Effects of high stimulus presentation rate on EEG template characteristics and performance of c-VEP based BCIs

Objective: This study aims at investigating the effects of high stimulus presentation rates on the characteristics of c-VEP responses along with the change in performance depending on the stimulus presentation rate by utilizing a c-VEP based speller BCI. Approach: Twenty subjects participated in three different experiments with refresh rates of 60 Hz (E1), 120 Hz (E2) and 240 Hz (E3), where a 127-bit m-sequence is used. To observe the change in frequency content of c-VEP responses, PSD estimates of c-VEP responses were evaluated. Principal component analysis (PCA) was applied to observe how many distinguishable responses could be evoked with a 127-bit length m-sequence for three different refresh rates. Main Results: Average ITR and accuracy values are 85.87 bits min−1 and 92% for E1, 94.21 bits min−1 and 97% for E2, and 78.65 bits min−1 and 87% for E3 respectively. The c-VEP responses are band-limited although the bandwidth of the input signal significantly increases as the refresh rate increases. The spectral densities of c-VEP templates are concentrated on several frequency intervals, especially for E3, which eventually results in a target misclassification. PCA shows that the number of well distinguishable responses decreases with the increasing refresh rate. 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 may be prefered for low number of targets. Significance: This study mainly investigates the alterations in the characteristics of c-VEP responses according to the stimulus presentation rate which have never been investigated thoroughly before. Our results show that increasing refresh rate does not necessarily increase the overall performance of the system due to the changes in characteristics of c-VEP responses. Further applications and designs of a c-VEP based BCIs will benefit from the observations and results of this study.