dramatically. Many scientists now think alpha is not just empty idling but an active 'standby' state — a way the brain quiets down regions it is not using, so attention can be aimed where it is actually needed.