Dear Henrik,
> So we suspect that we have done
> something wrong, - perhaps of the close temporal succession between
> fixation cross and the prime?
Indeed this may be the case. But I would also check that you have specified
the onsets of the events correctly w.r.t. fMRI_T0 as discussed in a few
recent posts. I also assume that you are using the 'hrf only' basis - if you
have used temporal derivatives also, that may be clouding the issue by
allowing the basis set to be too unconstrained temporally to separate two
closely spaced events.
> 2. Modell epochs.
> a) Modell the time from Onset of fixation cross till reaction as an
> epoch.
I would try this to start with, which treats everything from fixation onset
till probe offset as a 'mini-epoch'. Conceptually there is little
distinction between 'events' and 'epochs' in that the latter are just trains
of the former. This approach has the disadvantage that the difference
between each event type and (implicitly modelled) baseline will subsume not
just word processing but also fixation cross activity etc. However as
fixation is common to all conditions this is not true for differential
contrasts (e.g. A vs B) which should hopefully reflect the prime/probe
differences of interest.
> If we define a contrast, say a 1 for Condition A, then this contrast
> tells us what happens in the brain during condition A compared to what
> is not defined as an epoch. Or we can define a contrast say a 1 for
> condition A and a -1 for condition B. Then this contrast tells us what
> is activated during a trial epoch in condition A more than in condition
> B. Is this correct?
Yes.
Best wishes,
Geraint
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Dr. Geraint Rees
Wellcome Advanced Fellow, Lecturer,
Mailstop 139-74, Institute of Neurology,
California Institute of Technology, University College London,
Pasadena, 12 Queen Square,
CA 91125 London WC1N 3BG
voice (626) 395-2880
fax (626) 796-8876
web http://www.klab.caltech.edu/~geraint
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