Subject-Specific Methodology in the Frequency Scanning Phase of SSVEP-Based BCI

Steady State Visual Evoked Potentials (SSVEPs) often used in Brain Computer Interfaces (BCIs) differ across subjects. That is why a new SSVEP-based BCI user should always start the session from the frequency scanning phase. During this phase the stimulation frequencies evoking the most prominent SSVEPs are determined. In our opinion not only the stimulation frequencies specific for the given user should be chosen in the scanning phase but also the methodology used for SSVEP detection. The paper reports the results of a survey whose aim was to find out whether using subject specific methodology for identifying stimulation frequencies would increase the number of frequencies found. We analyzed three factors: length of time window used for power spectrum calculation, combination of channels, and number of harmonics used for SSVEP detection. According to the outcome of the experiment (performed with 6 subjects) the mean drop in the number of SSVEPs detected with any other but the best combination of factors was very large for all subjects (from 31.52 % for subject S3 to 51.76 % for subject S4).