[Eeglablist] Referencing active electrode EEG data

[Makoto Miyakoshi]

Dear Marissa,

Do we also have CMF channel in our recording system? If so and if you know
what to do with it, would you please telling us about it
(inclusion/exclusion in rereference or average reference, in ICA, etc.)

Makoto

2013/2/18 Alistair Walsh <6119123 at student.swin.edu.au>

> 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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-- 
Makoto Miyakoshi
JSPS Postdoctral Fellow for Research Abroad
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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