Dear Dr. Friston,
Thank you for for your help. Your reply has told me, that I know very
little about SPM, which is why I have some more questions - sorry.
>Strictly speaking this is true but ingnoring temporal correlations
>this gaves an appropriate fixed-effects model. If correlations are
>present then post hoc temporal smoothing should minimize any effect >of
appending different time-series.
Would this design not have the same flaw as the group analysis, which I
erroneously performed? A boxcar design would not account for the
interscan variability either, which is why I expect to compare the tall
woman with the small man in this design as well? Is the difference
between this design and the one below, that I do not compare the mean
activation of both conditions, but the change in activation over time
(10 min)? It appears that a boxcar functions is not suitable, can I
provide SPM with a function, which models the excepted response?
>You have made no mistake but you cannot attribute this effect to TMS.
Sorry, but I call that a mistake - and what a mistake it is!
>You have used a fixed-effect model to make an inference that >requires
a random effects analysis. Although it may be the case >that the two
sessions differ this is as meaningful as saying that >women are
significantly taller than men on the basis of measuring a >tall woman
and a short man 60 times each. To make an inference >about TMS you
would need to average the scans from several (say 8) >TMS and sham
sessions and then perform a simple group comparison.
So, SPM will dump all scans of 8 subjects into one mean image in each
group and compare those; thus allowing me to make inferences with a
fixed-effect model, where I should be using Dr. Holmes random effects
kit? While the mean image of one subject may or may not be different, is
there a way to consider the timecourse of the signal intensity rather
than the average activation over ten minutes?
I really appreciate your help.
Yours, Christian Keller
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dept. of Neuroradiology
University of Tuebingen
Hoppe-Seyler-Str. 3
72076 Tuebingen
Germany
TEL +49 7071 298-5168 (5642)
FAX +49 7071 295638
[log in to unmask]
currently at the
Laboratory for Magnetic Brain Stimulation
Beth Israel Deaconess Medical Center
330 Brookline Ave, KS 452
Boston, MA 02115, U.S.A.
TEL: 1-617-667-0205
FAX: 1-617-975-5322
[log in to unmask]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|