Figure 2: The course of one trial. The target letter is indicated by a green circle, for 1 second. Letters are flashed by group of 6 letters. After a delay, the letter chosen by the P300 classifier is displayed for 1.3 seconds. If the letter is the last of the word, a 4.5 second break occurs during which a display indicates the next word and the difficulty level (slow or fast spelling mode). If not, there is a short 0.5 s break before the next green circle appears.