Accounting for phase drifts in SSVEP-based BCIs by means of biphasic stimulation

Hung Yi Wu, Po Lei Lee, Hsiang Chih Chang, Jen Chuen Hsieh

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


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. Hotellings 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 users 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.

Original languageEnglish
Article number5676185
Pages (from-to)1394-1402
Number of pages9
JournalIEEE Transactions on Biomedical Engineering
Issue number5
StatePublished - May 2011


  • Biphasic stimulation
  • brain computer interface (BCI)
  • electroencephalography (EEG)
  • steady-state visual evoked potential


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