mu rhythm (sensorimotor rhythm)

The mu rhythm is a brainwave of roughly 8 to 13 cycles per second, picked up over the sensorimotor cortex — the touch-and-movement strip near the top of the head. When that region is idling, the rhythm hums along steadily, a bit like an engine ticking over at rest.

Its useful quirk is what happens when you act. The moment you move — or even just vividly imagine moving — the mu rhythm drops in strength, as if the engine revs and the idle hum fades. After you stop, it rebounds, often overshooting its baseline before settling. This reliable dip-and-rebound is the same idea as event-related desynchronization and resynchronization.

Because that dip is so dependable and shows up even for imagined movement, the mu rhythm is the workhorse signal of motor-imagery BCIs. A user pictures squeezing the left or right hand, the system spots which side's mu rhythm dropped, and uses it as a command — no actual movement required.