Biomedical Engineering Reference
In-Depth Information
the exposure to oscillating light. For their participation in the experiment participants
received a small reward.
Experiment was performed in a laboratory where only artificial light was present (the
light screens were closed). The room was not specially shielded or controlled against
environmental noise (to resemble real-life situation). Participants sat in a comfortable
chair, at a distance of roughly 60cm from the screen. The RVS was rendered using an
LED panel with 4 LEDs positioned at its corners, measuring 25cm on the diagonal. The
LEDs were switched on and off simultaneously. The experimenter had an extensive
experience in EEG measurement and analysis.
The study comprised two sessions per participant. Each session lasted for about two
hours and consisted of a preparation segment, dry electrode evaluation segment, water-
based electrode evaluation segment, gel electrode evaluation segment, and debriefing
segment. The order of evaluation segments was chosen to enable testing of all three
setups in a single session. Having to remove the gel after the usage of gel electrodes, or
to dry the hair after the usage of water-based electrodes in a different order of segments,
would make the study design more complex and lengthy. Also this stresses further the
impracticality of especially gel solutions for daily applications.
During the preparation segment the procedure was explained to the participants and
the three setups were shown to them. The participants then filled in the questionnaire ex-
pressing their perception of different electrode types and EEG acquisition systems that
were used in the investigation. This procedure was only done during the first session.
Dry electrode evaluation segment consisted of positioning the textile patch with dry
electrodes on participant's head. The experimenter visually inspected the EEG signal
quality (high-pass filtered at 1Hz) and in case of no signal, larger impact of noise,
and/or severe presence of artifacts in some of the channels, the headband was adjusted
to improve the contact and achieve better EEG signal quality level. A chronometer was
used to record the time required for this activity. Then the EEG signal was recorded
while the participant was focusing on one of the
4
LEDs oscillating at
28
,
32
,
36
,and
40
Hz in one of the runs and
12
,
15
,
18
,and
21
Hz in the other run. The order of these
runs was randomly selected for both sessions. This procedure was repeated for all 4
frequencies per run, each having segments of 5 seconds where LEDs were switched on,
interspersed with segments of 4-6 seconds (randomized) where LEDs were switched
off. The participants were instructed not to blink during the segments while the LEDs
were on. The recorded EEG data, sampled at 2048Hz, for all frequencies were stored.
At the end of this segment the headband with electrodes was removed.
For the water-based electrode evaluation segment, the experimenter soaked the water-
based electrodes into the cup with tap water 5-10 minutes before the start of the setup.
The setup consisted in positioning the EEG cap on a participants head and attaching
the water-based electrodes to the EEG cap. In case the signal quality was not good
enough according to the experimenter, more water was added to some of the electrodes
and/or the electrode positioning was adjusted. The time required to perform this pro-
cedure was recorded (excluding the time required to soak the electrode in water before
the setup procedure). Then the EEG signal was recorded in two runs, the same way as
explained in the previous paragraph for the dry electrodes. After the recording the head
cap and the electrodes were removed.
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