May 3, 2004

Scalp impedance measurements

I made some measurements today using my cap - the results were not clear. Single-pin impedances ranged from 15 to 40 to 140 MΩ. I don't really have a clean setup yet, but at least the labview software seems to be working right. Measurements were made at DC using the Keithley 6430. AC measurements at 10-20 Hz will probably be more appropriate. I used a current source 1e-7 A. The limitation on acquisition (voltage/current) appears to be a little less than 4.8 ms per reading. That gives us about 14 points per cycle, which should be ok.

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February 4, 2004

Femtoamp measurement

Nice thread.

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January 9, 2004

TIA, INA, etc.

Wil Grover (972.644.5580) from TI called this afternoon, recommending, as I had expected, the ADS1255 for my EEG project. He had some other recommendations. He recommended that I use a transimpedance amplifier rather than an instrumentation amplifier. Specifically, he suggested building one from either an IVC102, OPA129 or DDC112. I'll have to look at those. These would be connected to the scalp. Then, we would have an instrumentation ampflier, outputing a single ended output into the ADC.

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Measure Bias Current rather than Impedance

Bob Pease is the man. In his book on analog circuitry, he suggests rather than measuring input impedance, measuring the input bias current. The equation he gives for an ordinary differential bipolar stage (with no emitter-degeneration resistors or internal bias-compensation circuitry) is 1/(20 x Ib) where Ib is the bias current [1]. Now, one problem is that if you are actively seeking op amps with low bias current, wouldn't you expect them to have some sort of bias compensation circuitry?

References
[1] R.A. Pease, Troubleshooting Analog Circuits, Boston: Butterworth-Heinemann, 1991, p. 96.

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January 8, 2004

Double layer

On one hand, it is good news, on the other hand, it is not. I found a paragrah on the ACamp website on how DC bias currents cause double layers to form.

AC-coupled amplifiers with true AC-coupled inputs have capacitors on the inputs that form high pass filters. In addition to rejecting DC and low frequency signals, the input capacitors also block all DC bias currents from flowing across the amplifier inputs. One advantage is that blocking input DC bias currents eliminates double layer charging across the electrode surface of an input transducer, which significantly increases the transfer of analog biomedical signals across the tissue/transducer interface. Input capacitors also create added patient safety during clinical measurements. Similar advantages also apply to other analog amplifier systems including audio amplifier.
I've been saying this for a long time! The inventor is Dr. Michael F. Suesserman, from Seattle, WA (Patent 5,300,896). I wonder how many people have licensed his technology... a search on Suesserman on flashpoint reveals that he wrote a number of articles with Francis Spelman, who in turn did a substantial amount of research on EEG.

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December 17, 2003

Economist

The Economist had an article which is related to my thesis! It concerns the research of Prof. Klaus-Robert Müller at Franhofer Institute for Computer Architecture and Software Technology (FIRST). The article goes over some of the recent successes the group has had on single trial classification. Some encouragement (and impetus) for me! Incidentally, the classification is based on Bereitschaftspotential (readiness potential), a negative potential in EEG preceding movement. This is different from the type of BCI that we want to do in our lab, which is language based.

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October 1, 2003

Confusion

3 items of confusion - this is for me. First, why does S32 have a gap in the last third? Second, why does S24 yield such poor results? Finally, what makes S10 dry not as good as S10 wet? Is there a way of measuring a difference?

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September 7, 2003

S10 Visual

Doing some preliminary work on S10 I found something interesting. There were two experiments, a 24 sentence experiment and a 48 sentence experiment. I chopped up the 48-sentence experiment into two. Here are the results using a very very coarse grid (l,h,s,e). The interesting thing is that the best grid point seems to be about the same...

24-sentence
1,5,0,2,7
1,9,0,2,7
1,13,0,2,8
1,17,0,2,8
5,9,0,2,6
5,13,0,2,6
5,17,0,2,5
9,13,0,2,4
9,17,0,2,3
13,17,0,2,2

48-sentence I
1,5,0,2,12
1,9,0,2,11
1,13,0,2,13
1,17,0,2,11
5,9,0,2,5
5,13,0,2,4
5,17,0,2,4
9,13,0,2,3
9,17,0,2,4
13,17,0,2,4

48-sentence II
1,5,0,2,12
1,9,0,2,10
1,13,0,2,11
1,17,0,2,12
5,9,0,2,9
5,13,0,2,4
5,17,0,2,9
9,13,0,2,4
9,17,0,2,4
13,17,0,2,5

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August 29, 2003

Dry Electroencephalography: Reading Brainwaves with Less Pain and Hassle

In conventional electroencephalography (EEG), scalp abrasion and use of electrolytic paste are needed to insure low-resistance between sensor and skin. By replacing this “wet” process with dry sensors, setup and cleanup time are drastically reduced. More importantly, the number and frequency of sessions are no longer limited by the amount of abrasion human scalp can tolerate. However, the unavailability of amplifiers with sufficiently low noise and high input impedance has held back the development of dry EEG sensors until the very recently. While single-sensor comparisons of wet versus dry exist, there has yet to be a multi-sensor study. Our goals are to quantitatively evaluate dry EEG sensors under multi-sensor conditions and to develop specifications for high fidelity recording.

Dowload the poster.

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Electrical and magnetic readings of mental functions

Kutas, M. and Dale, A. Electrical and magnetic readings of mental functions, in Cognitive Neuroscience, M.D. Rugg (Ed.), University College Press, 1997, pp. 197-237.

Ever since Berger's (1929) discovery that brain electrical activity (electroencephalogram (EEG)) can be measured at the human scalp, it has been assumed that in these voltage fluctuations are hidden the mysteries of the workings of the human mind. While classical neurophysiologists questioned the likelihood that such "simple" fluctuations could be the key to the complexities of understanding, talking, reasoning, imagining and supposing, the past 70 years have proven otherwise. A large body of evidence has shown that electrical and magnetic activity (human or otherwise) encode information about brain states and brain processes and, by inference, about mental states and mental processes. (p. 197)

This is a nice point. At first glance, it is surprising that signals in the 10's of Hertz or less can provide meaningful information about brain processes. Individual neurons fire at a rates orders of magnitude higher, from 250-2,000 Hz [1].

The net flow of current across the neural membrane generates an electric potential in the conductive media both inside and outside the cells. It is this electric potential that forms the basis for the electrophysiological recordings made both invasively, by lowering elecrodes into the brani, and non-invasively, by placing electrodes on the scalp for EEG/ERP (Nicholson & Freeman 1975, Nunez, 1981). The same transmembrane current flows are also responsible for the magnetic fields recorded outside the head for MEG (the magnetoencephalograph). (p. 199)

This doesn't feel completely accurate. I would, instead, paint the following picture. Whenever a neuron discharges, the net electric field in the brain fluctuates because the distribution of shielded versus unshielded charges will change. Information about the new field is propagated to all charges (as well as the scalp) at nearly the speed of light. To minimize the energy of the system, free ions, i.e., those unconstrained by cell walls, then redistribute in response to the field. Kutas and Dale's statement makes it seem like this redistribution, the "flow of current", generates the electric potential at the scalp. My conjecture is that this larger flow of current will be very very slow, much slower than changes associated with billions of neurons charging and discharging. Assuming this to be true, and accepting that the local electric field can influence neuron firing rate, it is not to far fetched to consider that it is in fact the fluctuation in the electric field can encode information.

