BCI virtual keyboard for free communication. (a) Participants operated the real-time feedback loop to freely type words and phrases using their brain activity alone. Participants selected characters in sequence by focusing their attention and fixating their gaze on sinusoidally flickering keys of a virtual QWERTY keyboard on a computer display, which evoked oscillatory SSVEP responses at the corresponding flicker frequency/phase in the EEG. EEG time-locked to flicker was extracted, bandpass filtered to five harmonic ranges and then submitted to a filter-bank CCA with respect to a bank of individualized training templates. The classified frequency was the template most highly correlated with the real-time EEG, with the corresponding character displayed as feedback at the top of each key. Participants were free to select the next character, or to select the backspace key [<] to make a correction. The image of the head was created by Dr. David J. Lloyd. (b) Example timeline of visual stimulation and evoked EEG involved in BCI typing of the word “SENT”. Each key flickered at a unique frequency/phase for 1.5 s, followed by a 0.75 s flicker-free period, during which the letter was classified and participants shifted their attention to the next key. Focusing attention on a key potentiated the corresponding SSVEP response, increasing the likelihood that the corresponding letter would be classified. τ refers to the SSVEP delay relative to flicker onset, calculated separately for each frequency and harmonic. (c) Spatial organisation of the virtual keyboard’s flicker frequencies/phases. Each key flickered at a unique frequency/phase, ranging from 10 Hz/1.5π – 15.4 Hz/0.95π.