title: Multi-phase cycle coding for SSVEP based brain-computer interfaces.
abstract: BACKGROUND
  Brain-computer interfaces (BCIs) based on Steady State Visual Evoked Potential (SSVEP) have attracted more and more attentions for their short time response and high information transfer rate (ITR). The use of a high stimulation frequency (from 30 Hz to 40 Hz) is more comfortable for users and can avoid the amplitude-frequency problem, but the number of available phases for stimulation source is limited. To circumvent this deficiency, a novel protocol named Multi-Phase Cycle Coding (MPCC) for SSVEP-based BCIs was proposed in the present study.
  
  
  METHODS
  In MPCC, each target is coded by a block word that includes a series of cyclic codewords, and each block word is corresponding to a certain flickering visual stimulus, which is a combination of multiple phases from an available phase set and flickers at single frequency. The methods of generating block code and extracting phase were presented and experiments were performed to investigate the feasibility of MPCC.
  
  
  RESULTS
  The optimal stimulation frequency was subject-specific, and the optimal duration was longer than 0.5 s. The BCI system with MPCC could achieve average discrimination accuracy 93.51 ± 5.62% and information transfer rate 33.77 ± 8.67%.
  
  
  CONCLUSIONS
  The MPCC has the error correction ability, can effectively increase the encoded targets and improve the performance of the system. Therefore, the MPCC is promising for practical BCIs.
authors.forename: Jijun
authors.initials: J
authors.lastname: Tong
authors.affiliation: State Key Laboratory of Diagnosis and Treatment of Infectious Diseases, Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, The First Affiliated Hospital Of Zhejiang University, Hangzhou 310003, China. zdhla@126.com.
authors.forename: Danhua
authors.initials: D
authors.lastname: Zhu
mesh_terms: D062207:Brain-Computer Interfaces; D005074:Evoked Potentials, Visual; D006801:Humans; D010775:Photic Stimulation; D012815:Signal Processing, Computer-Assisted; D013997:Time Factors