IEEE Systems, Man and Cybernetics Magazine - July 2020 - 24

three 30-s stages: 1) open eyes, 2) presentation of a blinking sphere at 12.5 Hz, and 3) closed eyes. The signal-tonoise ratio (SNR) was computed for the stimulus
frequency (12.5 Hz) from the power spectrum as the ratio
of the power at the stimulus frequency over the mean
power of spectral components, 2 Hz over and under the
stimulus frequency, for the three electrodes located in
positions O1, Oz, and O2 [13]. Figure 5 presents the distributions of the average SNR values for each of the three
stages. As expected, the SNR for the SSVEP is higher
than for the other two conditions, thus validating the
EEG modality.
Validation of the ECG Signal
In our experiments, the ECG electrode was placed on
the left collarbone (see Figure 4). Although this location

differs from standard ECG leads, it was shown to provide accurate R peaks, thus allowing us to calculate
HR and HR variability (HRV). Such metrics have been
used often to measure different mental states, particularly in VR [35]. To validate the accurate registration of
the R peaks, a research-grade ECG monitor (Zephyr
Bioharness3 chest band) was used alongside the developed BMI-HMD, and both ECG signals were simultaneously acquired for 10 min. Figure 6 depicts an 8 -s
segment of the ECG obtained via the collarbone sensors and the reference ECG signal. As can be seen,
highly accurate cardiac measurement can be achieved,
and the correlation between the instantaneous HR
derived from both sensors was 0.996, thus validating
the ECG modality.
Validation of the EOG Signals
In this validation scenario, we used the acquired signals
from the seven flat electrodes placed in the face plate of
the HMD, as depicted in Figure 3(b). Here, we were interested in identifying the direction of the eye movements
and blinks. Eye blinks have been shown to be useful in
predicting cybersickness [23], whereas eye gaze detection could be particularly useful for foveated applications
and gaze-adaptive content [36]. For this validation, users
were asked to follow a target in the virtual environment
for 5 min. The target moved from the center of the FOV to
one of eight positions: right, right-up, up, left-up, left, leftdown, down, and right-down; each position was repeated
12 times.
Figure 7 depicts the average response over the 12 trials
for each of the seven electrodes, for each of the eight eye
gaze positions as well as the average of a series of
eye blinks. As can be seen in Figure 7(b), the acquired
waveforms present different patterns depending on the

Figure 4. The final BMI-HMD system encompassing

16 multimodality sensors and a wireless amplifier
module attached to the top of the HMD straps.

Relative Frequency

0.8
0.6

Open Eyes
SSVEP 12.5 Hz
Closed Eyes

0.4
0.2
0
-25

0

-20

-15 -10 -5
0
SNR (dB)

5

10

15

Figure 5. The distributions of the combined SNR values

for the O1, Oz, and O2 electrodes for the eyes-open
(yellow), eyes-closed (blue), and SSVEP (red) stages.
24	

IEEE SYSTEMS, MAN, & CYBERNETICS MAGAZINE Ju ly 2020

1

2

3

4
5
Time (s)

ECG Reference

6

7

8

ECG Collarbone

Figure 6. The ECG signals for the reference (blue)

and prototype (red) devices. The locations of
detected R peaks are indicated by circles. The y-axis
units are arbitrary.



IEEE Systems, Man and Cybernetics Magazine - July 2020

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