Page 141 - Read Online

Page 4 of 15 Li et al. Soft Sci 2023;3:22 https://dx.doi.org/10.20517/ss.2023.11

Impedance measurement
Before impedance measurement, the forehead and mastoid areas (A1 and A2) were cleaned with Nuprep
skin preparation gel. PMA sponge electrodes were placed on the specified locations according to the
standard 10-10 EEG system, and two solid gel electrodes were placed on A1 and A2 as the reference and
ground electrodes. The contact impedance spectrum was measured on the Autolab PGSTAT204 (Metrohm,
Switzerland) in the range of 1 Hz to 1000 Hz and the Brain Vision Recorder software (Brain Products
GmbH, Munich, Germany) at 15 Hz.

EEG recording and analysis
EEG signals were recorded from PMA sponge electrodes integrated on the Meta Quest 2 headset, fitted with
a Meta Quest 2 Elite Strap (Meta, California, USA). All analysis was performed using custom MATLAB
[37]
scripts and functions from EEGLAB .
Eye close vs. open comparison
EEG signals were recorded using the Brain Vision LiveAmp 32 amplifier and the Brain Vision Recorder
software (Brain Products GmbH, Munich, Germany) and analyzed using custom MATLAB scripts. The
experimenter recorded triggers on the Brain Vision Recorder software while giving instructions to the
subject. The EEG signals were initially filtered using a bandpass filter ranging from 1.5 Hz to 50 Hz, and the
alpha band was defined as 8 to 12 Hz.

VR experiment analysis
Three healthy male subjects aged 23-28 participated in the CNV recording experiment. The VR task was
designed and implemented in Unity (editor version 2020.3.11f1) and was conducted using the Meta Quest 2
headset and controllers. The grand average analysis and decoding analysis were performed using signals
from the Cz channel in MATLAB. The signal was preprocessed by bandpass filtering 0.1 to 1 Hz using a
fourth-order Butterworth filter. Baseline correction was applied using a 100 ms window prior to Stimulus 1.
Trials with an absolute sample value exceeding 50 μV were defined as artifactual and rejected from further
analysis. For the decoding analysis, the EEG time samples between the two stimuli (-4.3 s before Stimulus 2
to Stimulus 2 onset) were extracted after bandpass filtering from 0.01 to 3 Hz. The extracted samples were
used as features for the decoder after min-max normalization to values between 0 and 1. The decoding
accuracy was computed using leave-one-out block-wise cross-validation from five blocks, using diagonal
linear discriminant analysis (LDA).

RESULTS AND DISCUSSION
The design of our sponge electrode integrated VR headset for simultaneous EEG recording and VR display
is illustrated in Figure 1A. The sponge electrodes were integrated onto the VR headset using a custom
flexible connector array (FCA) that runs along the headset straps. Once the user wears the VR headset, the
electrodes, and FCA are concealed from view. The FCA provides a soft and stable electrical connection
between the electrodes and the recording system, and it is also compatible with the VR headset. In contrast,
previous VR headset-integrated EEG electrodes typically rely on rigid stud connectors that have to puncture
the strap to establish a connection with the recording system . Moreover, the FCA enables the sponge
[17]
electrodes to be distributed across various scalp locations, allowing EEG recording from multiple brain
regions, including frontal, frontocentral, central, centroparietal, and occipital regions. Notably, the headset
straps can apply pressure on the sponge electrodes, enabling them to deform and make better contact with
the hairy scalp, thereby reducing electrode impedance. Figure 1B shows a photograph of the PMA sponges,
which are 12 mm in diameter and 10 mm in height. Figure 1C shows a photograph of the FCA, comprising
nine copper O-ring connectors with seamless copper traces. The entire system is fabricated on a 25 μm-
thick polyethylene terephthalate (PET) substrate that can bend to conform to the headset straps, ensuring