Effects of High Stimulus Presentation Rate on c-VEP based BCIs

This study aims at investigating the eects of high stimulus presentation rates (refresh rate of the monitor) on a c-VEP based speller BCI. To this end, a 6×6 speller BCI was designed using a monitör with a refresh rate up to 240 Hz. Six subjects participated in three dierent experiments with refresh rates of 60 Hz (E1), 120 Hz (E2) and 240 Hz (E3), where a 127-bit m-sequence is used. Information transfer rate (ITR) and accuracy values were calculated for each experiment and subject. Power spectral density (PSD) estimates of c-VEP responses and results of principal component analysis (PCA) were evaluated. Average ITR and accuracy values for E1 are 84.24 bits/min and 91%, for E2 are 90.68 bits/min and 95% and for E3 72.16 bits/min and 82% respectively. Also 5 out of 6 subjects stated that E3, and 1 subject stated that E2 is the most comfortable experiment. The c-VEP responses are band-limited although the input m-sequence is a wide-band signal. The spectral densities of c-VEP templates are concentrated on several frequency intervals, especially for E3. This periodicity leads to target misclassification. PCA shows that only 73, 52, and 26 well distinguishable responses can be obtained with a 127-bit length m-sequence for E1, E2, and E3 respectively. Considering all results and observations, we suggest that 120 Hz refresh rate is best to use in BCIs with high number of targets whereas 240 Hz refresh rate is reasonable for low number of targets.