Multi-colour stimuli to improve information transfer rates in SSVEP-based brain-computer interfaces

Most brain-computer interface (BCI) systems based on visual evoked potentials (VEPs) rely on the use of a number of flashing stimuli with differences in their frequency or phase characteristics. These repetitive visual stimuli give rise to oscillatory electroencephalographic (EEG) activity which is correlated with the characteristics of the observed stimuli, allowing for target discrimination. The colour of the visual stimuli is also known to affect the elicited steady state VEPs (SSVEPs), and a number of BCI studies have investigated which colours give rise to the strongest SSVEPs. However, none of these studies have considered the possibility of identifying which visual target a user is observing based on the colour of the stimulus. In this work, the use of stimulus colour as a discriminative characteristic for SSVEP-based BCI systems is considered and it is shown that by introducing differently coloured targets in SSVEP setups, the information transfer rate (ITR) of such systems can be enhanced. The common spatial patterns (CSP) and analytic CSP (ACSP) methods were used for feature extraction, and for both methods the results obtained indicate that differently coloured stimuli can be discriminated and used to improve the information transfer rate (ITR) in BCIs.