Lambda

[Marco SPERDUTI]

Thank you Michael,

Marco

Quoting Michael Wibral <wibral at BIC.UNI-FRANKFURT.DE>:

> 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 10
> Optimising 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
>
>> -----Ursprüngliche Nachricht-----
>> Von: "Marco SPERDUTI" <marco.sperduti at UPMC.FR>
>> Gesendet: 02.02.09 18:28:50
>> An: FIELDTRIP at NIC.SURFNET.NL
>> Betreff: [FIELDTRIP] Lambda
>
>
>> Dear users,
>>
>> i'm using the function sourceanaylis, which is the meaning of the
>> lambda parameter?
>>
>> my results change a lot depending on the value i use, so i would like
>> to know in which way i should choose this value.
>>
>> thank you,
>>
>> Marco
>>
>> ----------------------------------
>> The aim of this list is to facilitate the discussion between users   
>> of the FieldTrip  toolbox, to share experiences and to discuss new   
>> ideas for MEG and EEG analysis. See also   
>> http://listserv.surfnet.nl/archives/fieldtrip.html and   
>> http://www.ru.nl/fcdonders/fieldtrip.
>>
>
>
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users   
> of the FieldTrip  toolbox, to share experiences and to discuss new   
> ideas for MEG and EEG analysis. See also   
> http://listserv.surfnet.nl/archives/fieldtrip.html and   
> http://www.ru.nl/fcdonders/fieldtrip.
>

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.