If possible, leave other electrically connected sensors (such as EMG, EKG, SC) unconnected
during EEG-Z impedance checking, or else perform a test to see whether their connection causes
a significant change in the reading.
Sensors (even EEG-Z sensors) attached to the same client but connected to
another encoder also
have the potential to affect EEG-Z readings. In particular, do not attempt to perform impedance
checking on two encoders on a single client, at the same time. Here again, test for the effect of
other connected sensors if you are concerned about receiving precise impedance readings.
10-20 EEG Electrode Placement System
To get a full understanding of the standard 10-20 EEG electrode placement system, please refer
to professional publications on EEG. This section will only give you an introduction on the subject.
Since no two people have the same head size and shape, the first step in defining correct
electrode locations is to make some measurements. Two measures are required: The distance
between the supra-nasal grove (nasion) and the little bump in the mid occipital region (inion)
and, the distance between the two pre-auricular notches (this is the dimple just in front and
slightly above the tragus. Once these two distances are measured, you need to find the spot over
which the two lines bisect at their mid-point. Calculate 50% of both measures and find the point,
on the client's head, where they cross over. That point is called Cz, and it is the first 10-20
location that you need to identify. Use a washable marker to mark the spot.
From Cz, you can easily find C3 and C4 by moving laterally towards each ear, along the
imaginary line that you drew between the pre-auricular notches. C3 is 20% of the full distance
towards the left and C4, 20% of the full distance to the right. C3 and C4 are also very important
points for EEG biofeedback.
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