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Using EEG with MEG

We have a 64 Channel EEG system, use EASYCAPs specifically designed for our TRIUX system, purchased from Brain Products UK, and have 6 caps available in 4 different sizes. The caps use the 10/20 system of electrode placement.

  • 1 x 54cm, 2 x 56cm, 2 x 58cm, 1 x 60cm

64Ch BrainCap-MEG for Triux - Electrode Layout/Channel Assignment

64Ch BrainCap-MEG for Triux - Table of Coordinates

Digitisation

To digitise the 64 electrodes, follow the Instructions for Digitisation copied from MEGIN's EEG Cap User's Manual
The EASYCAP layout closely follows the MEGIN caps, but there are some differences in electrode usage (EASYCAPs have T9 & T10, but no PO5 & PO6) , and when certain electrodes are digitised, and be wary of the extra EOG electrodes built into the EASYCAPs
- See the 64 BrainCap-MEG electrode layout above.

NOTE: One additional important difference between the two systems: the channel numbering (channel index mapping) is not the same.
For example, channel Fpz ...
- Is registered as EEG020 for the MEGIN caps, whereas it is ...
- EEG002 for the EASYCAP.
(The data is recorded as EEG001, EEG002, etc., rather than as Fp1, Fp2, and so on).

If using EASYCAPs ...

The eeg_digit_maps file in /neuro/dacq/setup has been edited to create a new layout to help digitise the EASYCAPs.

  • Select all EEG channels in megacq (click the EEG on button).
  • After Coordinate frame alignment and HPI Coils digitisation, select EEG EasyCap 64 from the dropdown sub-menu, then click on EEG ecectrodes to start digitising each electrode.
    • The REF electrode is always the first one to digitise ("0").

The new digitisation order (taken from eeg_digit_maps) is as follows ...

# A map for 64-channel EASYCAP (05/02/2026 - JLW & CZ)
EEG EasyCap 64:0:1:2:3:4:5:6:7:8:9:10:11:12:13:14:15:16:17:19:20:21:22:18:23:24:25:26:27:28:29:30:31:32:33:34:35:36:37:41:42:43:44:45:46:47:48:49:53:54:55:56:57:58:59:60:61:64:38:39:40:62:50:51:52:63

If using MEGIN Caps ...

To help in digitisation, three pages have been copied from MEGIN's EEG Cap User's manual above ...

She says ...

"I exchanged numbers of green electrodes so that they go from 1-32 instead of 33-64.
This new numbering is compatible with electrode numbering on SIGGI device.
So one can save a little bit of grey matter energy and skip subtracting number 32 when checking the impedances.
We should however not get rid of the old layout, which in turn is compatible with electrode numbering in the acquisition software (numbers going from 1 to 64)."

Procedure

Not many MEG Operators have undertaken EEG & MEG, but those that have used the following procedure/digitisation order:
Many thanks to Oscar Ferrante & Tara Ghafari @2020

They used the MEGIN caps
"We attached the HPI coils to the MEGIN cap because that way we could easily reach them during digitisation.
If they are placed on the head (as in normal usage), this needs to be done so that the Polhemus pointer can reach them under the EEG cap with a perpendicular orientation - it needs to be very well thought out."

When queried if it was possible to digitise the head shape before the 64 electrodes ...
"I’m not sure it’s possible to digitise the head shape before doing it for the EEG electrodes in the MEGIN software.
If so, it’s worth a try. But the HPI coils would then need to be attached to the head, adding the problems mentioned above.
Doing the head shape digitisation once the cap is in place is not perfect as it introduces some noise, which can make the coregistration less accurate.
It’s a trade-off. In our analysis (i.e., visual response in posterior and prefrontal areas), this extra noise did not have a strong impact, but I guess it depends on the specific experimental question."

NOTE: Autumn 2025: Coen & Helena found that using the BIO GND electrode (which they're using for EMG recording), rather than the EASYCAP GND electrode, produced better signal quality overall when plugged into the gantry side panel.
The side panel REF connection was made using the EASYCAP REF electrode (no BIO REF electrode was attached to the participant).