Figure 1

Program flow of the Center Speller with ErrP detection. Spelling starts with a countdown (a), followed by the stimulation sequence (b), wherein each symbol is presented sequentially in a randomized order. Then, the feedback (selected symbol group) is presented to the user (c), and the ErrP classifier evaluates the brain response (ErrP present/absent). If an ErrP is detected, a red 'X' indicates the error (d) and the stimulus sequence repeats. Otherwise, the algorithm proceeds with level 2 (e) wherein the selection process is repeated on the single character level (f-h). Finally, the selected character is appended to the phrase shown at the top of the screen and the next trial is started.