Neuromag data processing

Christian Hesse c.hesse at FCDONDERS.RU.NL
Wed Feb 21 09:02:31 CET 2007

Hi Eve,

> I can use meg-pd toolbox now. However, it seems all the functions
> in FieldTrip are for evoked data while my raw data is continuous data.

FieldTrip handles a variety of data, and the low level reading
functions can deal with continuous or epoch based raw data.

When you analyze your data in FT, you usually start by defining your
trials, i.e. epochs in your continuous data that are of interest,
w.r.t. to some sort of trigger or event. The function PREPROCESSING
then scans through your raw data file and returns a data structure
which contains all of the specified epochs (for a particular trial
definition) in a field data.trial. Please note that this has nothing
to do with evoked vs induced responses just yet.

You can use the output of PREPROCESSING as input to either
TIMELOCKANALYSIS or FREQUANALYSIS which look at evoked and induced
phenomena, respectively. Afterwards you can call various statistical
functions such as FREQDESCRIPTIVES, etc ...

So the reason why I have been giving you the examples using
read_header and read_data is because I needed to determine the nature
of the error you were getting without really knowning anything about
what sort of data is in your file. In your use of FT you do not
actually use these functions yourself, but basically use functions
like DEFINE_TRIAL and PREPROCESSING to start with.

>  I tried your suggestion. read_head works but read_data doesn't
> work. I don't know this error just due to memory or continuous data
> format. I remember previous discussion (2-feb-2006) in this
> emaillist mentioned that read_data couldn't deal with continuous
> data. The error shows as below:
> ??? Out of memory. Type HELP MEMORY for your options.
> Error in ==> F:\MATLAB6p5\toolbox\fieldtrip-20070213\private
> \read_data.m
> On line 529  ==>         dat(:,((i-begepoch)*hdr.nSamples+1):((i-
> begepoch+1)*hdr.nSamples)) = buf(chanindx,:);
> Error in ==> F:\MATLAB6p5\toolbox\fieldtrip-20070213\read_fcdc_data.m
> On line 49  ==> [dat] = read_data(varargin{:});
Just as one final double check, can you try the following to confirm
that you can read a block of data from your .fif file:

filename = 'fullpath/name.fif';
dat = read_fcdc_data(filename, 'begsample', 1, 'endsample', 100);
% this should be an nchan x 100 matrix

Please let me know if this works, and then look at how to use the
PREPROCESSING and DEFINE_TRIAL funcitons to extract relevant epochs
from your raw data file (see


Christian Hesse, PhD, MIEEE

F.C. Donders Centre for Cognitive Neuroimaging
P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands

Tel.: +31 (0)24 36 68293
Fax: +31 (0)24 36 10989

Email: c.hesse at

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