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dc.contributor.advisorİder, Yusuf Ziya
dc.contributor.authorBaşaklar, Toygun
dc.date.accessioned2018-09-14T10:26:35Z
dc.date.available2018-09-14T10:26:35Z
dc.date.copyright2018-09
dc.date.issued2018-09
dc.date.submitted2018-09-13
dc.identifier.urihttp://hdl.handle.net/11693/47882
dc.descriptionCataloged from PDF version of article.en_US
dc.descriptionThesis (M.S.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2018.en_US
dc.descriptionIncludes bibliographical references (leaves 54-62).en_US
dc.description.abstractObjective. Previous studies on code-modulated visual evoked potentials (c- VEP) have yielded important results regarding the performance of c-VEP based brain-computer interfaces (BCIs) in recent years. Since, speed is the key factor in BCI applications and since the monitor refresh rate limits the stimulation time and thus limits the performance of the system, this study aims at investigating the effects of high stimulus presentation rates (refresh rate of the monitor) on a c-VEP based speller BCI. Furthermore, Robinson's corticothalamic model, which has not yet been studied for c-VEP responses, is used to simulate the salient behaviors that are observed in our experiments. Approach. Six 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. Canonical Correlation Analysis (CCA) was used in the training stage to obtain 36 target templates from 100 averages of 8 EEG channels. Information transfer rate (ITR) and accuracy values were calculated for each experiment and subject. Subjects also answered a questionnaire asking at which refresh rate they felt more comfortable. Robinson's corticothalamic model was used to simulate the c-VEP experiments. Power spectral density (PSD) estimates of c-VEP responses and results of principal component analysis (PCA) were evaluated both for the simulation data and the experimental data. Main Results. Average ITR and accuracy values for E1 are 86.17 bits/min and 93%, for E2 are 90.68 bits/min and 95% and for E3 are 70.89 bits/min and 81% 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. The results from simulations shows great similarity with the results from experiments. 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. Results from modeling study suggest that the response of the visual system to the high frequency components in the input at higher refresh rates tends to diminish. Significance. Important results are obtained regarding characteristics of c-VEP responses and the effects of high refresh rates on c-VEP based BCIs. Robinson's corticothalamic model is found to be capable of explaining some of the salient behaviors in the experiments and this could be a basis for practical studies on improving the performance of c-VEP paradigm.en_US
dc.description.statementofresponsibilityby Toygun Başaklar.en_US
dc.format.extentxv, 62 leaves : illustrations ; 30 cm.en_US
dc.language.isoEnglishen_US
dc.rightsinfo:eu-repo/semantics/openAccessen_US
dc.subjectEEGen_US
dc.subjectCode-Modulated Visual Evoked Potentialen_US
dc.subjectc-VEPen_US
dc.subjectPrinciple Component Analysisen_US
dc.subjectPCAen_US
dc.subjectMonitor Refresh Rateen_US
dc.subjectBrain-Computer Interfaceen_US
dc.subjectBCIen_US
dc.subjectCorticothalamic Modelen_US
dc.subjectMathematical Cortex Modelen_US
dc.titleExperimental and model based investigation of the effects of high stimulus presentation rate on code-modulated visual evoked potential based brain-computer interfacesen_US
dc.title.alternativeKod-modülasyonlu görsel uyarılmış potansiyel tabanlı beyin-bilgisayar arayüzlerinde yüksek uyaran gösterimi hızlarının etkilerinin deneysel ve model tabanlı incelemesien_US
dc.typeThesisen_US
dc.departmentDepartment of Electrical and Electronics Engineeringen_US
dc.publisherBilkent Universityen_US
dc.description.degreeM.S.en_US
dc.identifier.itemidB158953
dc.embargo.release2019-03-15


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