title: The cost of space independence in P300-BCI spellers.
abstract: BACKGROUND
  Though non-invasive EEG-based Brain Computer Interfaces (BCI) have been researched extensively over the last two decades, most designs require control of spatial attention and/or gaze on the part of the user.
  
  
  METHODS
  In healthy adults, we compared the offline performance of a space-independent P300-based BCI for spelling words using Rapid Serial Visual Presentation (RSVP), to the well-known space-dependent Matrix P300 speller.
  
  
  RESULTS
  EEG classifiability with the RSVP speller was as good as with the Matrix speller. While the Matrix speller's performance was significantly reliant on early, gaze-dependent Visual Evoked Potentials (VEPs), the RSVP speller depended only on the space-independent P300b. However, there was a cost to true spatial independence: the RSVP speller was less efficient in terms of spelling speed.
  
  
  CONCLUSIONS
  The advantage of space independence in the RSVP speller was concomitant with a marked reduction in spelling efficiency. Nevertheless, with key improvements to the RSVP design, truly space-independent BCIs could approach efficiencies on par with the Matrix speller. With sufficiently high letter spelling rates fused with predictive language modelling, they would be viable for potential applications with patients unable to direct overt visual gaze or covert attentional focus.
authors.affiliation: Department of Clinical Neurosciences, University of Cambridge, Cambridge, UK. sc672@cam.ac.uk
authors.forename: Srivas
authors.initials: S
authors.lastname: Chennu
authors.forename: Abdulmajeed
authors.initials: A
authors.lastname: Alsufyani
authors.forename: Marco
authors.initials: M
authors.lastname: Filetti
authors.forename: Adrian M
authors.initials: AM
authors.lastname: Owen
authors.forename: Howard
authors.initials: H
authors.lastname: Bowman
mesh_terms: D000328:Adult; D062207:Brain-Computer Interfaces / Q000191:economics; D004569:Electroencephalography; D005071:Evoked Potentials / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D055815:Young Adult
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