Bichromatic visual stimulus with subharmonic response to achieve a high-accuracy SSVEP BCI system with low eye irritation

Abstract

ObjectiveWhile various Steady State Visual Evoked Potential Brain-Computer Interfaces (SSVEP BCI) with very high performance have already been developed, eye irritation is a significant burden in their practical use. Generally, there is a tradeoff between eye irritation and BCI accuracy. However, this study proposes a bichromatic visual stimulus with low eye irritation while providing accurately detectable electroencephalogram responses. MethodsThe proposed visual stimulus, which is high-frequency in terms of luminance but with a subharmonic color component, was compared to the conventional high-frequency SSVEP on 16 subjects. Canonical Correlation Analysis, Least Absolute Shrinkage and Selection Operator, Multivariate Synchronization Index (MSI), and Extended MSI were used separately to recognize aimed visual stimuli in a 4-target BCI system. ResultsThe proposed stimulus resulted in significantly (p-value = 0.024) higher accuracy (96 ± 4.1%) compared to the reference (87.1 ± 14.6%) for a processing window of 4.5 s using the Extended MSI algorithm. Eye irritation scores (3.8 ± 1.0 on a 1–9 scale) were also significantly lower (p-value = 0.035) than that of the reference stimulus (4.3 ± 1.1). No cases of illiteracy were observed with the proposed waveform, whereas there were 2 with the reference stimulus. ConclusionThe proposed bichromatic waveform provided significantly higher accuracy, especially for users who do not have strong high-frequency SSVEP responses. Eye irritation was also significantly lower than with conventional reference visual stimulus. The proposed visual stimulus can generate electroencephalogram responses, not only in the high-frequency range but also at half the main frequency. SignificanceThe results are significant for realizing practical, accurate SSVEP BCI systems.