From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-02-02 14:58:02
Dear Jim Peters:
I looks like you got some real good information from Andreas and Joerg.
As Joerg pointed out, you need to build a microvolt calibrated source. He uses a 555 to generate a square wave of known amplitude. In my case I useda 8038 sinewave generator chip. The sine wave lets you test your frequency response, the FFT accuracy, and the Nyquist rejection problem. You needto build something so you know what you are doing.
50 Hz pickup: Most standard EEG system do not uses shielded wire. They eliminate the problem with a sharp cutoff filter and a notch filter set to 50or 60 Hz. That limits the top frequency to about 35 Hz. For my research I wanted to see all the data so that made shielding mandatory. My experience was that I had to put the input stage in its own shielded box ( see my website for details ). All the power lines in and the audio out lines are bypassed with 0.01 uf capacitors. I used 4 standard RCA female connectors on the input to the box. I found a ready made audio RCA cable with four shielded wires molded together. I cut off one end and connected them to the electrodes. I tried to keep the unshielded part to be less than 1/4 inch,not just for shielding but also to reduce charge current coupling. I built a shield for the electrodes by using a double sided PC board. One side holds the electrode connector and the other is the ground.
Electrode types: Whatever type of electrode you use, you have to clean theskin to get rid of the oil. A paper towel with rubbing alcohol will work.Some people actually abrade the skin to get lower ohms. My first electrodes were made from the black conductive foam that IC chips are shipped in to reduce electrostatic charge damage. I used a needle to pull a # 30 wire through the foam a few times and then solder that to the shielded wire. That worked quite well. Using salt water improves the performance to some degree, but salt water dries out too fast and is not a good solution. Conductive paste works better.
Over the years I have been using the Mind Mirror electrode system. You live in England? You should get some from them. Also get the book by MaxwellCade "The Awakened Mind".
Probe Placement: It is quite critical as to where you place your electrodes. The professional 10-20 system ( skull cap ) is very specific so that the data is to be meaningful. As an amateur have followed the Mind Mirror probe placement.
This is a quote from an earlier Email about probe placement.............
My first introduction to EEG measurement was meeting Dr. Edith Jurka and working with her Mind Mirror. She had studied with Maxwell Cade in England. I noticed that the book "The Awakened Mind" by Maxwell Cade is listed on your webpage. I strongly recommend that you read that. The book by Anna Wise is much quoted, but to me it is a very poor copy of the Cade book. The Cade book suits the engineer in me while the Wise book is too "right" brained and full of new-age fluff.
With the majority of my EEG testing over the years, I have stayed with the Cade probe placement. Now I suppose you want to know what that is? Well, the ground goes to the middle of the forehead. Then there is a pair of right and a pair of left channel electrodes. The technical placement is F1 - O1 and F2 - O2. On the forehead, locate the scalp hair line or where it used to be. Put the right channel electrode directly over the right eye at the hair line, and similarly for the left channel. The occipital region is a little more difficult to describe. There is a place on the back of the head called the innion (sp?). That is a little bony protrusion at the back of the head, in the center at the top of the neck and where the scull starts. You should be able to feel it. Draw an imaginary line one inch above this. The rear electrodes go on this line and are spaced the same as the eye distance. Getting through the hair and making good low ohm connections isa serious challenge.
End of quote.
If you have trouble locating the Mind Mirror electrodes, let me know and I will try to tell you how to make them.
Probe material: I would be careful about putting lead solder near your skin.
The Mind Mirror buttons are nickel plated brass. The conductive paste is quite corrosive and after a few cleanings with Scotch Brite, they are essentially brass buttons. I am aware that this may not sit well with the purists on this group, but for my research I did not find it necessary to go to chlorinated silver electrodes.
If you need any more info please let me know.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
----- Original Message -----
From: Jim Peters
To: buildcheapeeg_at_yahoogroups.com
Sent: Thursday, January 31, 2002 8:13 PM
Subject: [buildcheapeeg] NaffEEG
I've now built the BrainMaster input stage (using AD620 + OP90), and
it seems to be working. I had to buy 5 of the polypropylene
capacitors, so I picked a good matching pair anyway, just in case it
made a difference.
I'm now getting 50Hz hum, no problem at all! I'm sampling at 5000Hz
(5kHz), and piping this through to my filterbank display that I posted
PNGs of earlier.
I've improvised some electrodes from 1 pence pieces tinned with solder
on one side. I also experimented with little wrappers for them dipped
in saline. (However, I'm guessing now that probably it's not a good
idea to use electrodes with lead (Pb) in them).
I'm not seeing any brain-waves yet, though.
With electrodes on my forehead (ground in the middle, other two either
side), I get a clean trace with 50Hz hum in the middle. If I tap the
left or right electrode, I get a pulse, but the middle one doesn't
give one, or it only gives a little one. This seems to be more or
less as expected.
If I frown or clench my teeth, I get patterns appearing, and they peak
around two to four times the amplitude of the 50Hz hum.
So, what do I need to do to improve this situation ? Do I need to buy
some proper electrodes and paste, or should I look at reducing this
hum first ?
At the moment the electrode connection is passing through about 3m of
microphone cable, with about 8 inches unshielded at the electrode end.
Would shortening this cable make a significant improvement ?
Can I improvise better electrodes ? I'm still looking for a UK
supplier of Ag/AgCl electrodes.
Jim
-- Jim Peters (_)/=\~/_(_) Uazú (_) /=\ ~/_ (_) jim@ (_) /=\ ~/_ (_) www. uazu.net (_) ____ /=\ ____ ~/_ ____ (_) uazu.netYahoo! Groups Sponsor ADVERTISEMENT
To unsubscribe from this group, send an email to: buildcheapeeg-unsubscribe_at_egroups.com
Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:38 BST