Experimental and model based investigation of the effects of high stimulus presentation rate on code-modulated visual evoked potential based brain-computer interfaces
Author
Başaklar, Toygun
Advisor
İder, Yusuf Ziya
Date
2018-09Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Objective. 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.
Keywords
EEGCode-Modulated Visual Evoked Potential
c-VEP
Principle Component Analysis
PCA
Monitor Refresh Rate
Brain-Computer Interface
BCI
Corticothalamic Model
Mathematical Cortex Model