[Eeglablist] Inter-Electrode Distance

Tue Dec 18 03:23:35 PST 2012

Iman, let's clarify further: are you asking how to compute the *geodesic
distance* between two vertices on a mesh? i.e. you have a scalp mesh
segmented from a subject's MRI using Freesurfer or NFT or whatever, and you
want to know the distance between two vertices along the mesh, each of
which is the nearest point on the mesh to a given electrode location?

Obviously elec_distance.m will not do this. But if this is your question,
you can find plenty of Matlab programs and tutorials for computing geodesic
distance on a mesh by googling "matlab geodesic distance mesh." Also, a
2-sec google search revealed that Freesurfer's mris_pmake() function will
return the inter-vertex distance on a mesh (tho' not exactly geodesic
distance) using Djikstra's algorithm:
https://mail.nmr.mgh.harvard.edu/pipermail//freesurfer/2011-April/017978.html.(Ironically,
this was also a question that was re-posted several times
there). EEGLAB or NFT may have a function for this as well, but I don't
recall specifically.

You stated "there is +/- 4 cm difference between using spherical model and
real head on average".  There is a 4 cm difference in what? Co-registered
electrode locations? Inter-electrode distance? If the latter, how are you
calculating distance (given that this is the topic of your post)? And
of course, warped electrode locations (and thus electrode distance) may be
fairly different if you warp to a spherical model versus an MNI head model
versus using an individual subject's head model.

It seems like there are at least two separate issues here and it's not
clear which you're asking about: (1) how to best localize/co-register each
electrode to it's nearest point on the scalp surface (i.e. vertices of a
suitable scalp mesh) or (2) how best to measure/approximate the distance
between two points on the scalp (i.e. mesh geodesic distance, shortest path
length (Dijkstra's algorithm), radial distance, euclidean distance, etc).

Other important factors: are your electrode locations digitized (e.g. using
a Polhemus digitizer) and co-registered to the individual subject head
model? Or are you assuming standard 10-5 (or whatever) locations and either
aligning/warping the electrode locations to the head model (e.g. EEGLAB's
coregister.m) or warping the head model to the electrode locations (e.g.
NFT's warping_main_function.m)? If you don't have digitized locations you
may easily have >>4 cm of error between assumed and true electrode
locations, which will propagate into any distance measurements,
irregardless of the distance metric you use.

Tim

On Tue, Dec 18, 2012 at 1:25 AM, Iman M.Rezazadeh <irezazadeh at ucdavis.edu>wrote:

> Thanks Tim for your response ! and your two cents: ) ****
>
> **·        **Ok let me be clear. If we have electrode location file and
> fit anatomical head model ( not spherical ) the results will different .
> So, my question is " Is there ANY ANATOLICAL MODEL based on the electrode
> locations/labels that can help us to measure the inter-electrode distance?"I have done a test over 3 different individual scalp and there is +/- 4 cm
> difference between using spherical model and real head on average.****
>
> **·        ** ****
>
> Best,****
>
> Iman****
>
> *From:* Tim Mullen [mailto:mullen.tim at gmail.com]
> *Sent:* Tuesday, December 18, 2012 1:04 AM
> *To:* Iman M.Rezazadeh
> *Cc:* Makoto Miyakoshi; UCSD EEG List
> *Subject:* Re: Inter-Electrode Distance****
>
> ** **
>
> No worries, Iman -- just my two cents :) The email mailing list format is
> also not the easiest format to search and follow up on old posts. But I do
> think it helps reduce everyone's inbox clutter (and likely ensures you get
> a faster/better response) if new posts make clear how the given problem is
> not addressed by previously posted solutions.****
>
> ** **
>
> In the 2010 thread Arno describes a solution for the electrode distance on
> the scalp assuming a spherical head model, and this is implemented in the
> function elec_distance.m, which you referred to. Asking "does
> elec_distance.m work properly?" is something you can probably quickly
> answer yourself :) Do you have reason to think this function is not working
> properly? Is this not the solution you were hoping for? The more specific
> and clear your question, the more help can be provided by the community.**
> **
>
> ** **
>
> Tim****
>
> ** **
>
> On Mon, Dec 17, 2012 at 4:51 PM, Iman M.Rezazadeh <irezazadeh at ucdavis.edu>
> wrote:****
>
> Tim, I have   searched the forum and if you look at my email you can see
> that I mentioned the function name as well. Also what Steve pointed at is
> sth different ! ****
>
> Anyhow, thanks for the reminder for reading the archives and internet
> search J  and I would add reading more carefully the emails J ****
>
> Iman****
>
>  ****
>
> *From:* Tim Mullen [mailto:mullen.tim at gmail.com]
> *Sent:* Monday, December 17, 2012 4:18 PM
> *To:* Makoto Miyakoshi
> *Cc:* Iman M.Rezazadeh; UCSD EEG List
> *Subject:* Re: Inter-Electrode Distance****
>
>  ****
>
> Iman, a general rule of forums and mailing lists (such as EEGLAB) is to
> first search the forum/list archives for a solution. In this case, the
> question has been addressed at least once before: i.e. search for
> 'inter-electrode distance' in the subject lines for 2010 --
> http://sccn.ucsd.edu/pipermail/eeglablist/2010/subject.html. If you can't
> find a solution in the archives (or after a little internet searching),
> then by all means post to the list :)****
>
>  ****
>
> Also, I just saw that Steve noted this topic came up a few days ago on the
> list...****
>
>  ****
>
> Tim****
>
>  ****
>
> On Mon, Dec 17, 2012 at 3:54 PM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
> wrote:****
>
> Dear Iman,****
>
>  ****
>
> Why don't you try it first since the function is already there :-)****
>
>  ****
>
> Makoto****
>
>  ****
>
> 2012/12/17 Iman M.Rezazadeh <irezazadeh at ucdavis.edu>****
>
>  ****
>
> Hi,****
>
> Could you please let me know how can I measure the inter-electrode
> distance on the head surface? Does elec_distance.m woeks properly? ****
>
>  ****
>
> Iman M.Rezazadeh, PhD****
>
> Postdoctoral Research Fellow****
>
> Center for Mind and Brain****
>
> University of California, Davis****
>
> irezazadeh at ucdavis.edu****
>
> Cell:310-490-1808****
>
> Skype: Imanmr****
>
>  ****
>
>
>
> ****
>
>  ****
>
> --
> Makoto Miyakoshi
> JSPS Postdoctral Fellow for Research Abroad
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego****
>
>
>
> ****
>
>  ****
>
> --
> ---------  αντίληψη -----------****
>
>
>
> ****
>
> ** **
>
> --
> ---------  αντίληψη -----------****
>

-- 
---------  αντίληψη -----------
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20121218/2fbff075/attachment.html>