References
[1] V. Gerasimov, "Information Processing in Human Body," [Online document], 1998, [cited 10 Sept 2001].

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August 28, 2003

Brain wave recogntion publications

Patrick Suppes and Bing Han
Brain-wave representation of words by superposition of a few sine waves
PNAS 2000 97: 8738-8743.

Patrick Suppes, Bing Han, Julie Epelboim, and Zhong-Lin Lu
Invariance of brain-wave representations of simple visual images and their names
PNAS 1999; 96: 14658-14663.

Patrick Suppes, Bing Han, Julie Epelboim, and Zhong-Lin Lu
Invariance between subjects of brain wave representations of language
PNAS 1999; 96: 12953-12958.

Patrick Suppes, Bing Han, and Zhong-Lin Lu
Brain-wave recognition of sentences
PNAS 1998; 95: 15861-15866.

Patrick Suppes, Zhong-Lin Lu, and Bing Han
Brain wave recognition of words
PNAS 1997; 94: 14965-14969.

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August 20, 2003

An overview of Babak's dry EEG sensor

In mid-90s, Alizadeh-Taheri et al. produced an insulated electrode designed specifically for EEG and built using conventional semiconductor fabrication equipment [1].  Several interesting features were incorporated including multiple electrode contacts for redundancy, electrostatic discharge (ESD) circuitry, and an application-specific ultra-low noise amplifier.  Silicon nitride (Si3N4) was selected from materials common to thin-film capacitors as the insulating material because of its excellent resistance to chloride corrosion and ease of deposition [2].  Other materials considered were silicon monoxide, silicon dioxide and tantalum pentoxide.  Silicon monoxide and silicon dioxide were rejected because of their reactivity with chloride ions.  Tantalum pentoxide had been shown previously to be impervious to chloride corrosion [3], but the temperatures required for proper deposition would have complicated fabrication.  In this device, DC bias is set by the source through the resistance of the insulating layer (20-40 MΩ) [2].

The necessity of an ultra-low noise amplifier is particular to EEG recording. Unlike ECG, which is measured in mV, EEG is measured in μV, orders of magnitude smaller. This poses a problem related to our preference for high amplifier input impedance. To satisfy this requirement, the input stage was designed using metal-oxide-semiconductor FETs (MOSFETs). However, with this type of transistor, given the low-level signals of EEG, one needs to carefully manage noise from the amplifier, in particular flicker or 1/f noise.

1/f noise is so-named because its associated power spectral density (PSD) is inversely proportional to frequency. In MOSFETs, 1/f noise is attributed to the random trapping and detrapping of charge carriers in oxide defects at or near the semiconductor-insulator interface [4,5]. This trapping-detrapping process alters the channel carrier density. In addition, the trapped charges act as Coulombic scattering sites, inducing fluctuations into the channel mobility. Consideration of these two factors has been shown to accurately model 1/f noise in standard MOSFET transistors.

There are a number of ways to deal with this type of noise. The brute force approach is to create MOSFETs which by design have sufficiently low 1/f noise. MOSFETs are typically operated in saturation [6] for which the 1/f input referred noise is well-described by [5]

S Vg q 2 C ox 2 N ot WL 1 f MathType@MTEF@5@5@+=feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uamaaBaaaleaacaWGwbGaam4zaaqabaGccqGHfjcqdaWcaaqaaiaadghadaahaaWcbeqaaiaaikdaaaaakeaacaWGdbWaa0baaSqaaiaad+gacaWG4baabaGaaGOmaaaaaaGcdaWcaaqaaiaad6eadaWgaaWcbaGaam4BaiaadshaaeqaaaGcbaGaam4vaiaadYeaaaWaaSaaaeaacaaIXaaabaGaamOzaaaaaaa@4602@ .

(1.1)

Here, q MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyCaaaa@36E3@  is the electron charge, C ox MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBaaaleaacaWGVbGaamiEaaqabaaaaa@38D2@  the gate oxide capacitance per unit area, and W MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4vaaaa@36C9@  and L MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamitaaaa@36BE@  the width and length of the channel respectively. N ot MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOtamaaBaaaleaacaWGVbGaamiDaaqabaaaaa@38D9@  , the equivalent density of oxide traps, is defined by

N ot [c m 2 ]= kT N t (E) γ MathType@MTEF@5@5@+=feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOtamaaBaaaleaacaWGVbGaamiDaaqabaGccaGGBbGaam4yaiaad2gadaahaaWcbeqaaiabgkHiTiaaikdaaaGccaGGDbGaeyypa0ZaaSaaaeaacaWGRbGaamivaiaad6eadaWgaaWcbaGaamiDaaqabaGccaGGOaGaamyraiaacMcaaeaacqaHZoWzaaaaaa@4707@

(1.2)

where N t (E)[c m 3 e V 1 ] MathType@MTEF@5@5@+=feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOtamaaBaaaleaacaWG0baabeaakiaacIcacaWGfbGaaiykaiaacUfacaWGJbGaamyBamaaCaaaleqabaGaeyOeI0IaaG4maaaakiaadwgacaWGwbWaaWbaaSqabeaacqGHsislcaaIXaaaaOGaaiyxaaaa@4330@  is the density of oxide traps per unit volume and unit energy, kT MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4Aaiaadsfaaaa@37B6@  is the Boltzmann constant times the temperature, and γ MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4SdCgaaa@3794@  is a parameter related to the effective mass of the tunneling carrier and the barrier height of the trap. (1.1) and (1.2) suggest three possibilities: (1) reduce the temperature, (2) select a process minimizing the density of oxide traps, and (3) maximize the width and length of the channel. Alizadeh-Taheri elected to implement (2) and (3), choosing p-doped MOS (PMOS) and fabricating channels with large dimensions [2]. It is well-established that the effective density of oxide traps in PMOS transistors is much lower than n-doped MOS (NMOS) transistors. As a result, S Vg MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uamaaBaaaleaacaWGwbGaam4zaaqabaaaaa@38B8@  in PMOS devices can be several orders of magnitude lower than equivalent NMOS devices [2,5]. With regards to channel size, Alizadeh-Taheri set W=220 MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4vaiabg2da9iaaikdacaaIYaGaaGimaaaa@3A01@  mm and L=10 MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamitaiabg2da9iaaigdacaaIWaaaaa@3939@  mm. For comparison, transistors in an Intel Pentium 4 processor have dimensions nearly a thousand times smaller, e.g., L=60 MathType@MTEF@5@5@+=feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamitaiabg2da9iaaiAdacaaIWaaaaa@393E@  nm [7].

References
[1] B. Alizadeh-Taheri, R. L. Smith, and R. T. Knight, "An active, microfabricated, scalp electrode array for EEG recording," Sensors and Actuators A, vol. 54, pp. 606-611, 1996.

