Event related potentials (ERPs) are based on the measurement of brain responses created by specific activation within the brain as a response to an external or internal event. Dr. Niels Birbaumer explains that today's brain-computer interfaces based on ERPs can enable a paralyzed person to select letters on a computer screen. He further describes that P300 waves, or positive signals, are generated automatically when people see something they recognize. If an error is detected then a large negative response - often called error related potential - can be observed. Using these signals, simple “yes” or “no” responses can be identified solely based on brain activation.