Naturally embedded SSVEP phase tagging in a P300-based BCI: LSC-4Q speller

This paper proposes a P300-based BCI speller called LSC-4Q that takes advantage of steady state visual evoked potentials (SSVEP) that appear naturally in the brain as a side effect of the inter-stimulus interval of P300 visual paradigms. The LSC-4Q speller has a circular layout divided into quadrants, and symbols flash individually with a given pseudo-random strategy. Controlling the sequence of the events such that consecutive flashes alternate between sides or quadrants of the speller, we research the possibility of detecting the SSVEP phase associated with the side or quadrant for which the user is focusing on the target symbol, without explicitly incorporating a SSVEP flickering stimulator. The SSVEP phase is extracted using a statistical spatio-spectral Fisher criterion beamformer (SSFCB) implemented in the frequency domain. Results show that SSFCB efficiently extracts phase tags from the SSVEPs embedded on the visual evoked potentials of the oddball paradigm. Preliminary results with 4 participants suggest that it is possible to detect with high accuracy the side of the screen to which the user is looking, and with less precision the detection of the quadrant. Main issues are related to phase variability across sessions. Online results show that some participants can benefit from the combined P300-Lateral approach, improving the overall classification when P300 misclassifications occur.