[2] B. Alizadeh-Taheri, "An Active Micromachined Scalp Electrode Array for EEG Signal Recording," Ph.D. dissertation, University of California Davis, Davis, CA 1994.

[3] C. H. Lagow, K. J. Sladek, and P. C. Richardson, "Anodic insulated tantalum oxide electrocardiograph electrodes," IEEE Trans Biomed Eng, vol. 18, no. 2, pp. 162-4, Mar 1971.

[4] K. K. Hung, P. K. Ko, C. Hu, and Y. C. Cheng, "A unified model for the flicker noise in metal-oxide-semiconductor field-effect transistors," IEEE Transactions on Electron Devices, vol. 37, no. 3, pt.1, pp. 654-65, 1990.

[5] Y. Nemirovsky, I. Brouk, and C. G. Jakobson, "1/f noise in CMOS transistors for analog applications," IEEE Transactions on Electron Devices, vol. 48, no. 5, pp. 921-7, 2001.

[6] B. Razavi, Design of analog CMOS integrated circuits. Boston, MA: McGraw-Hill, 2001.

[7] S. Thompson, M. Alavi, M. Hussein, P. Jacob, C. Kenyon, P. Moon, M. Prince, S. Sivakumar, S. Tyagi, and M. Bohr, "130nm Logic Technology Featuring 60nm Transistors, Low-K Dielectrics, and Cu Interconnects," Intel Technology Journal: Semiconductor Technology and Manufacturing, vol. 6, no. 2, pp. 1-13, 16 May 2002.

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August 15, 2003

Dissertation Instructions

I'm certainly not there yet - but in case I need it later (or you need it), here are the dissertation instructions. Scary.

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August 12, 2003

More insight on EDL

The EDL is more complex than I ever imagined... in fact, all the existing models are deficient in some way. There are a number of possible mechanisms and the truth is probably a combination of everything. Since superposition works for potentials, we can break down the EDL into several sub-EDLs. A book I found (citation to come later) on this topic broke it up into ionic and electronic double layers. Let's start with the electronic double layer.

Electronic double layer
Imagine am uncharged conductor, say a piece of platinum. Electrons, being free, protrude a little bit outside the boundary of the ions. The classical way of thinking about this is that the electrons are kept from flying out by the positively-charged ions, but they have some room to go a bit further. The quantum mechanical way to think about this is that with any finite barrier, the probability density leaks out, decaying exponentially. Either way, electrons exist a bit outside the boundaries of the conductor. This exterior cloud of electrons forms one layer of the the double layer. The positive ions left behind form another. Now, outside the electron cloud no charge is seen, since the net charge of the conductor is 0. Remember that the surface integral of the electric field flux is equal to the net charge enclosed. So, this clarifies an earlier question I had. If you plunk down a non-reactive conductor into a liquid you will not cause an ionic layer to form around the conductor. This can only happen if the conductor is charged.

Ionic double layer
The ionic double layer forms when the electrode has extra charge, in which case, ions outside the electrode in the electrolyte redistribute themselves (statistically). Ions with charge opposite the charge of the electrode will have a greater concentration by the electrode, while ions with the same charge will have a smaller concentration. How can the electrode have extra charge? Here's where it gets a bit complex. If the electrode is polarizable, meaning that there are no reactions, then the only way to give extra charge is to do something like charging it up with a battery. If the electrode is non-polarizable, it can become charged up because a chemical reaction at the interface contributes or consumes an electron. Reactions which are energetically favorable will occur, leaving the electrode charged, and an EDL.

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Electric double layer II

Getting warmer, I found some good sounding articles:

Polarization of the electrical double layer. Time evolution after application of an electric field
Shilov, VN; Delgado, AV; Gonzalez-Caballero, E; Horno, J; Lopez-Garcia, JJ; Grosse, C
Source: JOURNAL OF COLLOID AND INTERFACE SCIENCE; DEC 1 2000; v.232, no.1, p.141-148

Dynamics of the electric double layer: Analysis in the frequency and time domains
Lopez-Garcia, JJ; Horno, J; Gonzalez-Caballero, F; Grosse, C; Delgado, AV
Source: JOURNAL OF COLLOID AND INTERFACE SCIENCE; AUG 1 2000; v.228, no.1, p.95-104

Will track them down tomorrow... its in the chemistry library.

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August 11, 2003

Electric double layer

The electric double layer or EDL occurs at the interface between electrolytes such as salt water and electrodes such as gold. How does this happen? Does it happen just between metals and electrolytes? Understanding this is crucial to addressing the swtich from a non-polarized electrode to a polarized electrode. I should have been documenting resources as I found them but I suppose it isn't too late to start. A good start, with pictures, can be found here. Ahh, and Daniel Laser at Stanford might know something....

It seems that quite a few resources, like the ones above, talk about the EDL forming because of some sort of oxidation or reduction reaction (even without current). However, I'm interested in seeing if an EDL forms without any such reaction, like in platinum. What happens if there is no reaction, is there EDL? What if the electrode is coated with an insulating material? What if the material is non-polarizable (is that really possible)? Help!

EDL and Microfluidics - Electroosmosis
I looked at bit closer at Daniel Laser's page and realized that it was about microfluidics. This also sort of answers my question about insulators and the double layer. In this case silicon oxide makes contact with water. A reaction occurs at the surface, which makes H+ and adds electrons to the surface of the silicon oxide. A double layer forms as a result. The focus here is not the EDL but the H+ ions that are formed. When a potential is applied across the channel, the H+ ions are pulled towards the cathode (or is it the anode - ugh!) which in turns drags everything else. I found more details here. This still doesn't answer the question I had about platinum, which does not react, but it does suggest that having an insulating layer is no good if it isn't something that won't react as well.

Ahh, something interesting. In this article about electroosmosis, it states that "most surfaces spontaneously develop an electric double layer when brought into contact with either weak or strong electrolyte solutions. This charge generation is caused by electrochemical reactions at the liquid/solid interface..." Ok, so hat if there is no reaction, like platinum, or stainless steel? Is there still an EDL without applied voltage? The reference for that statement is Hunter's "Zeta Potential in Colliod Science".

EDL and AFM
EDL has recently been used to improve atomic force microscopy (AFM). For those of you that are unfamiliar with the technique, AFM basically involves a micromachined cantilever with a very sharp tip jutting out at the end. As the cantilever is dragged across the surface it basically traces out the atoms on the surface allowing to you to see the topography of the surface. How does this relate to the EDL? I found this article by Sokolov et al. One of the problems in AFM is the attractive van der Waals force between the tip and the sample. Sokolov et al. propose using an EDL to shield some of the van der Waals forces.

The answer?
I looked up "electric double layer" and "platinum" on Google, and voila, I found what I was looking for... I think. I found an article from korea about an EDL-based battery which I had skipped over ealier. A polarizable electrode made of carbon is used. In figure 2, it shows the battery under discharged and charged conditions. Clearly, when the battery is discharged there is no double layer, but when the battery is charged there is. Nicely, this gives a good review of all the different double layer models AND uses a polarizable electrode. Still it isn't exactly what I'm looking for but the references will be of great value.

