Thanks Jesper
I have tested the categorical comparisons in another separate experiment,
so they are not included in this model.
I am unclear what the higher order polynomials represent in terms of likely
relationship with BOLD response; and whether other people have used them
with good effect.
regards
s
At 02:32 PM 9/7/00 +0200, you wrote:
>Dear Sukhi,
>
>
>
> Dear All
>
> I am analysing a fMRI data set in 8 subjects performing a language task at
> 3 different rates (60 times a minute, 30 times a minute and 15 times a
> minute) within one block of scanning; each task performed 3 times for 30
> seconds at a time. So effectively ABCCBACAB.
>
> I am interested in looking at regions which respond in line with the
> increased rate. Using the parametric modulation and linear model reveals
> little of interest. I assume that a) this is a true negative finding - no
> regions with associated activity OR b) that the regions associated with
> increasing rate do not respond in a linear fashion..
>
>
>One little warning here. What you test when you test a specific regressor (in
>this case the regressor that
>contains the presentation rates) is the variance that is explained by that
>regressor when all variance
>explicable by other regressors have been removed. Hence, if you have included
>the categorical regressors in
>the model, your "rate" regressor will be almost (due to finite length)
exactly
>a linear combination of the other
>regressors, and it will explain virtually nothing over and above those. My
>excuses if you haven't made this
>little mistake.
>
>
> My question is where to go from here?
> Is there a scientific rationale for using different models - further order
> polynomials as per the menu, or is there another smarter way of
defining an
>
> optimal model.
>
>
>Lets say you have included the categorical regressors, i.e. you have one
>column in the design matrix for each
>condition A, B and C. If you now specify an F-contrast, testing these three
>together, that will be equivalent
>to testing any polynomial (or other function) of your rate parameter.
>If that F-contrast doesn't show much, then it is probably not worth searching
>further for any effects of
>presentation rate.
>
>
> Many thanks
>
> Sukhi
>
>
> Good luck Jesper
>
>
>
>
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