Dear Mathias,
I'm not sure whether I've got your paradigm right, but I guess a contrast
[0 1 0] is the relevant contrast in your case, given the fact that the
second column in the design matrix is the parametric modulation (PM).
The interpretation may depend on how you have specified the parameter. If
PM is centered around '0', then this contrast gives you the areas that
showed more activation for positive values and less activation for
negative values of PM. If PM is not centered then PM gives you the
activation that comes on top.
Best regards,
Karsten
Den 04.06.13 20:05 skrev "Dr. Mathias Scharinger" <[log in to unmask]>:
>Dear experts,
>
>this topic has been discussed partially before, but for my particular
>case, there seems to be no clear consensus.
>I have a relatively simple auditory category learning design, where
>participants are presented with tones that differ in duration and
>frequency, with different spreads of duration and frequency values.
>Participants have to attribute each stimulus to one of two possible
>categories, and receive immediate feed-back. The task is done while we
>recorded 2-seconds of EPI volumes in a sparse design, i.e. after the
>participants button presses.
>In 25% of all cases, instead of tones, we would present silent trials
>(null trials) that should serve as baseline for the subsequent analyses.
>Behaviorally, we find that the Euclidean distance of each tone to a most
>ambiguous 'mid-point' in the acoustic space (of duration and frequency)
>modulates accuracy: The closer a given tone to this ambiguous point, the
>less accurate the response.
>For this reason, we wanted to modulate the Euclidean distance in SPM, on
>the first level, as parametric modulator of 'tone' (this distance is
>available for each tone).
>If modeling null trials, the resulting design matrix has three columns,
>one for 'tone', one for the PM 'distance' and one for 'nulltrial'.
>While it seems straightforward to test 'tone' against 'nulltrial' (with
>the vector 1 -1), it is less clear whether the same is allowed for the PM
>'distance'. I am not asking whether it is possible (that I figured out
>already), the question is, is it legal to compare the PM 'distance'
>against the 'nulltrial' (using the vector 1 -1)? Is it just a different
>baseline (e.g. if specifying the PM 'distance' with 1 and the nulltrial
>with 0, I understand that this tests against an implicit baseline)?
>
>Thanks for comments & help,
>
>Mathias
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