The problem.
This is what it says in Webster's book.

When a polarizable electrode is in contact with an electrolyte, space charge forms in two layers at the interface to produce the half-cell potential and a capacitive reactance for current flow. If the electrode is moved so that the space charge distribution is altered, then there is momentary change of the half-cell potential until equilibrium is reestablished. If
an electrode pair is in an electrolyte and one moves with respect to the other, a potential difference appears between the two electrodes during this movement. This potential is a motion artifact associated with a polarizable electrode and can be a serious interference source in recording biopotentials. From the equivalent circuit, there are two potential sources of motion artifact: Ehe and Ese. To minimize these effects, the polarizability of the electrode-skin interface must be reduced. The best way to do this is to insure good contact between the electrode and the inner layers of the epidermis, meaning that something must e done to the skin surface (i.e., abrasion) to minimize polarization [1].
What is this half-cell potential Neuman is talking about? If there is no oxidation or reaction, then where does this come from? Oh man, I'm getting tired.

Reference
[1] M. R. Neuman, "Biopotential Electrodes," in Medical instrumentation : application and design, J. G. Webster and J. W. Clark, Eds., 3rd ed. New York: Wiley, 1998, pp. 183-232.

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August 9, 2003

Measuring voltage without current

Fundamentally, what are we measuring when we measure voltage? I think that we are really measuring the difference in how charge is perceived by two particular points. Actually, we are only taking the normal, the normal being parallel to the connection.

In the shower today I was thinking about balloons, static and hair. When you charge up a balloon by rubbing it in your hair, you create a potential between the balloon and yourself. More than that, the balloon really is charged up more than everything around. By bringing yourself close to the balloon, your hair gets attracted to the balloon so much so that it physically moves. Now, who does the work, is it the balloon? It's not, it is you, because it will take more work for you to approach the balloon since you must also force your hair to move. Why am I saying all this?

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August 8, 2003

More electrochemistry

"In any practical measurement of potentials, current flows in the measuring circuit for at least a fraction of the period of time over which the measurement is made. Ideally this current should be very small. However, in practical situations, it is never zero. Biopotential electrodes must therefore have the capability of conducting a current across the interface between the body and the electronic measuring circuit." - Michael Neuman in Medical Instrumentation [1]
This is the case if the current which flows is used to measure the potential, for instance through a resistor. What if the potential is all that we are after? Do we need to still worry about facilitating this current? Is the potential meaningful without current flow? Or, can we measure potential without current flow?

We can accelerate electrons in vacuum from one metal plate to another by imposing a voltage across the two plates. In most instances of this experiment, the electrons are released from one of the plates, so current really is flowing. Assuming that it is possible to drop an electron into the the field, would it accelerate? It would, so based on the acceleration we could measure the potential across the plates. Perhaps an even simpler experiment would be to send an accelerated electron through a perpendicular field and use the deflection to measure the strength of the field. So, it is possible to measure potential without current.

Now if we think about it, there is some wierdness going on. Where does the energy come from to create this field which does real work on the electron? Isn't that strange? If this was the case we could deflect electrons all day long for free. I'm not understanding something here...

Reference
[1] M. R. Neuman, "Biopotential Electrodes," in Medical instrumentation : application and design, J. G. Webster and J. W. Clark, Eds., 3rd ed. New York: Wiley, 1998, pp. 183-232.

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August 7, 2003

The tale of two electrodes

I got stuck thinking-writing about wet electrodes. It's time to nail things down... which has been a bit fuzzy. The question is, how does one measure changes in potential on the scalp surface which result from some kind of electrical activity beneath the scalp. In tissue - in liquids - the movement of charge occurs via ionic currents. So, the goal is to take those ionic currents and to convert them to electrical currents to do our measurements. Another related question is what exactly causes these ionic currents? If it is an electrical field perhaps that is what we should measure, rather than the ionic current.

The textbook response is that there are two ways of doing this, using a non-polarizable electrode, or using a polarizable electrode. In the case of a non-polarizable electrode, oxidation or reduction reactions occuring at the electrode-electrolyte junction create or absorb free electrons. These reactions can either occur from the electrode or from the electrolyte. This type of electrode is modeled using a resistor, since real current flows. A polarizable electrode is one in which no oxidation or reduction reactions occur. Changes in ionic concentration at the electrode-electrolyte junction cause changes in electron concentration at the junction. In this case the measurement is based on displacement current. Typically noble metals such as platinum are used for this.

Ok, so given all this, what is actually happening? I thought about this on the way to Ranch 99 and it still puzzles me. Because of the underlying electrochemical reactions in the brain, a fluctuating field is built up. This field causes ions to move around (which also affect the field in someway). Now, at the surface of the scalp, we see some of this ion movement, and if we look at two locations, there may be a net charge difference, if there is, then there is a voltage between the two. By connecting the two locations using a wire, charge will flow... in some sense. If the connection is made using non-polarizable electrodes, then oxidation or reduction take place so that the charge build-up begins to be neutralized as electrons flow across the wire to neutralize the other side. If the connection is made using polarizable electrodes then effectively electrons in the wire bunch up to neutralize the positively charged side (and the other side). This current is measured using a galvanometer (for instance).

Now here's the interesting question, if it is some sort of potential that causes the ions to be in those locations in the first place, couldn't the same potential also cause current in the wire? You could imagine, for instance, some sort of cantilever with a fixed charge on one side and the scalp on the other. In this way you could measure fluctuations without a reference. Of course, it would be much more difficult to do differential measurements, but it could still be done. This is a way of measuring EEG in a completely different way.

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August 6, 2003

Introduction to dry electrodes

Motivated principally by the need for stable long-term ECG at NASA, scientists have explored the possibility of dry electrodes since the early-60s. However, due of the ultra-low voltages involved, it was not until the mid-90s that Alizadeh-Taheri, et al. first published on a dry EEG sensor [1]. More recently, Harland et al. presented an alternate design based on commercially-available amplifiers [2]. In the following section, I detail important insights gained over the development of dry ECG sensors as well as the transition from dry ECG to dry EEG sensors.

Credit for the original dry electrode belongs to Lopez and Richardson [3]. Built in the late-60s, their insulated electrode was the first not to rely on an electrolytic solution for electrical contact with the body. Subsequent dry sensors, including the ones I have cited for EEG, have been based on it. This is not to say that there were not efforts prior the insulated electrode to record ECG or EEG without electrolytic paste. However, these early attempts were discouraging, the resulting recordings being severely hampered by motion artifacts. Lopez and Richardson addressed this issue by coupling surface potential fluctuations to the amplifier using a capacitive rather than resistive connection [4]. The electrode is called insulated because in this scenario the body is insulated from the amplifier. To appreciate this insight, we can skip back number of years to lithium chloride (LiCl) impregnated balsa wood electrodes.

