[FieldTrip] Two questions regarding source analysis in Fieldtrip
Cristiano Micheli
michelic72 at gmail.com
Wed Jan 27 14:04:21 CET 2016
Hi Laith
On Wed, Jan 27, 2016 at 10:51 AM, Laith Hamid <lah at pedneuro.uni-kiel.de>
wrote:
>
> Dear Community,
>
> I have two questions regarding source analysis in Fieldtrip.
>
> 1- Is it possible to estimate the regularization parameter (lambda) in
> ft_timelockanalysis or ft_sourceanalysis? How is it automatically
> determined if it is not specified by the user?
>
>
I am reporting a mail by M Wibral (2-3-2009) from the FieldTrip mailing
list here below:
*Hi Marco,in a nutshell the effect of the lambda parameter is to smoothe
your solution in space, it also makes it more stable in the presence of
noise. You might know that the estimation of the covariance matrix for
beamforming requires quite a lot of data. CTF/VSM (a MEG manufacturer) used
to suggest to have your data satisfy the following relationship:3000 <
BW[Hz] * #trials *EffectLength[s]Where BW[Hz] is the bandwidth of your
effect of interest in Hz, #trials is the number of trials that contain that
effect, and EffectLength[s] is the length of your effect in seconds (NOT
ms!).Here's an example: You have an effect between 30 and 60Hz, so the
bandwidth of that effect is 30Hz. The effect is visible (say at the
electrode level) for 400ms=0.4s in each trial. Now you calculate the number
of trials to be:#trials > 3000 / ( BW[Hz] * EffectLength[s]) =
3000/(0.4*30)= 250. This means that you would need 250 artifact free, valid
trials. Choosing a larger lambda can help to reduce the amount of data
necessary, but you get a more smeared out solution.A good introduction and
simulation results for various values of lambda can be found in:Neuroimage.
2008 Feb 15;39(4):1788-802. Epub 2007 Oct 10Optimising experimental design
for MEG beamformer imaging.Brookes MJ, Vrba J, Robinson SE, Stevenson CM,
Peters AM, Barnes GR, Hillebrand A, Morris PG.Hope this helps,*
*Michael*
In short, if you have enough trials you do not need to smoothe your
solution. If not, then apply the rule of thumb described above. I believe
you have an option to declare lambda in percentages as well (e.g. lambda =
'5%'; )
You can refer to this tutorial too:
http://www.fieldtriptoolbox.org/tutorial/beamformer
2- The source analysis results of ft_sourceanalysis contain information
> aobut time series of the sources. How can I show a specific point in time
> (e.g. the epileptic spike's peak) in ft_sourceplot? or is this information
> lost in ft_sourceinterpolate?
>
Can you add your scripts/code? This will add context to the conversation
and allow users to help you further.
If I understand your question, you can try to use the ft_sourcemovie
function, for example like in this tutorial:
http://www.fieldtriptoolbox.org/tutorial/minimumnormestimate
Here it is applied to minimum norm solutions, but the same concept holds
for LCMV beamformer (which you are using right?)
I hope this helps
Cris
> Thank you very much for your help.
>
> Best,
>
> Laith Hamid
>
>
>
>
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