[Eeglablist] modifying head model for children in dipfit2

Dorothy Bishop dorothy.bishop at psy.ox.ac.uk
Fri Dec 31 07:03:45 PST 2010


I'm not sure about what you say because

a) Relatively minor changes to the head model have led to single dipoles in middle of head rather than sensibly placed dipoles. The residual variance was below 2% per dipole with the adult head model and is now much higher.
b) I have explicitly asked for 2 dipoles per component, yet can only see one.  I think I should be able to spot 2 superimposed dipoles in the output, even if the estimate is for a central location.

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/

________________________________
From: Andrew Dimitrijevic [adimitri at uci.edu]
Sent: 31 December 2010 14:42
To: eeglablist at sccn.ucsd.edu; Dorothy Bishop
Subject: Re: [Eeglablist] modifying head model for children in dipfit2

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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