Workflow of a P300 speller and an SSVEP speller [34]. For each speller, the user watches the stimulation interface, focusing on the character he/she wants to input, while EEG signals are recorded and analyzed by the speller. The P300 speller first identifies the row and the column that elicit the largest P300, and then outputs the letter at their intersection. The SSVEP speller identifies the output letter directly by matching the user's EEG oscillation frequency with the flickering frequency of each candidate letter.