title: Eliciting dual-frequency SSVEP using a hybrid SSVEP-P300 BCI.
abstract: BACKGROUND
  Steady-state visual-evoked potential (SSVEP)-based brain-computer interfaces (BCIs) generate weak SSVEP with a monitor and cannot use harmonic frequencies, whereas P300-based BCIs need multiple stimulation sequences. These issues can decrease the information transfer rate (ITR).
  
  
  METHODS
  In this paper, we introduce a novel hybrid SSVEP-P300 speller that generates dual-frequency SSVEP, allowing it to overcome the abovementioned limitations and improve the performance. The hybrid speller consists of nine panels flickering at different frequencies. Each panel contains four different characters that appear in a random sequence. The flickering panel and the periodically updating character evoke the dual-frequency SSVEP, while the oddball stimulus of the target character evokes the P300. A canonical correlation analysis (CCA) and a step-wise linear discriminant analysis (SWLDA) classified SSVEP and P300, respectively. Ten subjects participated in offline and online experiments, in which accuracy and ITR were compared with those of conventional SSVEP and P300 spellers.
  
  
  RESULTS
  The offline analysis revealed not only the P300 potential but also SSVEP with peaks at sub-harmonic frequencies, demonstrating that the proposed speller elicited dual-frequency SSVEP. This dual-frequency stimulation improved SSVEP recognition, increased the number of targets by employing harmonic frequencies, reduced the stimulation time for P300, and consequently improved ITR as compared to the conventional spellers.
  
  
  METHODS
  The new method reduces the stimulation time and allows harmonic frequencies to be employed for different stimuli.
  
  
  CONCLUSIONS
  The results indicate that this study provides a promising approach to make the BCI speller more reliable and efficient.
authors.affiliation: Interdisciplinary Program for Bioengineering, Seoul National University, Seoul, Republic of Korea.
authors.forename: Min Hye
authors.initials: MH
authors.lastname: Chang
authors.affiliation: Interdisciplinary Program for Bioengineering, Seoul National University, Seoul, Republic of Korea.
authors.forename: Jeong Su
authors.initials: JS
authors.lastname: Lee
authors.affiliation: Interdisciplinary Program for Bioengineering, Seoul National University, Seoul, Republic of Korea.
authors.forename: Jeong
authors.initials: J
authors.lastname: Heo
authors.affiliation: Department of Biomedical Engineering, College of Medicine, Seoul National University, Seoul, Republic of Korea. Electronic address: pks@bmsil.snu.ac.kr.
authors.forename: Kwang Suk
authors.initials: KS
authors.lastname: Park
mesh_terms: D000328:Adult; D000465:Algorithms; D062207:Brain-Computer Interfaces; D003143:Communication Devices for People with Disabilities; D004569:Electroencephalography / Q000379:methods; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D010775:Photic Stimulation; D012815:Signal Processing, Computer-Assisted; D055815:Young Adult
keywords: BCI speller; Brain–computer interface (BCI); Dual frequency; Hybrid BCI; P300; Steady-state visual-evoked potential (SSVEP)