References
[1] B. Alizadeh-Taheri, R. L. Smith, and R. T. Knight, "An active, microfabricated, scalp electrode array for EEG recording," Sensors and Actuators A, vol. 54, pp. 606-611, 1996.
[2] C. J. Harland, T. D. Clark, and R. J. Prance, "Remote detection of human electroencephalograms using ultrahigh input impedance electric potential sensors," Applied Physics Letters, vol. 81, no. 17, pp. 3284-3286, 2002.
[3] A. Lopez, Jr. and P. C. Richardson, "Capacitive electrocardiographic and bioelectric electrodes," IEEE Trans Biomed Eng, vol. BME-16, no. 1, p. 99, 1969.
[4] P. C. Richardson, F. K. Coombs, and R. M. Adams, "Some new electrode techniques for long-term physiologic monitoring," Aerosp Med, vol. 39, no. 7, pp. 745-50., 1968.

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July 29, 2003

Introduction

Electroencephalography (EEG) is a well-established method for studying human brain activity. Based on voltage fluctuations across the scalp, it is a fundamental tool in disciplines such as neurology, psychiatry, psychology and pharmacology. Hans Berger in 1929, using string galvanometers, reported the first human EEG recordings and showed that changes in the fluctuations were related to changes in cognitive state [1]. Two years later, Berger replicated his results using electronic amplifiers and a special oscillograph from the Carl Zeiss Foundation [2]. Since then, EEG has been based on electrodes and electronic amplifiers which require scalp abrasion and use of conductive paste or gel [3].

While tolerable when the recordings are short and infrequent, abrasion and conductive paste become a hindrance when long or frequent recordings are desired. Over time, the conductive paste can dry, requiring reapplication, or skin can regrow, necessitating re-abrasion. Frequent abrasion can cause irritation or even infection. Finally, application and cleanup take time, adding to the cost of frequent sessions. Recently, two groups [4,5] have reported on EEG sensors requiring neither abrasion nor conductive paste. The suitability of such sensors remains controversial. In this dissertation, I propose and evaluate a number of tests to fully characterize such “dry” sensors.

Background
Liverpool surgeon Richard Caton discovered electrical activity in the brain in 1875. Using Lord Kelvin’s reflecting galvanometer, he probed the exposed cortex of rabbit and monkeys and reported suppressed fluctuations in response to the interruption of light incident on the animals’ eyes. His discovery was to sit quietly until 1890, when Polish psychologist Adolf Beck independently replicated Caton’s work. By 1912, using Willem Einthoven’s string galvanometer [6], Russian physiologist Pravdich-Neminski had produced a skull-intact photographic record of electrical activity in dogs which he called an electrocerebrogram. Berger, who had unsuccessfully initiated his research using a capillary electrometer , made the first skull-intact recordings from humans in 1924 using a modified string galvanometer [7]. For these recordings he coined the term elektroenkephalogramm from which we get our more modern electroencephalogram. Ten years later, Adrains and Matthews cemented Berger’s claims by duplicating his results using copper gauze electrodes in saline-soaked lint [2,8,9].
Nearly forty years elapsed between the first electrocardiogram [10] and Berger’s electroencephalogram, and another decade for EEG to be completely accepted. Why? To measure electrical activity from the brain non-invasively, it was necessary to have microvolt sensitivity, while electrocardiography (ECG) required only millivolt sensitivity. This thousand-fold difference meant that Berger had to not only improve the sensitivity of his string galvanometer, but also to consider electrochemical noise and the impedance of his electrodes. Sterilized zinc-plated needles were used for his first human studies. Later, he used thin lead-foil electrodes wrapped in flannel saturated with a 20% sodium-chloride solution [7]. Conventional electrodes today are in principle based on Berger’s original metal-foil electrodes.

To be completely accurate, EEG measures not the electrical activity in the brain, but the resulting fluctuations in electric potential at the scalp. It has been shown that these fluctuations can be used to study brain processes in-vitro, but what exactly causes these fluctuations? Some would argue that it is the net effect of the human brain’s 20-100 billion neurons firing at different times. Others have proposed instead that it is the result of ionic currents flowing in the brain. Or, it may be that we are observing in EEG a field which carries information from location-to-location. If it were possible, one potential solution to this question would be to start measuring at the single cell level, working up to the scalp level, to see how the electrical field changes. If the recording instrument had a high enough bandwidth, one would be able to observe and simulate the summation of fields from neurons firing. Deviations from the simulations would indicate if other processes needed to be taken account of. The difficulty in this experiment is that the tools used to measure at the single cell level differ greatly from EEG. In EEG, one typically considers microvolt-level signals between 0.1 and 100 Hz, whereas in single cell recordings, hundreds of millivolts are the norm at thousands of hertz.

The modern brain researcher has at his disposal a wide array of non-invasive instruments including EEG, magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and computed tomography (CT). It is important to understand how EEG fits in. While fMRI, PET and CT have excellent spatial resolution, they have poor temporal resolution – on the order of seconds. Incidentally, PET involves ingesting radioactive compounds and CT entails exposure to harmful radiation, so their suitability for non-clinical research is limited. EEG, and its relative, MEG, have comparatively poorer spatial resolution, but have temporal resolution on the order of milliseconds. MEG has some theoretical advantages, such as improved localization, however, the cost of installation and maintenance can be orders of magnitude greater than EEG. For these reasons, cognitive studies tend to based on either fMRI or EEG, or, more recently, both. EEG is the tool of choice when the aspect of interest is temporal in nature. Two notable examples, both scrutinized by Berger, are brain wave rhythms and the wave patterns associated with epileptic seizures.

Berger’s original intent was to obtain objective brain measurements on psychic phenomena such as telepathy. Though in his lifetime he failed to do so, along the way he revolutionized brain research. After nearly 80 years, EEG remains the primary method for diagnosing epilepsy, analyzing sleep disorder, assessing brain damage after a stroke, monitoring levels of consciousness in response to anesthesia and investigating brain death. Cognitive psychologists continue to use it to study memory, attentional and language processing. There has been a growing movement to treat disorders such as attention deficit disorder and depression using EEG-based biofeedback instead of medication. And, with the reduced cost and increased availability of computing power, and data storage, researchers increasingly interested in brain-computer interfaces based on the recognition of EEG patterns.

Our interest in improving EEG acquisition is an extension of successes we have had in the recognition of the brain wave representations of language [11-15]. A hybrid of cognitive psychology and machine learning, our work has been enabled not just by advances in computer hardware, but in particular by the quantity of data collected. While many in psychology continue to average across subjects, we have focused increasingly on individuals. However, because of waning interest and mental fatigue, it is often infeasible to run subjects beyond several hours. In addition, as discussed earlier, the necessity of scalp abrasion and application of a conductive paste severely limit the number and frequency of recording sessions. In order to continue supporting our analysis with data, it was necessary to consider alternatives to conventional EEG.

