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Brain-computer interfaces (BCI) based on Steady State Visual Evoked Potential (SSVEP) can provide higher information transfer rate and require shorter calibration than BCIs based on other modalities. For safety and comfort, the frequency of the repetitive visual stimuli seliciting the SSVEP should be higher than 30 Hz. However, in such frequency range, only a limited number of frequencies can elicit sufficiently strong SSVEPs for BCI purposes. Thus, the conventional approach, consisting in presenting various repetitive visual stimuli at different frequencies, is not feasible for high frequencies. Indeed this would bring low communication bitrates. To increase the number of possible repetitive visual stimuli, we consider modulating the phase of the stimulus instead of the frequency. In this paper, we present an approach to reliably detect the stimulus phase from the recorded SSVEP.

Brain computer interfaces (BCI) that use the steady-state-visual-evoked-potential (SSVEP) as neural source, offer two main advantages over other types of BCIs: shorter calibration times and higher information transfer rates. SSVEPs elicited by high frequency (larger than30 Hz) repetitive visual stimulation are less prone to cause visualfatigue, safer, and more comfortable for the user. However in the high frequency range there is a practical limitation because only fewfrequencies can elicit sufficiently strong SSVEPs for BCI purposes.We bypass this limitation by using only one stimulation frequency and different phases. To detect the phase from the recorded SSVEP, weuse spatial filtering combined to phase synchrony analysis. We developed an online BCI implementation which was tested on six subjects and resulted on an average accuracy of 95.5% and an average bit rateof 34 bits-per-minute. Our approach has the advantage of entailing only minimal visual annoyance for the user.

Brain-computer interfaces (BCI) based on Steady State Visual EvokedPotential (SSVEP) can provide higher information transfer rate thanother 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 rangeabove 30 Hz, only a limited number of frequencies can elicit sufficiently strong SSVEPs for BCI purposes. Consequently, the conventionalapproach, consisting in presenting various repetitive visual stimuli having different frequency each, is not practical for SSVEP basedBCI 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 thefrequency. 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 termedphase synchrony. It consists in using as feature, to identify a subject's focus of attention, the phase difference between the SSVEP and the stimulus. The phase is extracted through the Hilbert transformapplied on an univariate signal resulting from spatially filteringthe electroencephalogram.