From: Doug Sutherland (wearable_at_earthlink.net)
Date: 2001-12-19 07:27:11
The differences affecting electrode material selection for EEG
recording is related to the type of recording being performed
(continuous EEG, long-latency EP’s or short-latency EP’s) and
the effect of DC drift potentials on each type of recording
(DC drift is always present in EEG recordings). Typically,
Ag/AgCl electrodes are non-polarizable and the most DC-stable
over time of all the available EEG electrodes. Ag/AgCl
electrodes are recommended for continuous EEG and long-latency
EP recordings, where the epoch length is over 2 seconds. The
reason is that half-cell reactions, which occur on the surface
of Ag/AgCl electrodes, can balance DC drift among the electrodes
in the recording set and this reaction minimizes the contaminating
effect on the recording. If short-latency EP recordings are being
made (time periods of less than 2 seconds), than other electrode
materials may be appropriate because the effect of DC drift is
not as significant due to the long time course of the artifact.
In addition, other electrodes require much less maintenance than
Ag/AgCl electrodes and due not need rechloriding. Both Tin and
Gold electrodes are considered polarizable and, therefore, are
subject to much larger DC potentials over time than Ag/AgCl
electrodes. However, since drift artifacts are not as significant
for relatively short time periods, these electrodes may be fine
for use under short-latency conditions. In this case, the primary
differences between Tin and Gold electrodes are that Tin is less
expensive than Gold, however, it is also much softer and can be
easily damaged. Since the EEG current does not pass through the
electrode and only passes over the electrode surface, any
scratches, pits or dents will cause large DC offsets between
electrodes in a recording set. This condition can make measuring
impedance very difficult, particularly, with amplifiers that have
a high front stage gain, like the Synamps. Gold electrodes are
much less susceptible to damage, but their cost is much higher
and they are also subject to sudden depolarization artifacts
referred to as “electrode pops”. This condition looks very much
like an ECG artifact in the EEG data except that it occurs
randomly. Therefore, given the constraints described above, one
must use caution when selecting the electrode metal to use.
http://pub2.ezboard.com/fneurosoftquickcapsupplies.showMessage?topicID=6.topic
-- Doug
PS. The cost of electrodes alone throws a wrench in the idea
of an EEG for < $100
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