Preliminary study for extraction of P300 and SSVEP by stimulus presentation using phase inversion technique in hybrid BCI

In this study, we propose a novel stimulation presentation method for the hybrid BCI of the P300 and steady state visual evoked potential (SSVEP) to separate the two components efficiently. The method produces the separation by generating the P300 at two time points whose phase difference is π radians in the SSVEP component corresponding to stimulus frequency. Assuming that the consecutive two P300 responses are identical and the SSVEP is sinusoidal, the P300 can be extracted as a summation of the above two responses by suppressing the SSVEP. Also, the SSVEP can be detected by the subtraction of the above two responses. Accordingly, this method is realized by a stimulus pair consisting of the above two stimuli. In an EEG experiment, we used a checkerboard stimulus and character presentation for obtaining the SSVEP and P300, respectively. The stimulus frequencies of the checkerboard were assigned to 5 Hz and 3 Hz to classify the target character from the two given characters. The results showed the appearance of a prominent P300 component from only one pair of stimuli, even though the fundamental and harmonic frequency components of the SSVEP for lower stimulus frequencies are not very stable. This is because of the asymmetry of the positive and negative potentials for the SSVEP. It is a good idea to use a stimulus frequency that overlaps with the P300 frequency band, because this method does not separate the P300 and SSVEP by EEG frequency difference. Moreover, it reduces the measurement time (i.e., it shortens the number of averagings required for P300 estimation) because the SSVEP cancels out if it is sinusoidal. We consider that this will be a useful method to estimate the P300 and SSVEP simultaneously from these aspects.