[Neuroscience] Re: Minimizing stimulus artifacts

Bill via neur-sci%40net.bio.net (by connelly.bill from gmail.com)
Wed Feb 28 23:12:47 EST 2007

> The biphasic vs. monophasic is important only if you have problems
> with electrode polarization from running currents always in one
> direction through the metal/solution interface.  If you have
> reasonable chlorided silver wires, you shouldn't have a problem.
> There are lots of different stimulus artifacts and eliminating them is
> about as easy (i.e. not at all easy!!!) as eliminating line frequency
> noise.  Is the artifact from a separate electrode or is it from
> stimulating through the recording electrode?  Those are two very
> different situations.  If you have problems with series electrode
> resistance, you will definitely have problems passing current through
> those electrodes.  "Automatic" compensation methods only work if the
> electrode resistance is constant and linear.

It's through a bipolar extracellular electrode.
Here is an example of what I'm talking about:
http://www.ilikethings.net/stimartifact.GIF   (4k in size).
That's a thalamocortical EPSP. The stim electrode must be nearly 1mm
from recording. The stimulus voltage is probably somewhere between
"30-60V" (at least that's where the dial is on the Grass). The
duration is between 40-80microseconds. Yet artifact clearly lasts for
over 2ms; 20 times longer. It has a classical capacitive discharge
looking falling phase, making me think a biphasic pulse to "discharge"
whatever the capacitor is, might help.

I'm recording in current clamp with a Rs of about <10MOhm, and an
input resistance in the 100-200MOhm range.

(In Voltage clamp, the artifact is shorter in duration, but biphasic
(even triphasic if I have correction/prediction up high) and up to
20nA in size)

Any comments appreciated.

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