Enhancing user experience in c-VEP-based BCI: Effects of visual stimulus opacity on performance and visual fatigue

Abstract

Brain-computer interfaces (BCI) based on code-modulated visual evoked potentials (c-VEP) enable users to control devices through brain activity. These systems typically employ black and white flashes encoded via pseudo-random binary sequences to synchronize brain responses with specific commands. However, this traditional encoding often induces visual fatigue in users. Additionally, the sharp contrast of black and white commands can obscure the background on which they are displayed, complicating the integration of this technology in dynamic environments. Using semi-transparent stimuli could address these issues by reducing eyestrain and enabling compatibility with diverse backgrounds. However, the impact of opacity in c-VEP stimuli remains unexplored. This study aims to assess how varying visual stimulus opacity influences system accuracy and user experience. Six different opacity and background combinations were tested with ten healthy participants, who rated visual fatigue on a scale from 0 (none) to 10 (extreme) after each condition. Results showed that traditional encoding achieved 100% accuracy but induced high fatigue (6.4 points). A configuration with 100% opacity for black and 50% for white maintained high accuracy (99.38%) while reducing fatigue to 3.7 points. Brain responses were consistent when both black and white stimuli were present, but patterns changed when one color was omitted. Spatial filters revealed stable c-VEP decoding from the parieto-occipital cortex, with slightly higher activation in low-contrast conditions. The findings from this study suggest that adjusting the opacity of stimuli in c-VEP-based BCI can optimize the balance between performance and user experience. Implementing a reduction in opacity not only improves visual fatigue but may also facilitate the integration of c-VEP systems into lifelike environments.