Browsing by Subject "Steady state visual evoked potential"

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    A model based investigation of the period doubling behavior in human steady-state visual evoked potentials
    (IOP, 2019-07) Tuncel, Yiğit; Başaklar, Toygun; İder, Yusuf Ziya
    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 isolatedfrom the rest of the model and analyzed with a describingfunction approach.Main Results.It has beenfound in general, for a wide range of parameter values, that if the excitationfrequency 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 exceptfor some discrepancies which are also discussed. The intrathalamic feedback loop is identified to be the potential source of subharmonic oscillations.When isolatedfrom 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 conditionsfor a resonance phenomenon, a semi-analytical method was developed by which one can predict the existence of subharmonic generationfor a given set of parameters and stimulusfrequency. 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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    Period doubling behavior in human steady state visual evoked potentials
    (Institute of Physics Publishing, 2018) Tuncel, Yiğit; Başaklar, Toygun; İder, Yusuf Ziya
    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 brain-computer interface applications would likely benefit from the utilization of subharmonics in classification.