Phase detection in a visual-evoked-potential based brain computer interface

Brain-computer interfaces (BCI) based on Steady State Visual Evoked Potential (SSVEP) can provide higher information transfer rate than other BCI modalities. For the sake of safety and comfort, the frequency of the repetitive visual stimulus (RVS) necessary to elicit an SSVEP, should be higher than 30 Hz. However, in the frequency range above 30 Hz, only a limited number of frequencies can elicit sufficiently strong SSVEPs for BCI purposes. Consequently, the conventional approach, consisting in presenting various repetitive visual stimuli having different frequency each, is not practical for SSVEP based BCI functioning. Indeed this would bring low communication bitrates. In order to increase the number of possible repetitive visual stimuli, we consider modulating the phase of the stimulus instead of the frequency. Thus, several stimuli, sharing the same frequency, but with different phase can be presented to the user. The approach presented in this document, to detect the phase of the stimulus is termed phase synchrony. It consists in using as feature, the phase difference between the SSVEP and the stimulus. The phase is extracted through the Hilbert transform applied on an univariate signal resulting from spatially filtering the electroencephalogram. The spatial filter is determined in such a way that the SSVEP energy is enhanced through a linear combination of the signals recorded at different positions on the scalp. Phase detection accuracy for seven subjects ranges from 70 to 94%.