[FieldTrip] Question regarding the suitability of source localization models for in-ear electrodes

[Schoffelen, J.M. (Jan Mathijs)]

Hi Pouria,

Forward (and thus inverse) modeling of extracranial EEG signals requires (algorithmically) that the electrodes are located on the outermost boundary of your volume conduction model. In other words, electrodes whose coordinates are not directly on top of the outermost boundary will be projected there before any additional computations (i.e. forward computations, a.k.a. leadfield calculation) can be performed. I have my doubts whether what you refer to as ’the standard model’ has sufficient anatomical detail to have ‘ear canals’, which means that your electrodes will end up (most likely) somewhere on the surface of the head. If you really want to model this properly, I think that you would need a volume conductor that has (and numerically allows for) this fine geometric detail. I suspect that this means you need to build a fine grained FEM model, ideally based on your individuals’ anatomical images (which then also allows for more accurate anatomy-to-electrode coregistration), but perhaps a template FEM-model may be good enough for your scientific question. Using FEMs also allows for a more fine grained modelling (per participant) of the tissues. Standard models (both BEM and FEM) may not do justice to the real anatomy.

Best wishes,

Jan-Mathijs


On 22 Jun 2026, at 15:06, Motaghi, P. (Pouria) via fieldtrip <fieldtrip at science.ru.nl> wrote:

Dear FieldTrip community/developers,

I hope my email finds you well.

My name is Pouria and I am in the third year of my PhD at Radboud. During my PhD we have designed an in-ear electrode and we would like to test the added value of it in detecting the sources involved in fear/threat processing. For this, I would like to compare the source models with and without the in-ear electrodes. We do not have individual head models and we have calculated the coordinates of the in-ear electrodes by estimating them using the length of the in-ear electrodes (20 mm). Here are the coordinates:
Left in-ear electrode: theta: -122, phi : 0, radi: 0.77
Right in-ear electrode: theta: 122, phi : 0, radi: 0.77

We were thinking if we give the models these inputs, would they take it into account that these electrodes are outside of the skull or because the radii coordinates are less than 1 the model might think that these two electrodes are located inside the skull and therefore not take the skull, scalp, etc into consideration in estimating the source? Also since these two electrodes are placed inside the ear canal, I was thinking if the head model makes a distinction between the tissue layer covering the ear canal and the tissue layer covering the skull (i.e., scalp). In general, I am not sure how accurate the standard head model is in taking the actual location of the in-ear canal electrodes into account. Would you be able to help me with this? Please let me know if you have any questions.

Thank you in advance for your help.

Kind regards/Met vriendelijke groeten,

Pouria Motaghi, M.D. | PhD Candidate | Behavioural Science Institute | Radboud University

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