title: Exploring Combinations of Different Color and Facial Expression Stimuli for Gaze-Independent BCIs.
abstract: BACKGROUND
  Some studies have proven that a conventional visual brain computer interface (BCI) based on overt attention cannot be used effectively when eye movement control is not possible. To solve this problem, a novel visual-based BCI system based on covert attention and feature attention has been proposed and was called the gaze-independent BCI. Color and shape difference between stimuli and backgrounds have generally been used in examples of gaze-independent BCIs. Recently, a new paradigm based on facial expression changes has been presented, and obtained high performance. However, some facial expressions were so similar that users couldn't tell them apart, especially when they were presented at the same position in a rapid serial visual presentation (RSVP) paradigm. Consequently, the performance of the BCI is reduced.
  
  
  METHODS
  In this paper, we combined facial expressions and colors to optimize the stimuli presentation in the gaze-independent BCI. This optimized paradigm was called the colored dummy face pattern. It is suggested that different colors and facial expressions could help users to locate the target and evoke larger event-related potentials (ERPs). In order to evaluate the performance of this new paradigm, two other paradigms were presented, called the gray dummy face pattern and the colored ball pattern.
  
  
  CONCLUSIONS
  The key point that determined the value of the colored dummy faces stimuli in BCI systems was whether the dummy face stimuli could obtain higher performance than gray faces or colored balls stimuli. Ten healthy participants (seven male, aged 21-26 years, mean 24.5 ± 1.25) participated in our experiment. Online and offline results of four different paradigms were obtained and comparatively analyzed.
  
  
  RESULTS
  The results showed that the colored dummy face pattern could evoke higher P300 and N400 ERP amplitudes, compared with the gray dummy face pattern and the colored ball pattern. Online results showed that the colored dummy face pattern had a significant advantage in terms of classification accuracy (p < 0.05) and information transfer rate (p < 0.05) compared to the other two patterns.
  
  
  CONCLUSIONS
  The stimuli used in the colored dummy face paradigm combined color and facial expressions. This had a significant advantage in terms of the evoked P300 and N400 amplitudes and resulted in high classification accuracies and information transfer rates. It was compared with colored ball and gray dummy face stimuli.
authors: Key Laboratory of Advanced Control and Optimization for Chemical Processes, Ministry of Education, East China University of Science and Technology Shanghai, China.
authors: Long
authors: L
authors: Chen
authors: Key Laboratory of Advanced Control and Optimization for Chemical Processes, Ministry of Education, East China University of Science and Technology Shanghai, China.
authors: Jing
authors: J
authors: Jin
authors: Brain Embodiment Lab, School of Systems Engineering, University of Reading Reading, UK.
authors: Ian
authors: I
authors: Daly
authors: Key Laboratory of Advanced Control and Optimization for Chemical Processes, Ministry of Education, East China University of Science and Technology Shanghai, China.
authors: Yu
authors: Y
authors: Zhang
authors: Key Laboratory of Advanced Control and Optimization for Chemical Processes, Ministry of Education, East China University of Science and Technology Shanghai, China.
authors: Xingyu
authors: X
authors: Wang
authors: Riken Brain Science InstituteWako-shi, Japan; Systems Research Institute of Polish Academy of SciencesWarsaw, Poland; Skolkovo Institute of Science and TechnologyMoscow, Russia.
authors: Andrzej
authors: A
authors: Cichocki
keywords: brain-computer interface (BCI); dummy face; event-related potentials; facial expression; fusion stimuli; gaze-independent