Dear SPMers
In the review of our paper on children (who move quite a lot, and we
used ART to regress out moved volumes) one of the Reviewers asked: “Did
the authors verify that the number of “rejected” scans was equally
distributed across conditions?” So we want to check it now.
In the experiment we had four conditions: two visual and two auditory.
The experimental design was quite rapid – visual trial lasted about 2
seconds and auditory about 4 seconds. The ITI was randomized and
differed between 4-7 seconds. Thus, after convolution of the design with
the standard HRF (which has max about 12 seconds after the onset), the
conditions in the model partly overlap, and “rejection“ of one volume
may affect few conditions in the same time.
My idea is to read the (absolute) values of predicted BOLD signal in the
moment of volume rejection across all conditions from design matrix in
the estimated SPM.mat (SPM.xX.X) and then to sum it within each condition.
1. First: do you think it is a good idea? Or maybe you have other
suggestions?
2. Assuming this is a reasonable idea, I have the second question. As I
have already mentioned the trials differed in length – thus the
predicted BOLD will have higher amplitude in longer (auditory) trials,
than in shorter (visual) trials. Should I normalize the values by the
length of the trials? As I understand GLM normalizes the condition to
calculate betas (am I right?), so to estimate the influence of motion on
betas I should also normalize it?
I’ll be grateful for your comments and suggestions.
Best regards
Marek
--
Marek Wypych PhD
Laboratory of Brain Imaging (LOBI)
Neurobiology Center
Nencki Institute of Experimental Biology
Pasteur 3, 02-093 Warsaw, Poland
Tel.: +48 22 5892 550
http://lobi.nencki.gov.pl/
|