band power

A brain signal is a jumble of rhythms at many speeds happening at once, like several musical notes played together. Band power asks a simple question about one of those notes: how loud is it? More precisely, it is how much of the signal's energy sits inside a chosen frequency band — for example the mu band around 8–12 cycles per second, or the beta band a bit faster.

Band power is a primary feature for rhythm-based BCIs because it rises and falls with what the person is doing in their mind. When someone imagines moving a hand, for instance, the mu and beta rhythms over the motor cortex tend to quiet down, so the band power in those bands drops. A decoder can watch that number and infer the intention behind it.

To compute it you take a short window of signal, work out how its energy is spread across frequencies, and add up the part that falls within your band. It is cheap to calculate and easy to interpret, which is part of why it has stayed popular for decades.