An Online Interactive Paradigm for P300 Brain-Computer Interface Speller

For each brain-computer interface system, efficiency is a key issue that considers both accuracy and speed. The P300 spellers built upon oddball paradigm are usually less efficient due to the repetitive stimulation of multiple characters for reliable detection. In this paper, based on the online EEG signal, we propose an interactive paradigm for P300 speller to improve its efficiency, primarily focusing within the single character P300 paradigm. Specifically, after each stimulation, we first evaluate the posterior probability of each character in the stimuli set to be the target. The low probability characters are then removed from the stimuli set in the subsequent round(s), as character flash continues until the probability of any character surpasses a predefined threshold. Then, the character is selected as the target and data collection for the trial terminates. By reducing stimulus sequence characters, the system efficiency can be substantially improved. The spelling accuracy is insignificantly affected as the characters being removed have low probability to be the target. The online experimental results from a total of eight subjects show that an average practical information transfer rate of 50.26 bits/min (i.e. 9.07 characters/min) has been achieved, with 91% average spelling accuracy rate.