[Eeglablist] Signal/noise loss, re-referencing Biosemi data

[Thomas Ferree]

LLoyd,

Thank you for the clarifications.

It is true that my comments ignored the issue of ground electrode
for simplicity.  You describe this issue very nicely.  We should
note that a ground electrode is also present in conventional EEG
systems (e.g., EGI and Neuroscan) for the same reason (CMRR).

Active noise suppression circuitry is also available in Synamp2,
although I do not know the design details.  Since the existence
of a ground and active noise suppression in the amps are not
unique to Biosemi, could you please clarify the main hardware
difference between the ActiveTwo system and conventional EEG
systems?

Regarding the choice of reference, would you agree that your use
of the term Linked Reference is more like the conventional term
Average Reference, in that there is no direct physical linkage
between different electrodes, except by going through the amps?
I wish to verify that your system satisfies the theeretical 
zero-current-flow-through-the-scalp condition, at least
approximately by virtue of the high input-impedance amps.

Thanks very much,
Tom.


>In response to Tom Ferree's post -- in brief, ActiveTwo stores a monopolar
>signal (channel) for each active electrode.  One must subtract one channel
>or the average of some number of channels (e.g. A1 & A2, or common average,
>which is the average of all scalp channels) to compute a referenced EEG
>signal.  Although you can display a referenced EEG signal in the ActiView
>data acquisition software, the signal is never saved in a referenced format.
>A reference subtration must be performed off-line.  The common mode
>interference is reduced (signal/noise is increased) by the act of
>subtracting an EEG reference.
>
>Below is a more thorough discussion of how the ActiveTwo system measures
>voltage and how the measurements are stored by Coen Metting Van Rijn of
>BioSemi.
>
>================
>
>In greater detail, voltage is defined as the difference in electrical
>potential between points (for example on the scalp).  In principle, you can
>measure a voltage with two inputs and two electrodes.  This is what you do
>when you use a meter to measure a battery voltage.  The problem with
>semiconductor amplifiers (transistor inputs) is that you need a third
>connection to archive a really high input impedance on the two inputs used
>for the voltage measurement.  This third connection is the "ground"
>electrode (for the technically interested: it provides the bias current for
>the two high-impedance inputs).  The amplifier now takes two voltage
>measurements: E1 with respect to GND, and E2 with respect to GND.  By
>subtracting the two voltage readings, the amplifier "calculates" the voltage
>between E1 and E2.  Both measurements with respect to GND are polluted with
>inference, because of interference currents flowing through the GND
>electrode. However, because this error signal is the same for both
>measurements - it is Common Mode - it cancels in the reference subtraction
>(common mode rejection). 
>
>In the ActiveTwo system, instead of a simple GND electrode, an active
>feedback circuit with two electrode is used: CMS and DRL.  This offers
>better CMRR and subject safety than a simple passive GND electrode.
>However, for easy understanding of the topic decribed now, you can regard
>the CMS as equivalent of the conventional GND electrode, and forget about
>the DRL.
>
>In general, the voltages E1-CMS and E2-CMS are large (polluted with common
>mode interference), while the voltage E1-E2 is small (clean EEG).  In the
>old days, analog EEG systems wrote EEG data directly to paper during
>acquisition, so it was important to be able to reject line noise on-line.
>Even in the beginning of the digital era, dynamic range remained very
>limited because only 12 bits were available in the ADC.  In addition, mains
>instead of battery power supply caused high levels of Common Mode
>interference.  The combination of high CM and small dynamic range, forced
>older EEG equipment to do the calculation to E1-E2 directly in the first
>amplifier stage.  The method used was analog subtraction by an
>instrumenation amplifier (differential amplifier with fixed gain, a clever
>transistor circuit that amplifies only the voltage difference between two
>inputs).  However, with increase of dynamic range of the electonic circuitry
>(mainly by the introduction of 24 bit ADCs), and the introduction of battery
>power supply, it became possible to process (amplify and digitize) the raw
>voltages (E1-CMS and E2-CMS), and postpone the subtraction to the end of the
>signal chain (with the advantages of a completely symmetric system setup,
>increased stability, and the ability to reject also interference and errors
>picked up after the input stage).  Thus, in the ActiveTwo we do the
>subtraction in a digital way in the PC.  It is important to realize that
>valid EEG signals only appear after the last subtraction step is made.  EEG
>is hidden in the raw signals (the voltages with respect to CMS), but it is
>still distorted and polluted with interference.  Only after the referencing
>step do all errors cancel and a valid voltage measurement appears.
>
>Up to this point, we have talked about measuring one voltage between two
>potentials.  In a typical EEG recording, many electrodes are used to
>determine many potentials.  Consequently, there are many reference options:
>
>- Monopolar: select one of the electrodes as a reference, calculate the
>voltage between every other electrode and the reference electrode (V1 =
>E1-REF, V2 = E2-REF, etc.)
>- Bipolar: select pairs of electrodes (V1 = E1-E2, V2 = E2-E3, etc.)
>- Linked reference: Use the average of several potentials (electrodes) as a
>reference. Calculate the the voltage of every electrode with respect to this
>reference. For example, use the average between ears as a REF, thus REF =
>(EA1+EA2/2), then V1 = E1 - (EA1+EA2/2), etc.
>
>In the old analog days, you only could use differential amplifiers to make
>these subtractions.  A differential amplifier can only calculate the
>difference between two inputs (the non-inverting and the inverting input).
>This is OK for bipolar reference schemes, but with Monopolar and Linked
>Reference schemes, accuracy and interference problems are inevitable.  In
>the Monopolar case, the reference electrode has to drive many inputs of
>differential amps, which causes a low input impedance for the reference
>input.  In the Linked Reference case, resistors have to be used to calculate
>the average potential, which means that the accuracy of the average
>calculation (the weight factors) depends entirely on the equality of the
>electrode impedances.  A last problem is that the reference scheme had to be
>determined in hardware, by switches changing signal paths.  This introduced
>unreliability, and meant that a file was always saved with a fixed reference
>scheme -- it couldn't be changed afterwards.
>
>The digital reference calculation used in the ActiveTwo fully eliminates all
>these problems: every electrode always sees a single amp input with
>optimally high input impedance (the active electrode circuit), every
>possible reference scheme can be chosen in software (no unreliable
>switches), and every possible choice of reference scheme remains possible
>off-line (raw signals are saved).
>
>================
>Lloyd Smith
>Owner
>Cortech Solutions, LLC
>
>Office: 910-362-1143
>Mobile: 910-431-2811
>Fax: 910-362-1147
>E-mail: LSmith at cortechsolutions.com
>Web: www.cortechsolutions.com
>
>
>
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-- 

Thomas Ferree, Ph.D.
(415) 353-9474
http://dnl.ucsf.edu/users/tferree