Dear Dr Henson,
Recently we are analyzing our fMRI data using spm2 and would like to
incooperate reaction times of the two levels (A, B) into the model as
parametric factor.
A and B are game tests of different categories, and which cause significant
difference of reaction time to them. For the possibility to specify model
with parametric modulation, we searched in the archive and foud several
piceses of your responses to similar qustions. In one of them as listed in
the following, it is allowed to specify a categorical modulation by +1/-1
assignment. Actually, with your suggestions, we tried categorical modulation
effects by [+1/-1] and [+1/0]. The results looked similar. However, we are
wondering: are they really the same? How about [+1/+2] ?
Best,
Kuo
-------------------------------
Date: Tue, 1 Jun 2004 16:11:46 +0100
Reply-To: Rik Henson <[log in to unmask]>
Sender: "SPM (Statistical Parametric Mapping)" <[log in to unmask]>
From: Rik Henson <[log in to unmask]>
Subject: Re: RT as covariate
Comments: To: [log in to unmask]
Content-Type: text/plain; charset=us-ascii
Andreas -
It is a little known fact that SPM (SPM99 and SPM2) can handle
more than one parametric modulation per trial-type. The trick
in the GUI is to enter several numbers (eg "1 1 1"), rather
than a single number, when SPM prompts for "which trial-type"
(to modulate). Entering "1 1 1" for example will then allow
you to enter 3 modulations, each of which is applied to the
first trial-type. (It is even easier in batch mode, where
you can give them different names and expansions.)
For an example from SPM2 (which again is also possible in
SPM99), in which three modulations similar to what you
would need - namely a continuous modulation (like RT),
a categorical modulation (a +1/-1 condition marker) and
a modulation for their interaction, see:
http://www.fil.ion.ucl.ac.uk/~wpenny/datasets/face-rep/SPM2/README-SPM2.txt
(further down the file in the "parametric analysis" example).
You can generalise this to your 3 condition + no-response trials.
Rik
|