Dear Matthijs,
> I want to analyze the data of one subject, which consists of two
> identical sessions. The second session immediately followed the
> first.
>
> The design matrix shows two separate collumns. Are the data from
> both sessions combined into one 'big' (common) session? Or are
> the two sessions analyzed separately and are their results
> somehow combined?
> Eg. how is the T-map created?
If you have specified the design matrix as containing two (identical)
sessions, then the design matrix will duplicate the columns for each
individual session (offset in time) and add columns which represent the
session effect. These are the long white bars to the right of the design
matrix that model effects that are constant within a session, but might be
different between sessions. One reason for modelling things this way, rather
than as a single 'big' session, is that it offers a more complete model if
there are session-specific differences in mean or evoked activity
I'm not sure what you mean about whether the two sessions are separately
analysed. SPM fits all columns to the data simultaneously, so in that sense
the sessions are not analysed separately. However, as the regressors are
session-specific, in that sense the regression slopes will be
session-specific.
One way to compute a t contrast is to specify the t contrast that you would
have input for a single session twice. Thus if your contrast for a single
session was [-1 1] then you would input [-1 1 -1 1] for two sessions
modelled separately.
best wishes,
Geraint
--
Dr. Geraint Rees
Wellcome Advanced Fellow, Lecturer,
Division of Biology 139-74, Institute of Neurology,
California Institute of Technology, University College London,
Pasadena CA 91125 London WC1N 3BG
voice 626-395-2880 020-7833-7472
fax 626-796-8876 020-7813-1420
web http://www.klab.caltech.edu/~geraint
--
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