1. Tripolar concentric ring electrode
electroencephalography using Signa gel for
impedance matching
Ivy Shen, Maciej Walkosz
Electrical Engineering
University of Rhode Island
Kingstown, Rhode Island
Ivy.x.shen.16@dartmouth.edu
Dr. Walter Besio
Electrical Engineering
University of Rhode Island
Kingstown, Rhode Island
Besio@ele.uri.edu
Abstract- Our goal is to develop a high-spatial resolution
and high signal-to-noise electroencephalography (EEG)
recording system. Conventional scalp EEG is contaminated by
noise represented by non-brain electrical activity such as
ocular artifacts, scalp muscle potentials, electrocardiogram,
etc.. Conventional disc EEG recordings also have reference
electrode problems.
To overcome the problems of the disc electrodes (Fig. 1. A)
we have developed the tripolar concentric ring electrode
(TCRE). The TCRE consists of three electrode elements - outer
ring, middle ring, and the central disc (Fig. 1, B). In the past
we have always used Ten20 paste to attach the TCREs and
match skin to electrode impedances. For many electrodes, say
64 or more, it would be time consuming to use Ten20 paste. In
this study we demonstrate that Signa gel on TCREs acts as an
effective means to match electrode impedances to the scalp.
We recorded physiological brain signals using Signa gel
and our 10 mm diameter TCREs. The EEG signals we
recorded with the TCREs (tEEG) originated from alpha wave
activity. A conventional disc electrode (CDE) next to the TCRE
was used to record EEG for comparison. We believe that 10
mm TCREs, and gel used for impedance matching, are
appropriate when many TCREs are needed for high spatial
sampling.
Keywords—electrode gel, tripolar concentric ring electrodes,
TCRE, alpha waves.
I. INTRODUCTION
Conventional scalp electroencephalography (EEG) is
contaminated by noise such as ocular artifacts, scalp muscle
potentials, electrocardiogram, etc., [31]. The noise can be as
large as, or even larger than, the brain potential of interest
[32], [33], [34]. Conventional disc electrode (CDE) EEG
recordings also have reference electrode problems [35].
Tripolar concentric ring electrodes (TCREs) have unique
capabilities which reduce the CDE problems. They perform
the second spatial derivative, the Laplacian, on surface
potentials. In previous experiments we have shown that
EEG recorded with TCREs (tEEG) has significantly better
spatial selectivity, signal-to-noise ratio, localization,
approximation of the analytical Laplacian, and mutual
information than EEG [18]-[20].
In the current study we tested the effectiveness of
recording alpha waves with 10 mm., 6.0 mm, and 4.0 mm
diameter TCREs using Signa gel. Next we describe in detail
our methods and results as well as a discussion about the
results and our conclusion.
II. METHODS
A. Subjects
Seven healthy subjects (four women) were recruited for
the experiments. The protocol was approved by the
University of Rhode Island Institutional Review Board
(IRB).
B. Electrode Attachment
Hair was moved aside to prepare for the electrode and
then NuPrep abrasive gel (D. O. Weaver, Aurora, CO) was
used to remove oil and dry skin. Four electrodes were
attached to the scalp. As shown in Fig. 2. one 10 mm, 6 mm,
or 4 mm diameter TCRE was placed on the occipital lobe in
the O2 area with Signa gel (Parker Labs) used for skin-to-
electrode impedance matching. The reference electrode was
a CDE attached with ten20 paste on the mastoid process. An
isolated ground (CDE) was secured to the forehead also
using ten20 paste. A third conventional disc electrode was
attached in the O2 region of the scalp using ten20 paste.
Next to the O2 CDE we positioned a TCRE with Signa gel.
Surgical tape was used to hold the TCREs in place. This set
up was repeated on all subjects without varying the
placement of electrodes.
We would like to thank the National Science Foundation (NSF) for
award IIP-1248654 to WGB. The content is solely the responsibility of the
authors and does not represent the official views of the NSF.
Fig. 1. Conventional disc electrode (A) and tripolar concentric ring
electrode (B).
978-1-4799-3728-8/14/$31.00 ©2014 IEEE

2. Fig. 2. Schematic representation of the electrode placement during
experimentation. A TCRE and CDE were attached to the O2’ and O2
regions (where the signals were collected from), the reference to the
mastoid process, and the ground to the forehead. After preampfification,
the TCRE is amplified and transmitted to the computer via Grass Aura
LTM64. Signals are displayed and recorded with Grass Technologies TWin
software.
C. Experimental Procedure
Impedances, between the TCRE elements, and to the
reference, were measured using an impedance meter.
Subjects were seated in a comfortable chair and were then
asked to open and close their eyes alternately every 15
seconds to determine if alpha waves appeared.
D. Signal Acquisition and Processing
The TCRE was connected to the custom built
preamplifier (gain = 48, high-pass = 0.5 Hz) and the outputs
connected to the Aura LTM64 amplifier (Grass
Technologies) for further amplification and digitization. The
rest of the electrodes were also connected to the Aura
LTM64. The signals were acquired and displayed with
TWin (Grass Technologies) running on a laptop computer at
1600 samples per second and 1 to 100 Hz bandwidth with
the 60 Hz notch filter active. The custom TCRE interface
combines two differential signal pairs from the TCRE as
described previously by Besio [6]:
16*(M-D) – (O-D), where M, D, and O are the potentials on
the middle, central disc, and outer TCRE elements,
respectively. To summarize, the algorithm is two-
dimensional and weights the middle ring and central disc
difference 16 times greater than the otuer ring and central
disc difference.
III. RESULTS
The average impedances of the 10 mm TCRE were lower
than the average impedances of the 6.0 mm and the 4.0 mm
(Table 1.). Alpha waves are neural oscillations in the
frequency range of 7.5-12.5 Hz arising from synchronous
and coherent (in phase) electrical activity of the human
thalamus.
Table 1. Impedence Recordings
Avr
1cm
Sd
1cm
Avr
6mm
Sd
6mm
Avr
4mm
Sd
4mm
Reference-outer 6.22 1.66 17.84 4.47 18.11 3.19
Reference-middle 6.42 1.54 19.84 4.15 8.83 2.42
Reference-disc 6.69 1.96 7.74 1.78 11.52 3.38
Outer-disc 1.51 .327 14.91 8.34 19.41 8.59
Outer-middle 1.12 .51 12.17 5.42 14.36 6.06
Disc-middle 1.46 .496 14.08 8.84 12.74 2.91
* Units in kilo-ohms. Average impedances and standard deviation.
IV. DISCUSSION
Alpha waves from the TCREs were detected when the 10
0m electrode was used and coincided with the CDE EEG
alpha waves. This demonstrates that the Signa gel is not
overly conductive and that the custom TCRE interface is
sensitive enough to see the brain signals. Alpha waves
detected from the 6.0 and 4.0 mm TCREs were not as
consistent with the CRE EEG. This may be due to
connecting the 6.0 and 4.0 mm TCREs to the TCRE interface
with alligator clips which are very noisy.
V. CONCLUSION
We found that the 10 mm TCRE worked very well with
Signa gel recording alpha waves. Further work must be
performed to determine why the 6 mm and the 4mm were
inconsistent.
ACKNOWLEDGEMENTS
We would like to thank all of our volunteers for helping us
acquire the data.
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