online (real-time) decoding

Online decoding means reading the live brain signal and acting on it right away, as it streams in. This is the opposite of offline decoding, where you record everything first and analyse it later at your leisure — useful for research, but useless for actually steering a cursor or a wheelchair. To be online, the system has to take in fresh signal, extract features, run the decoder, and move the device, all on the fly.

The catch is speed. For the loop to feel natural and responsive, it usually has to close in well under a second — ideally in tens to low hundreds of milliseconds — from brain activity to action on screen. Any longer and the lag breaks the sense of control, much as a video call with a long delay makes conversation awkward.

Running in real time also forces hard choices the offline world avoids. The decoder cannot peek at the future, every step must be quick and reliable, and the user is reacting to the system's output even as it reacts to them, so small errors and delays feed back on each other. Making all of that smooth is much of the craft of building a usable BCI.