(1) Do not model the baseline. The signal due to baseline will be
attributed to the constant term in the model. Additionally, if you
started with baseline, then the model would say the BOLD response
increases because you started the scanner, which probably isn't the
case.
(2) The order doesn't matter. In the GLM, each column is treated as if
it was the last column added to the model. This is a property of the
model and its solution.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General
Hospital and Harvard Medical School
Office: (773) 406-2464
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On Fri, Apr 1, 2011 at 5:33 AM, Ben Becker <[log in to unmask]> wrote:
> Dear SPMers,
>
> I have a questions concerning parametric regressors in an event-related fmri working-memory task with three levels of memory load.
>
> In my first level design matrix I have specified one regressor for the working memory load :
>
> WM load 1 -> parametric modulation = 1
> WM load 2 -> parametric modulation = 2
> WM load 3 -> parametric modulation = 3
>
> Additionally I have a regressor for the baseline condition (fixation cross) and a regressor for the “missed” events.
>
> My questions:
>
> 1. Is it reasonable to include the regressors for the baseline condition and the “missed” events in the 1-level designa matrix?
> 2. Does the does the order in which I specify the regressors in the 1-level design matrix (e.g. baseline / working memory load / missed) matter?
>
> Thanks in advance & best regards
> Ben
>
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