User discomfort in SSVEP-based BCIs - can modulation depth offer a solution?

Steady-state visual evoked potential (SSVEP)-based brain-computer interfaces (BCIs) enable people to control external devices by focusing on repetitive visual stimuli (RVS). However, RVS can be visually annoying and cause user discomfort. To address this issue, this study proposes subtle flickering RVS to enhance visual comfort with SSVEP-based BCIs. Previous research on mitigating this discomfort has explored various stimuli characteristics, such as frequencies, colors, and patterns, but some studies are contradicting and have their limitations. This study investigates whether modulation depth (MD) can reduce user discomfort when using LCD-displayed stimuli. Specifically, MDs ranging from 1 down to 0.0125 and their effect on SSVEP response and BCI performance have been computed. Results demonstrate that stimuli with a 0.5 MD achieve accuracy and information-transfer rate (ITR) performance of 80.06% and 41.6 bpm which are at par with a baseline BCI having stimuli with a MD of 1, having 79.98% accuracy and 41.08 bpm ITR. These findings demonstrate that a lower flicker annoyance may be obtained with stimuli having a MD of 0.5. By reducing user discomfort, the proposed subtle flickering RVS can improve user experience without significant degradation in performance, potentially increasing the adoption and usability of SSVEP-based BCIs.