[FieldTrip] Source reconstruction - low-rank leadfield

Tue Oct 20 12:41:33 CEST 2020

Hi Tibor,

Let me deliberately ignore your follow up questions for now, and focus on the grand scheme of things, and ask a question back:

using the simbio headmodel, and the (it seems) cortically constrained source model as an input to ft_prepare_leadfield, does this result in dipoles which are inconsistently classified as ‘inside’, (as having a ’true’ for the corresponding entry in the inside-field) while the corresponding entry in the lead field cell-array is empty?

If so, this needs to be fixed, because it is incorrect behavior of the fieldtrip code. Again, looking forward to a PR that fixes this (possible) issue.

Best wishes,

Jan-Mathijs

On 20 Oct 2020, at 11:44, tibor.auer at gmail.com<mailto:tibor.auer at gmail.com> wrote:

Dear Diego and Jan-Mathijs,

Thank you for your valuable input.

I am using simbio headmodel with 5 tissue compartments.

I have looked into the code and data, and Jan-Mathijs’ suspicion is correct. I have attached a figure, which shows the location of the 0-ranked leadfield on the sourcemodel, and they are indeed mostly ‘outside normal brain’ (i.e. ventricles and one spot just outside the cortex).

Based on that, I think it is safe to exclude these locations. However, I wonder how these exlcusions might affect the analysis. Does it mean that I will not have an estimate for the affected locations, or will they be interpolated? If the former, then will these locations automatically excluded during group analysis, or do I have to explicitly exclude them?

I understand that the locations I am showing on the attached figure are not of interest anyway.

Kind regards,
Tibor

Auer, Tibor M.D. Ph.D.
tibor.auer at gmail.com<mailto:tibor.auer at gmail.com>
+44-7906-863837
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From: fieldtrip <fieldtrip-bounces at science.ru.nl<mailto:fieldtrip-bounces at science.ru.nl>> On Behalf Of Schoffelen, J.M. (Jan Mathijs)
Sent: 20 October 2020 09:49
To: FieldTrip discussion list <fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>>
Subject: Re: [FieldTrip] Source reconstruction - low-rank leadfield

Hi Tibor, and Jefe,

I guess that the road to a solution starts by looking into the code, and inspecting what line prompts the error (let me give this one away: line 145 in ft_inverse_eloreta).

Also, it might be useful to consider not only the source model, but also the volume conduction model, and the method that was used for the forward computation. This was not mentioned in your messages, so it is hard to comment on that.

Then, it may make sense to think about what matrix gives a rank of 0. Typically, the only way in which matlab returns a 0, is in the case of an empty matrix.

Thus, the suspicion will rise that the eloreta code is trying to operate on dipole positions for which the corresponding leadfield is an empty matrix.

The latter typically occurs if that dipole position is considered to be an ‘outside’ position in the forward computation step, e.g. when it is ’sticking out’ of the brain compartment boundary of the volume conduction model (but again, given the lack of details about the specifics of the volume conduction model etc., this is mere speculation, causing me to already type and think about this for longer than is probably necessary (in case the above is not true) ).

Then, the overall question might boil down to the question: why does the forward computation consider some dipoles to be outside (i.e. not returning a leadfield matrix for those locations), and why does the inverse computation code (at least for particular method) not take that into account?

Assuming that all my speculations above are valid, a principled solution will lie in ensuring that the eloreta code only considers the dipole positions with a valid lead field matrix.

I am looking forward for a PR that addresses this.

Best wishes,

Jan-Mathijs

On 20 Oct 2020, at 10:26, Diego Lozano-Soldevilla <dlozanosoldevilla at gmail.com<mailto:dlozanosoldevilla at gmail.com>> wrote:

Dear Tibor and FieldTrippers
I'm in the same situation. I excluded those positions but now I'm wondering whether this was a good decision. If somebody know what's happening and how to fix this, please drop a line!
Yours sincerely,
Diego

On Mon, 19 Oct 2020 at 20:17, <tibor.auer at gmail.com<mailto:tibor.auer at gmail.com>> wrote:
Dear FieldTrippers,

I have some questions regarding source reconstruction.

We use eLORETA, which runs fine for most participants, however, I receive this error message for quite a few:

the forward solutions have a different rank for each location, which is not supported

The rank of the leadfdield is reduced at each sourcemodel position to the default 3. However, it sometimes has an even lower rank (often 0) at certain sourcemodel positions, and I am not sure how to interpret it. We use a cortical sheet based sourcemodel, and I have noticed that only a handful (<<1%) of positions are affected. My educated guess would be to exclude those positions, but I am not sure how it affects the analysis, either.

Kind regards,
Tibor

Auer, Tibor M.D. Ph.D.
Research Fellow
School of Psychology, Faculty of Health and Medical Sciences
University of Surrey, Guildford GU2 7XH
T.Auer at surrey.ac.uk<mailto:T.Auer at surrey.ac.uk>
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