[Eeglablist] Referencing active electrode EEG data

[Alistair Walsh]

Dear experts,

I have EEG recorded with a prototype, active electrode system from Quasar
USA. My question concerns what is labelled the 'CMF' electrode. I want to
know how to process data in EEGLAB to remove noise detected by the CMF
electrode which I think is equivalent to a reference electrode on a passive
system. This 'CMF' electrode is in the P4 position and I am after
directions about how to remove the collected 'common mode signals' and
'body-ground potentials' from the data. Do I for example sum all the other
electrodes and subtract this from the CMF signal and then subtract the
remainder from the other electrodes individually?

Please excuse my ignorance in these techniques if that is a silly
suggestion but I am trying to process the data from the Quasar headset
without using the supplied software (which takes care of these
calculations).

I welcome general comments - as any guidance from experienced researchers
will be beneficial, but I especially need specific comments from anyone who
has used a Quasar system of a BIOSEMI Active Two system or similair.

Please also correct me if I've made incorrect assumptions about what the
CMF actually is.

Thanks in advance.

Alistair Walsh
Swinburne University
Australia

I have also attached relevant information from the EEGLAB website and some
excerpts from two Quasar papers.

>From the EEGLAB website:

Chapter 04: Preprocessing Tools - Re-referencing the data
The reference electrode used in recording EEG data is usually termed the
'common' reference for the data -- if all the channels use this same
reference. Typical recording references in EEG recording are one mastoid
(for example, TP10 in the 10-20 System, the electrode colored red in the
picture below), linked mastoids (usually, digitally-linked mastoids,
computed post hoc, the vertex electrode (Cz), single or linked earlobes, or
the nose tip. Systems with active electrodes (e.g. BIOSEMI Active Two), may
record data reference-free. In this case, a reference be must be chosen
post hoc during data import. Failing to do so will leave 40 dB of
unnecessary noise in the data!

>From a QUASAR paper:

A Novel Dry Electrode for Brain-Computer Interface
Eric W. Sellers, Peter Turner, William A. Sarnacki, Tobin McManus, Theresa
M.
Vaughan, Robert Matthews

The hybrid biosensor consists of an electrode, an ultra-high input
impedance amplifier circuit, a common-mode follower (CMF; a proprietary
technology for reducing common mode signals), and a wireless node that
contains a gain/filter module and a data acquisition/communications module.
 As the contact  impedance between the scalp and each finger can be as high
10 7 Ω, the amplifier electronics are shielded and integrated with the
electrode in order to limit interference caused by the pickup of external
signals.  The hybrid sensors and wireless node data acquisition channels
are closely phase and gain matched and can provide individual EEG signals
or high common-mode rejection ratio difference signals (CMRR > 70dB between
1Hz – 50Hz) between biosensors. The CMF is used to reduce the sensitivity
of the hybrid biosensor to common mode signals on the body.  The CMF is a
separate hybrid biosensor on or near the scalp that measures the potential
of the body relative to the ground of the amplifier system.  Ultrahigh
input impedance for the CMF (~10 12 Ω) ensures that the output of the CMF
tracks the body-ground potential with a high degree of accuracy.  The CMF
output is used in the wireless node as a reference for the EEG measurement
by the electrodes.  In this way, the common-mode signal appearing on the
body is dynamically removed from the EEG measurement.  This typically
achieves a CMRR of 50 to 70 dB.

and another:

Nonintrusive, Wearable Bioelectrodes for Monitoring the Heart and Brain
September 1, 2007
By: Robert Matthews et al., Quantum Applied Science & Research (QUASAR)
Sensors

To provide immunity to common-mode (CM) signals appearing on the body, we
operate the sensor in conjunction with the company's proprietary
common-mode follower (CMF) technology. Due to its ultra-high input
impedance (~1012 Ω), the CMF tracks the body-ground potential with a high
degree of accuracy. This can then be used to dynamically reduce any CM
signals by more than three orders of magnitude.
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