Effects of monitor refresh rates on c-VEP BCIs

Abstract

Brain-Computer Interfaces (BCIs) allow humans to form a physical symbiosis with computer systems. One use case scenario of BCIs is communication by brain activity. High spelling speeds have been achieved with BCIs based on code-modulated visual evoked potentials (c-VEPs). Typically, the flickering stimuli are presented on a standard 60 Hz monitor. Users can find VEP-based BCIs annoying and tiring due to the perceptible flickering. This is especially the case for multi-target systems designed for maximal communication speed. Higher monitor refresh rates allow a faster flickering rate for BCI targets, and thus a more subtle visual stimulation. In this paper, user friendliness and speed of c-VEP BCIs with different monitor refresh rates (60, 120 and 200 Hz) are compared. The experiment was comprised of three sessions (each consisting of training and spelling stages), one for each tested monitor refresh rate. Performance was assessed with ITR and accuracy and user friendliness was evaluated using a questionnaire. High flickering speed is usually accompanied by poorer BCI performance. In this study, the system utilizing the 200 Hz refresh rate surprisingly competed well in terms of ITR and accuracy. Regarding user friendliness it was preferred by most users, as expected.