A couple of additional comments:

What you want to do is to set up a parametric modulator. This will ensure that the modulator is (a) mean centered; (2) orthogonal to the task effect; (3) represents something of interest.

If column 2 is orthogonal to column 1, then column 1 will have the same values with or with column 2 in the model. If they are not orthogonal, then the values for column 1 will change.

If its a custom regressor, not defined through parametric modulation, but related to the task... I'm not sure how you would interpret it since you have to consider the hemodynamic response of the stimulus.

On Thu, May 28, 2009 at 9:28 AM, Michael Froelich <[log in to unmask]> wrote:
Hi SPM community,
Guillaume, Donald

this is a test set up question following up on previous responses.

Assume the following experiment design: block, one task, 2 sessions, 12 participants, one
custom regressor (R) that models the changing stimulus intensity of the task.

First level Analysis:
I have the following questions:
(1) Is there task related activity? I am setting this up as shown in the attached diagram
with a '1' for the task and all zeros for the rest of the parameters.

(2) Does the addition of the custom regressor (R) change my activation? That's a more
tricky question and I am not quite sure about it. Should I set that one up as a '1 1
followed by zeros' - contrast or a '0 1 followed by zeros' - contrast?

Alternatively, should I run 2 models, one including the custom regressor 'R' and one
without it and I am getting two .mat files in my 1st level analysis, how do I then set up
my model comparison in SPM? I understand that comparing nested models would be an F-
test, but how do I enter two .mat files in the contrast manager?

(3) If I figured out the proper way at the subject (1st) level, how do I use this information
at the second level. Presumably, this depends on the approach at the first level.

Thanks for you help and I hope this will be of interest two others.


Best Regards, Donald McLaren
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
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