References
[1] H. Berger, "Über das elektroenkephalogramm des menschen," Archiv für Psychiatrie und Nervenkrankheiten, vol. 87, pp. 527-570, 1929.
[2] "EEG - ElectroEncephaloGraph," Biocybernaut Institute, [Online document], 2000, [cited 25 July 2003]. Available: http://biocybernaut.com/tutorial/eeg.html.
[3] T. W. Picton, S. Bentin, P. Berg, E. Donchin, S. A. Hillyard, R. Johnson, Jr., G. A. Miller, W. Ritter, D. S. Ruchkin, M. D. Rugg, and M. J. Taylor, "Guidelines for using human event-related potentials to study cognition: recording standards and publication criteria," Psychophysiology, vol. 37, no. 2, pp. 127-152, 2000.
[4] B. Alizadeh-Taheri, R. L. Smith, and R. T. Knight, "An active, microfabricated, scalp electrode array for EEG recording," Sensors and Actuators A, vol. 54, pp. 606-611, 1996.
[5] C. J. Harland, T. D. Clark, and R. J. Prance, "Remote detection of human electroencephalograms using ultrahigh input impedance electric potential sensors," Applied Physics Letters, vol. 81, no. 17, pp. 3284-3286, 2002.
[6] W. Einthoven, "The string galvanometer and the measurement
of the action currents of the heart," Nobel Lecture, [Online document], 1925, [cited 30 July 2003]. Available: http://www.nobel.se/medicine/laureates/1924/einthoven-lecture.pdf.
[7] R. W. Thatcher, Functional neuroimaging : technical foundations. San Diego: Academic Press, 1994.
[8] O. D. Enersen, "Hans Berger," Who Named It?, [Online document], 2001, [cited 30 July 2003]. Available: http://www.whonamedit.com/doctor.cfm/845.html.
[9] J. D. Bronzino, "Principles of Electroencephalography," in The biomedical engineering handbook, J. D. Bronzino, Ed., 2nd ed. Boca Raton, FL: CRC Press, 2000.
[10] A. D. Waller, "A deomnstration on man of electromotive changes accompanying the heart's beat.," J. Physiol. (London), vol. 8, pp. 229-234, 1887.
[11] P. Suppes, Z. L. Lu, and B. Han, "Brain wave recognition of words," Proc Natl Acad Sci U S A, vol. 94, no. 26, pp. 14965-9, 1997.
[12] P. Suppes, B. Han, and Z. L. Lu, "Brain-wave recognition of sentences," Proc Natl Acad Sci U S A, vol. 95, no. 26, pp. 15861-6, 1998.
[13] P. Suppes, B. Han, J. Epelboim, and Z. L. Lu, "Invariance between subjects of brain wave representations of language," Proc Natl Acad Sci U S A, vol. 96, no. 22, pp. 12953-8, 1999.
[14] P. Suppes, B. Han, J. Epelboim, and Z. L. Lu, "Invariance of brain-wave representations of simple visual images and their names," Proc Natl Acad Sci U S A, vol. 96, no. 25, pp. 14658-63, 1999.
[15] P. Suppes and B. Han, "Brain-wave representation of words by superposition of a few sine waves," Proc Natl Acad Sci U S A, vol. 97, no. 15, pp. 8738-43, 2000.

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July 23, 2003

Why Minimize Interelectrode Impedance?

Introduction
In conventional electroencephalography (EEG), scalp abrasion and application of a conductive paste are unavoidable for quality recording [1]. These are needed primarily to minimize the electrode-scalp impedance, which, in turn, addresses the finite input impedance of the EEG amplifier. The electrode-scalp impedance is estimated by measuring the interelectrode impedance. This quantity informs the EEG technician whether there has been sufficient abrasion at a particular site. Here, I discuss in detail the theory behind the current methodology and explore alternative means of insuring quality recording.

To measure the interelectrode impedance, a low frequency, limited current source is connected across two electrodes. By recording the voltage across the two electrodes, the impedance between the two points is deduced by Ampere's law. This interelectrode impedance serves as an indirect measurement the electrode-scalp impedance since it includes the contact impedances for two electrode-scalp contacts, two layers of epidermis, and the tissue in-between. Tissue impedance can be ignored, since it is highly conductive and contributes little to the interelectrode impedance.

Experts recommend that (1) the interelectrode impedance be less than the input impedance of the amplifier by at least a factor of 100, (2) the interelectrode impedance be reduced to less than 10 kΩ as measured at the frequencies of interest (e.g., 10 Hz), and (3) when skin potentials are of concern, that is, the frequencies of interest are below 0.1 Hz, the interelectrode impedance be reduced below 2 kΩ by puncturing the skin with a needle or lancet [1]. In this case, the technician is advised to abrade until a drop of blood is seen!

The latter recommendation addresses a secondary concern linked not with the impedance of the electrode-scalp contact but the epidermis itself. Underlying the surface of the skin are ions which distribute themselves in a way that generates a voltage. This voltage can fluctuate when pores open due to heat or arousal, or when varying pressure on the skin causes the ions to redistributed.

Skin Impedance


References
[1] T. W. Picton, S. Bentin, P. Berg, E. Donchin, S. A. Hillyard, R. Johnson, Jr., G. A. Miller, W. Ritter, D. S. Ruchkin, M. D. Rugg, and M. J. Taylor, "Guidelines for using human event-related potentials to study cognition: recording standards and publication criteria," Psychophysiology, vol. 37, no. 2, pp. 127-52., 2000.

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July 22, 2003

High Impedance Electrode Techniques

This entry summarizes important points from the James Roman article discussing the NASA spray-on electrodes.

Introduction
James Roman in [1] describes, in particular, the electronic techniques used in NASA's spray-on dry electrode [2]. The last point in the introduction is most important, that the "acceptance of high impedance allows the use of small, or dry, electrodes." These electrodes were designed for electrocardiography and impedance pneumography.

The Requirement
The spray-on electrodes were tested at the USAF Aerospace Research Pilot School at Edwards Air Force Base in California. The requirements for the project were quite stringent, and serve as a good reference for dry EEG. They were as follows:

  1. Neither the electrodes nor the electrode wire should be felt by the subject.
  2. Electrodes must be resistant to motion artifact.
  3. Skin irritation must not result from frequent application of the electrodes.
  4. Shaving must not be a part of the procedure of applyign electrodes.
  5. Application of an electrode should take less than 30 seconds.

The NASA Flight Research Center Electrode
Summarized earlier, the NASA spray-on electrode consists of a layer of conductive glue applied by a spray gun or aerosol package. The site is prepared in 3 seconds using an oscillating toothbrush saturated in electrode paste. A thin, non-shielded wire is captured in the glue during application. Finally, an insulating glue is sprayed over the electrode. The final electrode is approximately twenty-thousandths of an inch thick and three-fourths of an inch in diamater.

Impressively, these sensors were tested for over 700 hours in-flight, and 500 hours on the ground over hundreds of patients with no cases of skin irritation, folliculitis, or contact dermatitis. Roman claims that the tracings obtained were either equal or superior to those obtained with more conventional electrodes but does not provide hard analysis for this.

