title: Gaze independent brain-computer speller with covert visual search tasks.
abstract: OBJECTIVE
  Brain-computer interface (BCI) provides a mean of communication for the patients that are severely disabled by neuromuscular diseases. The performance of the classical P300 speller, however, declines noticeably in the gaze fixation condition. The speller paradigm presented in this paper aims to release the gaze dependency at the cost of an extra covert visual search task.
  
  
  METHODS
  Clusters of characters were presented sequentially in the near-central vision as stimulation. Participants fixed their gaze on the center, searched and recognized the target character with covert shift of attention. Random position (RP) and fixed position (FP) presentation modes designed with different searching set size (6 for RP, and ≤ 2 for FP) were examined.
  
  
  RESULTS
  Online sessions using 10 stimulus sequences achieved character accuracies of 94.4% and 96.3% for RP and FP mode, respectively. For offline overall evaluation, the peak written symbol rate (WSR) of 1.38 symbols/min was obtained, with corresponding accuracies of 87.8% (RP) and 84.1% (FP). The P300 waveform of RP mode has evident longer latency and larger amplitude. Electrooculogram (EOG) analysis indicated that the performance was independent of gaze shift.
  
  
  CONCLUSIONS
  The proposed speller could be operated effectively and gaze independently by healthy participants.
  
  
  CONCLUSIONS
  The proposed gaze independent BCI approach promises reasonable communication capability for the profoundly paralyzed patients with head or ocular motor impairments.
authors.affiliation: Department of Automatic Control, College of Mechatronics and Automation, National University of Defense Technology, Changsha, Hunan 410073, People's Republic of China.
authors.forename: Yang
authors.initials: Y
authors.lastname: Liu
authors.forename: Zongtan
authors.initials: Z
authors.lastname: Zhou
authors.forename: Dewen
authors.initials: D
authors.lastname: Hu
mesh_terms: D000328:Adult; D001288:Attention; D001921:Brain / Q000502:physiology*; D001931:Brain Mapping; D004569:Electroencephalography / Q000379:methods; D004596:Electroretinography / Q000379:methods; D005074:Evoked Potentials, Visual / Q000502:physiology; D005260:Female; D005403:Fixation, Ocular; D006801:Humans; D007802:Language; D008297:Male; D009862:Online Systems; D010775:Photic Stimulation / Q000379:methods; D011237:Predictive Value of Tests; D011930:Reaction Time / Q000502:physiology; D014584:User-Computer Interface; D055815:Young Adult