Phase Coherent Detection of Steady-State Evoked Potentials: Experimental Results and Application to Brain-Computer Interfaces

Steady-state visual evoked potentials (SSVEP) have frequently been used in brain-computer interfaces (BCI). These BCIs commonly use non-coherent estimators for the discrimination of different visual stimuli. Here we present a novel phase coherent detection method for discrimination. This method utilizes information about the stimulus phase which leads to a significant reduction in classification error. In addition, using stimuli with identical reversal rate but different phases allow for a more flexible BCI system design. In this study, EEG-signals from two electrodes were recorded while subjects gazed at one out of two stimuli. These stimuli differed in reversal rate and/or relative phase shift. We compared the results obtained with the conventional non-coherent estimator with our phase coherent estimator in an offline analysis for different stimulus conditions. We found that including phase information into the analysis can reduce error probability by at least a factor of two. In addition we showed that stimuli of identical frequency that differed in phase, only, can reliably be discriminated. Similar low error probabilities as for stimuli that differed in phase and frequency were found