The electrodes are said the require ampliifiers with input impedance in excess of 2 MΩ. This seems quite low, though at the time it was considered quite high. At lower values of input impedance there was increased attenuation of lower-ferquency components. Roman claims that at 100,000 Ω distorition was "barely detectable without a comparison record". This comparison was made using signal averaging - though the number of signals used was not reported. Why would they have low frequency attenuation with insufficient input impedance? There must be a high-pass filter being implmented. And there is. There must be a capacitor between the electrode and the amplifier which acts as a high-pass filter. At low frequencies, the impedance of this capacitor is very large. If the input impedance is too low, most of the voltage drop occurs through this capacitor so we see that the low-frequencies will be attenuated. This is a good lesson.

Electrical Characteristics of Electrodes
The NASA dry-electrodes were speced to operate between 0.1 to 100 Hz. This is exactly what we are looking for in the dry EEG sensors. The model used for the skin-electrode combination was a parallel RC. Using this, Roman computed the change in gain over frequency for a given input impedance. He found that there were two plateaus. The model has two plateaus for the following reason: at low frequencies, the capacitor is effectively open, so the resistor R forms the voltage divider with the input impedance. At high frequencies, the capacitor acts like a short, and there is no voltage divider, so the gain is 1. These are the two plateaus. As long as the resistor R is small compared with the input impedance, the difference in level for the two plateaus will be minimal. This, in fact, is the traditional approach. One could argue that if you knew a priori what the values would be you could just correct it in software.

"The resistive component of an electrode is a function primarily of skin preparation and only secondarily of electrode size." The electrode paste under the dry electrode cuts the resistive component of the electrode impedance in half whether the thin film is dry or still wet. As the film is drying, resistance for the first hour is higher than right when the spray is applied. After one hour, this difference disappears. Electrodes with different types of conductive paste show similar capacitance but different resistance. With "vigorous preparation", Roman and his collaborators were able to obtain resistive components of less than 1 kΩ with electrodes 1/8 inch or less in diameter. They gave some important caveats. For instance, "variation between subjects for similar electrodes applied by similar techniques can easily be of the order of 500 percent." Also, as we already know, the location also is a factor in resistance.

For the spray electrodes, "parallel capacitance appears to be primarily a function of electrode size... and is liniearly related to electrode area." This is good to know. Skin preparation played a secondary role in determining capacitance.

Partially addressing a concern that I have had, Roman found that the distance between the electrodes had little effect on the parallel RC. He gives typical RC values for the electrodes in Table I, which I replicate below. Unfortunately, no details were given as to the dimensions of the electrodes.

Small Plastic CupConductive Spray
(electrolyte base)
Conductive Spray
(no electrolyte base)
R (Ω)C (μF)R (Ω)C (μF)R (Ω)C (μF)
73,0000.02157,0000.06690,0000.055

The last section discusses motion artifacts. Roman admits that "it is not known how the circuit parameters associated with electrodes operate to produce motion artifact." I suspect that this has to do with the skin potential effect. As the skin gets squeezed down, ions flow and there is a momentary disruption and recovery which causes the artifact. They found that electrodes which are more rigid show less artifact. Direct tapping would cause an artifact, however, having a plastic shield avoided this. They did conclude that change in impedance due to skin or electrode distortion was not responsible since using different shunt resistances did not alter the ratio of artifact amplitude to signal amplitude. Thus the artifact must be some sort of low source impedance ac generator.

References
[1] J. Roman, Flight Research Program III - High-Impedance Electrode Techniques, Nasa Technical Note D-3414 Supplement, National Aeronautics and Space Administration, Washington, D.C., June 1966. Preprint of article published in Aerospace Medicine, August 1966.
[2] C. W. Patten, F. B. Ramme, J. Roman, Dry Electrodes for Physiological Monitoring, Nasa Technical Note NASA TN D-3414, National Aeronautics and Space Administration, Washington, D.C., May 1966.

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July 8, 2003

Papers papers

We need to publish. My first paper will be on the the dry recordings I made using the CS5532, called "Characterization of Voltage Noise in Dry EEG Sensors". It will be a short paper, first outlining issues with the typical noise measurement (spectrum), and discussing tools that are useful, and issues with the tools. Finally, it will present some results.

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June 24, 2003

What is really needed?

I was talking to student of Bruce Wooley's yesterday, and it occurred to me that I need to nail down the requirements for a good dry eeg sensor. What's really needed?

Introduction
Three things are required to do dry electrodes. First, the input current must be small, or somewhat equivanlently, the input impedance must be large. Second, the voltage noise must be small in the frequencies of interest, which, in our case, ranges from 0.1 Hz to not more than 100 Hz. In these regions, flicker noise, or 1/f noise, usually dominates. Finally, there must be compensation for the input capacitance so that every electrode experiences exactly the same phase shift.

Input Current
Really the key, three approaches, original is a dielectric contact, stops the current from flowing, need to compensate for chargeup. This is the Richardson electrode. Second is metal contact, dielectric intermediate, then contact - this is Babak's patent. Finally, we have a capacitor made by air, with some support mechanism. This is the Sussex group. Alternative is to use something with low input current already (like the LMC662) as the basis of the sensor, then any complaints go to the chip designer, not us.

Noise
Solution is chopping. Also, we can use the same concept of chopping to take the signal further out - use two amplifiers.

Phase Shift
Need to compensate, review stuff from Bob Pease.

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June 12, 2003

Dry Elecrodes for Physiological Measurement

I finally located James Roman's Dry Electrodes for Physiological Monitoring. It turns out to be in the library after all! For future reference, in case you do not have access to Stanford Libraries, the place to look is the Center for AeroSpace Information Technical Report Server, DO NOT use NASA Technical Reports Server (see I didn't even provide a link). I finally located the technical note with help from Eric at Government Documents. The classification scheme for this sort of material is poor because it is bound together so that a search on NASA TN D-3414, for instance, will fail because the reference is actually NASA TN D-3403-3423. Even searching on that doesn't work, I don't know how Eric did it, but he is the man.

Introduction
In pursuit of a solution for the rapid application of electrocardiogram (ECG) electrodes for long term recording, Patten et al. [1-2] developed "dry" electrodes based on quick-drying conductive glue. The skin is first prepared by subjecting it to 3-seconds of an oscillating toothbrush soaked in electrode paste. Then, a thin layer of the glue is sprayed directly onto the skin using a spray gun or aerosol package. A thin non-shielded wire is captured in the spray. Finally, the electrode is sealed by spraying a second insulating coat. Because of the high impedance of these electrodes, amplifiers with input impedances in excess of 2 MΩ are required. At the time this was very large, though today people talk about trying to do 100s of GΩs or even TΩs.

Technical details
Conductive glue. The conductive glue consists of silver powder suspended in household cement. The exact recipe is given in [1] and is as follows: combine

in an 8-oz bottle, cap and shake until mixture is devoid of lumps.

Application. A DeVilbiss No. 156 atomizer with two valves and a glass supply bottle was modified to launch a lead wire into the spray using a spring-loaded release rod. Both valves were connected to a single air house, with one valve responsible for spraying the conductive glue and the other for blowing air for drying. The air pressure used was 20 pounds per square inch. The insulation-glue spray is applied using an aerosol container and dried using heated air.

Removal. Electrodes are removed by dissolving them in acetone. This is accomplished by patting them with saturated gauze sponges.

