[Eeglablist] modifying head model for children in dipfit2

Andrew Dimitrijevic adimitri at uci.edu
Fri Dec 31 06:42:12 PST 2010


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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