title: Accounting for phase drifts in SSVEP-based BCIs by means of biphasic stimulation.
abstract: This study proposes a novel biphasic stimulation technique to solve the issue of phase drifts in steady-state visual evoked potential (SSVEPs) in phase-tagged systems. Phase calibration was embedded in stimulus sequences using a biphasic flicker, which is driven by a sequence with alternating reference and phase-shift states. Nine subjects were recruited to participate in off-line and online tests. Signals were bandpass filtered and segmented by trigger signals into reference and phase-shift epochs. Frequency components of SSVEP in the reference and phase-shift epochs were extracted using the Fourier method with a 50% overlapped sliding window. The real and imaginary parts of the SSVEP frequency components were organized into complex vectors in each epoch. Hotelling's t-square test was used to determine the significances of nonzero mean vectors. The rejection of noisy data segments and the validation of gaze detections were made based on p values. The phase difference between the valid mean vectors of reference and phase-shift epochs was used to identify user's gazed targets in this system. Data showed an average information transfer rate of 44.55 and 38.21 bits/min in off-line and online tests, respectively.
authors.affiliation: Institute of Brain Science, National Yang-Ming University, Taipei 112, Taiwan. bard1026@gmail.com
authors.forename: Hung-Yi
authors.initials: HY
authors.lastname: Wu
authors.forename: Po-Lei
authors.initials: PL
authors.lastname: Lee
authors.forename: Hsiang-Chih
authors.initials: HC
authors.lastname: Chang
authors.forename: Jen-Chuen
authors.initials: JC
authors.lastname: Hsieh
mesh_terms: D000328:Adult; D004569:Electroencephalography / Q000379:methods*; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D005583:Fourier Analysis; D006801:Humans; D008297:Male; D010775:Photic Stimulation; D015203:Reproducibility of Results; D012656:Self-Help Devices; D012815:Signal Processing, Computer-Assisted; D014584:User-Computer Interface