Real-time EEG system

5.3.1.

Real-time EEG system

For this second experiment a commercially available consumer grade EEG monitoring device was chosen. The Muse brain-sensing headband (Figure 5.3.1) is a simple non-invasive 4-channel wireless EEG headset produced by InteraXon (InteraXon Inc.). The headset has seven dry sensors that go on the skin; two on the left and right of the forehead (AF7 and AF8), two behind the ears worn like spectacle frames, and three reference sensors in the middle of the forehead. The Muse is a low cost consumer device that can record and display real-time EEG with minimal preparation. Muse has online artifact detection of eye blinks, muscle tension (Thompson, Steffert, Ros, et al., 2008) or bad connections and freezes the signal when these are detected. The Muse can sample the EEG at 220 Hz or 500 Hz and can output the raw EEG, or filtered frequency bands at 10Hz, as well as providing three channels of accelerometer.

Figure 5.3.1: Muse EEG Headset from InteraXon Inc. This is a 4 channel dry electrode consumer grade Bluetooth EEG system.

5.3.2.

Sonifications of the Electroencephalogram

The free Muse software development kit (SDK), captures the Bluetooth data from the headset and uses a „Windows PowerShell‟ Script to send the open sound control (OSC) data over „User Datagram Protocol‟ (UDP) „localhost‟. to any compatible software. OSC is a networking protocol that originated in sound and music computing.

In Muse-io the „Preset 14‟ was selected, which outputs the EEG data at a sampling rate of 220 Hz at a bit depth of 10, with a Notch filter of 45 to 65 Hz inclusive. The PowerShell Script was:

“muse-io --device-search Muse-354B --osc osc.udp://localhost:5000 --preset 14 - -50hz”

The Muse calculates the relative band powers by dividing the absolute linear- scale power of a band by the sum of the absolute linear-scale powers in all bands and gives a value range from 0 to 1. The band powers are then averaged over 100 ms to return a value 10 times a second and sent to Pure Data (Puckette, 2002).

In Pure Data the Audio sample rate was set to 48,000 Hz and the relative alpha band (7.5 to 13 Hz) power from the left frontal electrode (AF7) of the Muse headset was then sonified in real-time and the EEG data and sound files were saved to disk.

was found in the previous experiment, only the 261.6 Hz (Middle C) carrier frequency was used in this Experiment.

For the AM sonification method, in Pure Data the Alpha power was linearly interpolated over 100 ms to up sample the EEG data to the Audio sample rate and multiplied by a sine carrier wave of 261.6 Hz (see the grey section „AM_AF7‟ in figure 5.3.2.1) to modulate the amplitude of the sine wave by the power of the Alpha EEG.

For the FM sonification method, the Alpha power was multiplied by 600 and the output added to by 261.6. The value was then linearly interpreted over 100 ms and sent to a „cosine wave oscillator‟ (osc~) to give an output frequency range of 261.6 to 861.6 Hz, (see the grey section „FM_AF7_261.6‟ in figure 5.3.2.1).

Figure 5.3.2.1: Shows the Pure Data patch used in experiment 2 for the real-time sonification of the EEG.

The Alpha EEG is received as an OSC message and the „Horseshoe‟ section in light blue, shows if there is a bad signal. The two light blue vertical sliders display the real-time Alpha power for AF7 (left) and AF8 (right). The grey section labelled „AM_AF7‟ computes the AM sonification and the grey box labelled „FM_AF7_261.6‟ generates the FM sonification. The „Timer‟ in the turquoise box, runs each trial for 180 seconds and the „File Name‟ in grey generates the unique filename that is used to save the files.

The „Sound Output‟ in the purple box, controls the output volume, the „Record Sound‟ records the sound output and the „Save_AF7_Alpha‟ saves the EEG data.

The following section will evaluate the sonifications output from this Pure Data patch. Figure 5.3.2.2, shows the Alpha EEG and the output of the AM sonification from participant 111 during a training trial, where they were trying to reduce their alpha activity by lowering the volume of the AM sonification.

Figure 5.3.2.2: Shows the original Alpha EEG from participant 111, on the top. The sound output from the AM sonification with the 261.6 Hz modulation frequency in the middle and the upper envelope of the sound, extracted using Hilbert transform, on the bottom.

As seen in figure 5.3.2.2, the Pearson‟s r correlation between the EEG and the upper amplitude envelope of the AM sonification was 0.918 with a p< 0.001 and the Spearman's Rho was 0.911 at p< 0.001.

Figure 5.3.2.3: Shows the Welch Power Spectral Density Estimate of the AM Sonification from participant 111 in trail 7, vertical grey line shows 261.6 Hz.

Figure 5.3.2.3, clearly shows the peak frequency of the AM modulation is 261.6 Hz, with no other extraneous spectral components. Thus this suggests that the Pure Data patch has performed adequately at producing a real-time AM sonification from the EEG data. The next section will present the evaluation of the FM sonification used in experiment 2.

Figure 5.3.2.4: Shows the original EEG data in black in the top subplot, the spectrogram (short-time Fourier transform) of the FM sonification with the 261.6 Hz modulation frequency in the middle and the FM demodulated signal in purple on the bottom.

Again as seen in figure 5.3.2.4 all the correlation measures are extremely high, with a Pearson‟s r correlation between the EEG and the FM demodulated signal of the FM sonification of 0.912 at p< 0.001 and Spearman's Rho of 0.907 with a p< 0.001.

Figure 5.3.2.5: Shows the Welch Power Spectral Density Estimate of the FM sonification from F3 with the 261.6 Hz modulation frequency. The blue box shows the maximum output frequency range of 261.6 to 861.6 Hz of the FM sonification.

In figure 5.3.2.5 the output frequency range of the sonification is from 261.6 to around 450 Hz. This output range is dependent on the amplitude of the Alpha EEG that each participant produces and normalized relative alpha measure was taken from the Muse headset with a range between 0 and 1.

Thus looking at both figures 5.3.2.4 and 5.3.2.5 of the FM sonification, again it can be concluded that the Pure Data patch has performed an adequate job of the FM modulation. But for participant 111 in trail 7 the potential full frequency range of the sonification was not used because they had a relatively low amplitude of Alpha EEG.