# tutorial for EEG electrical parameters?

Dan Marquez dmarquez3 at socal.rr.com
Sat Sep 28 00:08:08 EST 2002

"Jean-Etienne Poirrier" <jepoirrier at nospam.free.fr> wrote in message
news:amuhqd\$hpu at aix4.segi.ulg.ac.be...
> Hello,
>
> I am using an electronix device to record EEG and EMG from rats. I want to
> know what's inside the "black box" : what is a filter, a bandpass, gain,
> etc. Do you know any interesting paper, book or website providing this
kind
> of information?
>
> Jean-Etienne

I can also offer brief descriptions here.  If you look at a voltage on an
oscilliscope, lots of times you see a wobbly line with a lot of noise. To
get rid of the noise, filters are typically used.  Each filter has an input,
where the noisy voltage is connected, and an output, which is the same
voltage without the noise.

But, there are different kinds of filters, and some are better than others.
These are common filters...

Lowpass filter - it allows low frequency voltages to pass through it,
blocking high frequency noise

Bandpass filter - it allows only a selected range of frequencies to go
through

Highpass filter - it allows only high frequencies to go through

Notch Filter - it allows only one frquency to go through

Gain is a number that you use to calculate the magnitude of an output if you
know the magnitude of an input. For instance, an amplifier with a gain of
1000 will increase the input voltage by a factor of 1000. If you have a 1
millivolt signal at the input, you would expect to see a 1 volt output.

Amplifiers are very easy to make. A common component used to make amplifier
is another amplifier called an "operational amplifier" (op amp).  This
sounds complex but they are easy to use.  There are lots of cartoon-style
books that teach how to use operational amplifiers.  Just to show how easy
it is to make an amplifier, here's an example. Say you want a gain of 100.
Just get a 100 kilo-ohm resistor and a 1 kilo-ohm resistor and hook them up
to an operational amplifier.  Done.  Well, sort of. You might see some noise
and so you might add a filter.

There are different kinds of operational amplifiers. Some are very low power
while others are very fast. You can configure op amps to make many common
types of amplifiers, ranging from stereo amplifiers to ultra tiny
operational amplifiers.

EEG and EMG often use instrumentation amplifiers.  These are fancy names to
op amp circuits that have really nice characteristics.  For instance, they
don't drain a lot of current (like an audio amp might). They are generally
resistant to static electricity, and they don't mess up when there is a
noise called common mode noise. (But you don't need to know that.) But they
will monitor voltages acoss different parts of the body.

Very often the voltages are converted to digital voltages so a
microprocessor or computer can make pretty graphs or blinking lights.

Hopefully your rats are generally happy with lots of room to play and lots
of wood to chew. I believe lab animals should be treated with etopian
courtesy.

Dan