EGI Cz channel label in a bipolar montage

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1

Hi Brainstorm team

We are using HydroCel Geodesic Sensor Nets (EGI). We would like to preprocess the recorded HD-data using a bipolar montage with 52 channels:

Fp1-Fp2 : Fp1, -Fp2
Fp1-F7 : Fp1, -F7
F7-T3 : F7, -T3
T3-T5 : T3, -T5
T5-O1 : T5, -O1
Fp2-F8 : Fp2, -F8
F8-T4 : F8, -T4
T4-T6 : T4, -T6
T6-O2 : T6, -O2
Fp1-F3 : Fp1, -F3
F3-C3 : F3, -C3
C3-P3 : C3, -P3
P3-O1 : -O1, P3
Fp2-F4 : Fp2, -F4
F4-C4 : F4, -C4
C4-P4 : C4, -P4
P4-O2 : -O2, P4
Fz-Cz : Fz, -Cz
Cz-Pz : Cz, -Pz
Fp1-Fz : Fp1, -Fz
Fp2-Fz : Fp2, -Fz
F7-F3 : F7, -F3
F3-Fz : F3, -Fz
F8-F4 : F8, -F4
F4-Fz : F4, -Fz
T3-C3 : T3, -C3
C3-Cz : C3, -Cz
T4-C4 : T4, -C4
C4-Cz : C4, -Cz
T5-P3 : T5, -P3
P3-Pz : P3, -Pz
T6-P4 : T6, -P4
P4-Pz : P4, -Pz
Pz-O1 : -O1, Pz
Pz-O2 : -O2, Pz
O1-O2 : O1, -O2
F7-C3 : F7, -C3
C3-Fz : C3, -Fz
F8-C4 : F8, -C4
C4-Fz : C4, -Fz
T5-C3 : T5, -C3
C3-Pz : C3, -Pz
T6-C4 : T6, -C4
C4-Pz : C4, -Pz
T3-P3 : T3, -P3
P3-Cz : P3, -Cz
T4-P4 : T4, -P4
P4-Cz : P4, -Cz
T3-F3 : T3, -F3
F3-Cz : F3, -Cz
T4-F4 : T4, -F4
F4-Cz : F4, -Cz

Brainstorm has the channels defined as integers 1 through 128. In order to integrate the desired montage into Brainstorm, we had to translate the traditional electrode identifiers into the corresponding numerical values according to the "HCGSN, 128-Channel Map" available in: https://www.egi.com/images/HydroCelGSN_10-10.pdf. No numerical value for "Cz" is listed on the map, however, we consulted their manual and have tried to replace "Cz" with #129. Unfortunately, this did not work since Brainstorm has not assigned the number 129 to any channel. In addition, we didn't use an adult cap for the recording we are trying to preprocess.

2

There is no need of a channel for Cz. In EGI caps the reference electrode is placed on the position for Cz So all the measures are with respect to it.

The montage F8-F4 is equal to E124-E122. These two signals are with respect to the Ref (Cz), thus it is the same as (E124-Ref)-(E122-Ref). So, a bipolar montage in your list that includes Cz for example Fz-Cz is equal to Fz alone, equal to E11. In the same way Cz-Pz is equal to -Pz or -E62

image
image640×641 61.2 KB

Image from: How context influences the interpretation of facial expressions: a source localization high-density EEG study on the “Kuleshov effect” | Scientific Reports

3

Hi Raymundo

Thank you very much for your reply and the image! My advisor assumed all the signals are with respect to the Ref and that I could write Fz instead of Fz-Cz.
However, it is helpful to have confirmation.