Browsing by Subject "SSVEP"
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Item Open Access Design and development of an SSVEP based low cost, wearable, and wireless BCI system(Bilkent University, 2019-08) Waheed, AbdulIt has become a challenging research topic to design and develop cheap and wearable brain-computer interface (BCI) systems but not compromising the performance. In this thesis, the design and development of a steady state visually evoked potential (SSVEP) based BCI system has been presented which is a low cost, wearable BCI system and gives highly accurate target identi cations with good information transfer rate (ITR). It is a battery powered, wireless BCI system and ensures the complete isolation to the subject. Like all the BCI systems, it is designed and implemented in ve major parts: (i) stimulator which is a microcontroller based circuit and provides the frequency modulated visually evoked potential (f-VEP) and code-modulated visually evoked potential (c-VEP) stimulations (ii) dry active electrodes which capture the electroencephalography(EEG) signals from the O1, O2, and Oz head positions (iii) high sampling rate, 4-channel EEG data acquisition hardware which acquires the EEG signals, amplify them, converts them to digital data, and transmits the data using wi communication (iv) the data processing unit (DPU) which is a MATLAB script to process the raw EEG data and displays the results and (v) the headset which mounts all the components except DPU and is developed using 3D printing technology. The rst prototype of the proposed BCI system has been developed in 331 USD and tested for both the f-VEP and c-VEP modalities on six human subjects. For f-VEP modality, it exhibits an average accuracy (live accuracy) of 92.1% and average ITR (live ITR) of 69.5 bits/min on the basis of target identi cations done on 1:04 s data recordings. If we extract one message character from ve consecutive target identi cations, the average message accuracy goes to 98.8% and average message ITR to 17.2 bits/min. In case of c-VEP modality, it exhibits live accuracy of 70.1 % and live ITR of 23.5 bits/min while message accuracy of 90.7 % and message ITR of 12.4 bit/min.Item Unknown Experimental and model based investigation of period doubling phenomenon in human steady state visual evoked potential responses(Bilkent University, 2018-07) Tuncel, YiğitObjective. 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. There also is lacking information about the consistency and repeatability of the occurences of subharmonic oscillations. The neural mechanism of such oscillations have also not been explored. To elaborate these rather unknown aspects of the PD behavior in SSVEP responses, an experimental and model based approach has been taken. Approach. The experimental side of the study aims at obtaining experimental and statistical data regarding the frequency range of PD generation and also investigates other characteristics of PD. In two sets of experiments, seven subjects were presented a sinusoidal flickering light stimulus with frequencies varying from 15 Hz to 42 Hz. To observe the short term repeatability in PD generation, another set of 5 successive experiments performed on five subjects with 10-minute breaks in between. To obtain the SSVEP responses, filtering, signal averaging and Power Spectral Density (PSD) estimation were applied to the recorded EEG. From the PSD estimates, Subharmonic Occurrence Rates (SORs) were calculated for each experiment and were used along with ANOVA for interpreting the outcomes of the short term repeatability experiments. The model based side of the study aims at explaining the observed phenomena in mathematical terms. For this purpose, Robinson’s Corticothalamic Model was implemented in both C and Simulink. The experimental procedure was reanimated on the model and the subharmonic generation in the model depending on different values for parameters was observed. The feedback loop that is responsible for the generation of subharmonic components was identified in the model, and this loop was isolated from the rest of the model and further analyzed with a describing function approach. Main Results. The experimental results showed that 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 Hz 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. The modelling results showed that the subharmonic oscillations in the model are of resonance nature and that they can be obtained virtually in any frequency interval depending on the values of the parameters in the system. The intra-thalamic feedback loop in the model is identified to be the potential source of subharmonic oscillations in the system output. When isolated from the rest of the model and examined 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 containing the necessary conditions for this resonance phenomenon, a semi-analytical method by which one can find the existence of these oscillations has been developed. Significance. From the experimental studies, important results and statistical data are obtained regarding PD generation. Our results indicate that modelling studies should attempt to generate PD for a broader range of stimulus frequencies by adjusting the parameter values. It is argued that SSVEP based BCI applications would likely benefit from the utilization of subharmonics in classification. Our modelling study is the first to investigate the source of subharmonic oscillations on a mathematical brain model. An experimental verification of the potential origin of such oscillations, which was identified to be the intra-thalamic loop, would be an important work. The proposed semi-analytical method could potentially be used to speed up a future parameter sweep study. We observed that in the current model alpha oscillation and subharmonic oscillations are in some way interrelated and they can not be generated together for any stimulation frequency. This is referred to as alpha entrainment, and is visible only for some stimulation frequencies in experimental results. Thus, we claim that the model is insufficient in explaining the PD phenomenon in SSVEP responses.Item Unknown 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 ZiyaObjective.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.Item Unknown Period doubling behavior in human steady state visual evoked potentials(Institute of Physics Publishing, 2018) Tuncel, Yiğit; Başaklar, Toygun; İder, Yusuf ZiyaObjective. 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.