[Neuroscience] Re: Excessive DC drift during field potential recording

r norman via neur-sci%40net.bio.net (by r_s_norman from _comcast.net)
Fri Nov 30 09:42:16 EST 2007


On Fri, 30 Nov 2007 06:05:47 -0800 (PST), "J.A.Legris"
<jalegris from sympatico.ca> wrote:

>On Nov 28, 4:37 pm, Bill <connelly.b... from gmail.com> wrote:
>> Hi,
>>
>> We're trying to record cortical spreading depression in rat brain
>> slices, you microinject some 26mM KCl solution and it induces a 5-20mV
>> spreading wave of depolarization across the cortex.
>>
>> The problem is we're getting really bad DC drift, the potential drifts
>> up and down, sometimes by as much as +/-10mV.  Turning ON the
>> temperature controller makes it worse (presumably because of
>> temperature swings). The hight of the tissue bath doesn't seem to be
>> changing.
>>
>> Any ideas on what could be causing the drift, and hence how to get rid
>> of it?
>>
>> (Oh, and I can't just AC couple the data, the spreading depression
>> waves are 10+s long).
>
>Can you post a link to circuit diagram of your setup?

The most important thing to check is whether you get drift if you
leave out the brain slice.  That is, with the same apparatus in the
same arrangement with the same electrodes and stimulation, does it
drift?  It is a LOT easier to fix things if the biology is not
involved!  Of course, if the electrodes are not in situ, they will
have a different impedance and that can change things so you have to
keep that in mind.

Drift is probably from the metal-liquid interface.  Do you have good
chlorided silver interfaces that are in themselves stable?
Rechloriding them and storing them in pairs shorted together can help.
Of course if the ground electrode is a chlorided silver in the bath
that sees the change in K+, it will change its electrochemical
potential so you have to make sure the ionic composition of whatever
touches the metal is constant.  I believe that silver chloride can
also get poisoned with protein accumulation on the surface and that
can also be a problem.  Cleaning and keeping the wires isolated from
the biological solutions with liquid junctions can help.

Of course, there is drift in the electronics, but that was more likely
in the olden days of tube equipment and is more easily detected.

Are there room temperature changes that can be affecting things
(including electronics)?  Silver chloride is also light sensitive so
the afternoon sun coming through the window changing to night (if you
are that dedicated!) is conceptually a problem.  Yes, I am grasping at
straws, here, in an attempt to be thorough.



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