Yes, the statistics should be exactly the same, because SPM
mean-corrects parametric modulation values, making [1 0] (and [2 1])
equivalent to [1 -1], bar a scaling which affects the parameter
estimates but not the statistics. ([1 2] will have the same statistics
except reversed sign.)
Rik
Nissen W-J Kuo wrote:
> 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
--
-------------------------------------------------------
DR RICHARD HENSON
MRC Cognition & Brain Sciences Unit
15 Chaucer Road
Cambridge, CB2 7EF
England
EMAIL: [log in to unmask]
URL: http://www.mrc-cbu.cam.ac.uk/~rik.henson
TEL +44 (0)1223 355 294 x522
FAX +44 (0)1223 359 062
MOB +44 (0)794 1377 345
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