[Eeglablist] modifying head model for children in dipfit2

[Scott Makeig]

It is important to weight one's interpretation dipole fitting results from
dipfit (or other inverse methods) by how much or little residual scalp map
variance the component map has from the scalp projection of the equivalent
dipole model - We often ignore independent components whose scalp maps have
residual variances above 15-20% (though this is not a hard and fast
distinction, and should depend in some way on the number of channels
recorded).

ICA gives many components that are highly 'dipolar' (with low residual
variances) -- and, in results we will soon submit for publication, gives
more such components the more mutual information ICA  removes from the data,
though the number of such components identified by ICA varies somewhat from
dataset to dataset depending, first, on the number of data channels, and
then on other (less well defined) 'data quality' factors. Removing stretches
of spatially non-stereotyped ('messy') data from the data to be decomposed
by ICA will definitely increase the numbers of dipolar components identified
in the data.

Scott Makeig



On Fri, Dec 31, 2010 at 6:42 AM, Andrew Dimitrijevic <adimitri at uci.edu>wrote:

>  Hi Dorothy,
>
> This is a common problem (at least with auditory evoked potentials). Some
> suggestions that worked for me in the past:
>
> 1) use "initial seed points" that are approximately in the auditory cortex
> (you can set the initial x,y,z points) before you do the dipole fit
> 2) make sure you have a good SNR (general comment for source analysis, but
> seems to be extra important for DipFit)
>
> the reason that you're getting dipoles in the centre of the head is that it
> "truely" is the best explanation of the variance, albeit it doesn't make
> sense physiologically.
>
> cheers
>
> andrew
>
>
>
> On 12/30/2010 3:01 AM, Dorothy Bishop wrote:
>
>  I am learning to use Dipfit2 with eeglab9_0_4_5s.
>
> All works well if I accept default head model.
>
> I would like, however, to modify the head model for children.
>
>
>
> I followed the instructions for doing this that I found in:
>
> http://sccn.ucsd.edu/pipermail/eeglablist/2007/001690.html
>
>
>
> I kept everything the same as for spherical 4 shell model, except that I
> subsituted a new head model file with altered vol.r values to correspond to
> 8 yr old child. I also, on advice from a colleague, made
>
> vol.c(3)=.0084
>
>
>
> Output coordinates default to MNI when you select custom model file, and I
> kept this.
>
>
>
> I then went to Edit Channel locations to set head radius to be same as last
> vol.r value, i.e. 70.3
>
>
>
> I then ran autofit dipoles , specifying 'fit bilateral dipoles'.
>
> But I end up with single dipoles in the middle of the head.
>
>
>
> I am not sure if the problem is with my model specification, or whether
> there is a bug, and would be grateful for advice.
>
>
>
> thanks
>
>
>
>  Dorothy Bishop, Professor of Developmental Neuropsychology,
> Dept of Experimental Psychology, University of Oxford, OX1 3UD.
> tel +44 (0)1865 271369; fax +44 (0)1865 281255;
> WEB: http://www.psy.ox.ac.uk/oscci/
> Blog: http://deevybee.blogspot.com/
>
>
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-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0559,
http://sccn.ucsd.edu/~scott<http://sccn.ucsd.edu/%7Escott>
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