Different lengths of the window and lengths of overlaps were investigated to allow for the signal processing and ensuing cooperative task to be performed in real-time. To try to compensate for a longer delay due to longer window length, the number of overlapped samples between the previous and next DFT was increased. In particular, we considered windows of length 440 samples with an overlap of 330 samples (110 new EEG samples added to each window). Thus, with a sampling frequency of 220 Hz, the length of each window being Fourier Transformed was two seconds long. The BCI system updated each time the 110 new EEG samples were added; new classification of EEG signals twice every second due to a sampling frequency of 220 Hz.
The EEG signals acquired during concentrated and relaxed states were measured and observed, with general EEG phenomology agreeing with the literature [5]. The two-second-long segments of the EEG signals for both states were plotted and shown in Figure 9. While the subject is relaxed, the alpha EEG frequency band is expected to be dominant in the signal. Note there were about 19 large oscillations within 2 second period. Peaks at around 10 Hz was
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was relaxed, the signal while the subject was focused had much larger high frequency contents. Magnitude of the DFT of these two signals are shown in Figure 9.
Figure 9: Segments of EEG signal while the subject is under relaxed and focused states are plotted. Higher frequency contents are expected while the subject is focused. Lower frequency contents are expected while the sujbect is relaxed. Compared with the spectrum with the relaxed subject, the spectrum while subject is focused has higher powers at frequencies higher than 30 Hz. There is also high power at around 10 Hz in the spectrum while subject is relaxed.
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More intense gamma activities were observed in the spectrum while the subject is focused as compared to the spectrum of the relaxed subject. Previous findings on gamma activities in the frontal lobe inform that the subject’s state of mind is under concentration, focused attention, or learning [5]. The levels of gamma activities varied every time the headsets were newly put on. We speculated that there exists a subject varing electrode position where the noise of the EEG signals is minimized and that the signal to noise ratio can vary depending on where the electrodes are placed. Thus, it was impractical to record systematic numerical responses for each mental states; The power of each frequency bins varied each time the
headsets were newly worn. As expected from the number of oscillations in the EEG signal while subject is relaxed, there was a peak at around 10 Hz in the spectrum. These two observeations in each electrode were used as the feature variables in the classification algorithm.
In order to visualize how the power spectral density varied with time, we analyzed spectrograms: As before, each spectrogram utilized a Hamming window, with a sliding window length of 440 samples, and the overlap of 330 samples. Figure 10 shows spectrograms while the subject was in a focused state. Figure 11 shows spectrograms while the subject was in a relaxed state. Each figure has two spectrograms, one from the EEG recordings on the right side of the frontal lobe (Electrode 1) and the other from the left side of the frontal lobe (Electrode 2).
Qualitatively, there is minimal difference in the spectrograms across the two different electrodes. There are more activities in the right hemisphere during comparison task and in the left
hemisphere during the multiplication and bilateral during subtraction task [18]. In this regard, it is worth noting that the sources of stimulating concentration were randomly generated
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quantitavely observed in our data. This was mainly due to the poor contact between the scalp and the electrodes.
Figure 10: Spectrogram of the EEG signals while the subject was focused with Hamming window with window length of 440 samples and overlap of 330 samples. The high gamma power is observed at frequencies above 30 Hz at most time interval. The stripes in the spectrogram and the irregular high powers at low frequencies were artifacts from the data
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Figure 11: Spectrogram of the EEG signals while the subject was relaxed with Hamming window with window length of 440 samples and overlap of 330 samples. Most of the power at frequencies above 30 Hz at most time interval were realatively low. High power at around alpha frequencies were observed for most time interval. The high power artifacts
at low frequencies were low compared to the the Figure 10.
In the spectrograms while the subject was relaxed, we observed relatively high alpha activities on the frontal lobe. This narrow band-like behavior at around 10 Hz was systematic across all the spectrograms while the subject was relaxed. Much higher gamma activities (> 30 Hz) were observed during concentration. In an effort to characterize delays in switching from one brain state (focused/relaxed) to the other, the subject was told to switch states whenever a beep sound was heard (beep at t = 40, t = 90, t = 120, t = 170, and t = 200). The result of this experiment is depicted in Figure 12.
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Figure 12: Spectrogram of the EEG signals while the subject was relaxed and focused for different intervals with Hamming window with window length of 440 samples and overlap of 330 samples The intervals colored in grey in the
spectrograms were the intervals while the subject was told to relax. High power at around 10 Hz and low powers at
frequencies higher than 30 Hz were observed in these intervals.
From t = 0 to t = 40, t = 90 to t = 120, and t = 170 to t = 200 seconds, the subject was told to be relaxed. During the time intervals in between when the subject was relaxed, the subject was in a focused state. Bands at around 10 Hz were observed while the subject was relaxed. The gamma activities during focused states were much higher than the gamma activities during relaxed states. The powers from 22 Hz to 110 Hz were summed to observe total beta and gamma activities in the frontal lobe. For this specific dataset, the summed power when the subject was relaxed averaged at around 5000 for both electrodes. The summed power when the subject was concentrating averaged at around 9000.
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The Weber contrasts were higher for the relaxed state, and lower, negative number for the concentrated state. Referring back to equation 13, the contrast is expected to be high and positive when the subject is relaxed because the power at the alpha band are large. The numerator of equation 13 is large compared to denominator. However, while the subject is concentrating, the power at the alpha bands is low relative to the adjacent frequency bands. The numerator of equation 13 becomes negative (refer to Figure 13).