References
[1] C. W. Patten, F. B. Ramme, J. Roman, Dry Electrodes for Physiological Monitoring, Nasa Technical Note NASA TN D-3414, National Aeronautics and Space Administration, Washington, D.C., May 1966.
[2] J. Roman, Flight Research Program III - High-Impedance Electrode Techniques, Nasa Technical Note D-3414 Supplement, National Aeronautics and Space Administration, Washington, D.C., June 1966. Preprint of article published in Aerospace Medicine, August 1966.

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June 10, 2003

P. C. Richardson

This is an update on Philip Richardson, who, with Alfredo Lopez, invented capacitive electrodes. His email bounced, but I was able to locate his phone and address in San Francisco (he mentioned that he opened a practice in SF on his website). One of these days when I'm brave enough I'm going to call him. Hope that he is in good health.

I found Richardson's classmates from Rensselaer (1960, EE).

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Dry Electrodes from NASA

I'd like to get this:

DRY ELECTRODES FOR PHYSIOLOGICAL MONITORING , Technical Note
Authors:
C. W. Patten, F. B. Ramme and J. A. Roman
Report Number: NASA-TN-D-3414
Performing Organization:
NASA Dryden Flight Research Center, Edwards, CA
Availability:
Currently this document is not available on-line.
From the NTRS FAQ: Where do I go for a hardcopy of the report?

Report Date: May 1966
No. Pages: 40

for free, how!

Another article, written by the third author, James Roman, is available in Aerospace Medicine 37. Unfortunately, while I was at Lane last week this volume was not there, and the attendants said that it had not been checked out. Perhaps I'll check again today. This isn't as substantial as the first report (40 pages) but should shed some light. I can get the first article at ntrs (nasa technical reports server), but it costs $30. I suppose I can get it... I looked around the Stanford Catalog but could not obtain it. There's yet another one called Method of making dry electrodes which does not have a price or number of pages.

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SRICO

Today I found a document about some work SRICO had been doing with the US Army on dry biopotential acquistion. Essentially their device uses lithium niobate to alter the polarization of a beam of light traveling through a fiber. The major technical challenge is noise from the fiber-optic since microphonic noise can affect the beam. How the contact is made is not completely clear though. The sensor is large, about 5 cm square.

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June 5, 2003

Richardson's Bioelectric Capacitive Electrode

Although the patent (US3500823) belong's to both Philip Richardson and Alfredo Lopez, Jr., the earliest substantial paper includes Franklyn Coombs and Robert Adams. With some effort, I was able to obtain the original patent from the USPTO. As far as I know, this is really the first capacitive biopotential sensor, though there are some references that talk about dry biopotential sensing prior to this. Amazingly, Richardson has a web presence and even an email address!

Introduction
The development of capacitive biopotential measurement began in the late 60's with Lopez and Richardson's capacitive electrocardiographic sensor [1-3]. Based on a black stained anodized aluminum electrode and an ultra-high input impedance circuit (30,000 MΩ), the sensor was attached to unprepared skin using an elastic strap.

Theory
Preparation of the skin by abrasion and application of a conductive paste or gel are, for the most part, necessary in biopotential recordings such as electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG). The reason is to minimize the variation of the impedance between electrode and skin with respect to the input impedance of the amplifier. This, in turn, minimizes the amplitude of motion-artifacts. In ECG and EMG, studies inevitably involve movement, e.g., the aerobic stress test. The impact of this movement is somewhat alleviated by the fact that ECG and EMG signals are measured in the millivolts (mV). In EEG, though much less movement is involved, micro-motion artifacts can still be bothersome, as EEG is measured in microvolts (μV).

In addition to electrode-skin impedance variation, artifacts linked to motion can also be coupled into the signal by polarization effects at the electrode due to non-zero DC current. When the electrode is stationary, non-zero DC current from the sensor causes the system to polarize. Because the system is biochemical, there will be a finite response time. Any sudden shifts in electrode position will require the stystem to restabilize. The voltage shifts resulting from this restabilization will contribute to motion artifacts. This will be very complicated - I would guess that the speed at which the system recovers is related to the amount of input current. The more current there is, the faster the polarization will stabilize. So, it makes sense either to have a lot of input current, or none at all.

The claim of Richardson and Lopez is in convential electrodes, change in ohmic contact (via the paste) is responsible for motion artifacts. Since in a capacitive electrode there is no ohmic contact, then there will be no motion artifacts. That this is true is not completely clear. If there is motion the capacitance may change, which would induce a voltage change.

Electrode
Richardson and Lopez used an anodized aluminum disk as the electrode, though in their patent [3], they claim any conductive material such as "copper, aluminum, or stainless steel having an insulation on its outer or skin contacting surface." In this case, the insulating coating was produced using an anodizing process. Here they claim that the aluminum oxide is used so that the film will be "free from pores or grain structure". To produce the film, they immerse the electrode in a standard sulphuric acid anodizing bath for 1.1 hours. The voltage is brought up to 100V using 100A/sq. ft. The process is finalized by dying the oxide, and immersing in hot water for oxide sealing.

To obtain the dimensions of the insulating layer, they measured the capacitance and back-calculated the thickness. For the said electrode, the resistance was greater than 4 GΩ and the capacitance was 5000 pF at 30 Hz. It is unclear why the capacitance is given in terms of frequency. Assuming a dielectric constant of 9, they calculated the thickness to be 0.7 mil.

Conclusions
Richardson, et al. [1] end with a very important (and honest) point, that "the production of motion artifacts caused by change incapacity coupling... limits the use of this type of electrode." In fact, this will the be the problem with a capacitive electrode-skin junction. With any sort of movement, the capacitance will change because the contact area will change. One way of addressing this is to make the contact area conductive, but to have a fixed capacitor between the amplifier and the contacting electrode.

Further Thoughts
One thing that was not mentioned above is the issue of skin potential artifacts in EEG. Present at low frequencies (<0.1 Hz), these artifacts result from biochemical changes in skin and are related to the galvanic skin response. When necessary, the most common way of eliminating such artifacts is to abrade the scalp aggressively, to the point of drawing blood [4].

References
[1] P. C. Richardson, F. K. Coombs, and R. M. Adams, "Some new electrode techniques for long-term physiologic monitoring," Aerosp Med, vol. 39, no. 7, pp. 745-50., 1968.
[2] A. Lopez, Jr. and P. C. Richardson, "Capacitive electrocardiographic and bioelectric electrodes," IEEE Trans Biomed Eng, vol. BME-16, no. 1, p. 99, 1969.
[3] P. Richardson and A. Lopez, Jr., "Electrocardiographic and Bioelectric Capacitive Electrode," U. S. Patent 14,860,040, March 17, 1970.
[4] T. W. Picton, S. Bentin, P. Berg, E. Donchin, S. A. Hillyard, R. Johnson, Jr., G. A. Miller, W. Ritter, D. S. Ruchkin, M. D. Rugg, and M. J. Taylor, "Guidelines for using human event-related potentials to study cognition: recording standards and publication criteria," Psychophysiology, vol. 37, no. 2, pp. 127-152, 2000.

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