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 127bit 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.

Objective. Previous human steady state visual evoked potential (SSVEP) experiments have yielded different results regarding the range of stimulus frequencies in which period doubling (PD) behavior is observed. This study aims at obtaining experimental and statistical data regarding the frequency range of PD generation and also investigates other characteristics of PD. Approach. In two sets of experiments, seven subjects were presented a sinusoidal flickering light stimulus with frequencies varying from 15 to 42 Hz. To observe the short term variations in PD generation, another set of 5 successive experiments were performed on five subjects with 10 min breaks in between. To obtain the SSVEP responses, filtering, signal averaging and power spectral density (PSD) estimation were applied to the recorded electroencephalogram. From the PSD estimates, subharmonic occurrence rates were calculated for each experiment and were used along with ANOVA for interpreting the outcomes of the short term repeatability experiments. Main results. Although fundamental (excitation frequency) and second harmonic components appear in almost all SSVEP spectra, there is considerable inter-subject and intra-subject variability regarding PD occurrence. PD occurs for all stimulus frequencies from 15 to 42 Hz when all subjects are considered together. Furthermore, the statistical analyses of short term repeatability experiments suggest that in the short term, PD generation is consistent when all frequencies are considered together but for a single frequency significant short term differences occur. There also is considerable variation in the ratio of subharmonic amplitude to fundamental amplitude across different frequencies for a given subject. Significance. Important results and statistical data are obtained regarding PD generation. Our results indicate that modeling studies should attempt to generate PD for a broader range of stimulus frequencies. It is argued that SSVEP based braincomputer interface applications would likely benefit from the utilization of subharmonics in classification.

Objective. This study aims at investigating the potential mechanism of period doubling (PD) (subharmonic generation) in human steady-state visual evoked potentials (SSVEPs) by using a mathematical model. Approach. Robinson's Corticothalamic Model, which includes three main neuronal populations (cortical, thalamic reticular, and thalamic relay neurons) was employed. SSVEP experiments were simulated using this model and dependence of PD behavior in relation to the values of model parameters was investigated. The feedback loop in the model that is responsible for the generation of subharmonic components was thus identified, and this loop was isolated from the rest of the model and analyzed with a describing function approach. Main Results. It has been found in general, for a wide range of parameter values, that if the excitation frequency or half of it is close to the native oscillation frequency of the system, the native oscillation ceases to exist and oscillations at either the excitation frequency or half of it are observed. This observation is in line with the experimental findings except for some discrepancies which are also discussed. The intrathalamic feedback loop is identified to be the potential source of subharmonic oscillations. When isolated from the rest of the model and simulated by itself, it has been found that this feedback loop can show a resonance phenomenon at the subharmonic frequency. By deriving a set of equations based on the necessary conditions for a resonance phenomenon, a semi-analytical method was developed by which one can predict the existence of subharmonic generation for a given set of parameters and stimulus frequency. Significance. This study is the first model-based investigation of the mechanism of subharmonic oscillations. The proposed semianalytical method can replace extensive time and memory consuming parameter sweep studies.

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