EEG source localisation questions

[Ajay Halai]

My apologies, I have searched through the archives, and found a similiar
problem by Floris de Lange, to which Robert.O replied. I will try that
suggestion. Sorry to fill up your inbox.

best
Ajay

Roberts reply:

"
Last week I discussed similar issues with Jan-Mathijs. Sofar at the  
FCDC we have not really optimized the data handling for LCMV  
beamforming. Markus Bauer is one of the few who had a go at it, and  
for him it did not really work that well. Jan-Mathijs mentioned that  
sofar we have been working with the data covariance that was estimated  
based on the single trialsd, and not with th edata covariance  
estimated on the average (note that the order matters for the  
covariance computation and averaging). Right now I don't know the  
details of your experiment and data any more, but you might want to try
   data = preprocessing(cfg)
   avg1 = timelockanalysis(cfg, data) with keeptrials=no, covariance=no
   avg2 = timelockanalysis(cfg, avg1)  with keeptrials=no,  
covariance=yes
and then use avg2 which includes the covariance of the average in the  
sourceanalysis.

best regards,
Robert
"

On Wed, 28 Jul 2010 12:43:30 +0200, Ajay Halai
<ajay.halai at POSTGRAD.MANCHESTER.AC.UK> wrote:

>Dear FT users,
>
>I am a 1st year PhD student and new to FIELDTRIP, and any help will be very
>welcome. I have managed to run simple ERP analyses on a language experiment.
>I am now hoping to localise the peaks (P100 and N400) but am coming across
>some problems.
>
>I will try my best to explain briefly my current method of analyses and
>would greatly appreciate any comments or suggests, as I fear I am making a
>mistake somewhere.
>
>The problem I encounter is that the values returned for the avg.pow are the
>same for each condition. Naturally, I made sure that I specified different
>conditions at the ft_sourceanalysis (cfg, data) stage, but I did specify
>different conditions. I also checked the avg.noise, and was surprised to see
>these values were the same as the avg.pow, which leads me to think I have
>done something wrong or am failing to interpret this. Furthermore, I tried
>to then use loreta, and selected the timelock data, with the same cfg but
>get an error (Undefined function or method 'loreta' for input arguments of
>type 'struct') and wonder if I need to specify other cfg.
>
>I used the 'lcmv' method. projectnoise 'yes', rawtrial 'no', lambda = 0, and
>using the vol, grid and elec files from the created leadfield.
>
>Any suggestions on what I may be doing wrong would be greatly appreciated.
>Additionally, I have provided some more details about the stages before
>this, which may point to the problem.
>
>Best wishes 
>Ajay Halai
>
>analysis details:
>
>I have 7 conditions in all, but take 2 conditions for 1 subject as an
>example. I have two types of speech, which show significant differences at
>P100.  I used a bandpass filter (1-40hz), padding (0.1), baseline correction
>(-0.1 0), detrend and average EEG reference at the preprocessing stage. I
>removed artifacts by first removing components using the 'fastica' method,
>and then a visual rejection. Following this, I specified the time of
>interest limit to the P100, and used the timelockanalysis. I have not used
>keeptrial or keepindividual cfg, although I don't think this is the cause of
>the problem.
>
>I do not have individual MRIs, therefore I have used a template from the
>MNI. I have applied a segmentation (using volsegment). I specified the elec
>positions (64 sensors), and prepared leadfields using reducerank 2,
>threshold 0.1, smooth 5, units 'mm'
>
>___________________________________________________________________
>Neuroscience and Aphasia Research Unit (NARU)
>School of Psychological Sciences (Zochonis Building)
>University of Manchester 
>Brunswick Street Manchester
>M13 9PL
>UK
>
>----------------------------------
>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/neuroimaging/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/neuroimaging/fieldtrip.