Dear Darren and others,
I came across a peculiarity in the combination of data extraction via
the VOI button and subsequently setting up a PPI.
Here the basic design specs:
- single subject analysis with 3 sessions
- extract data with VOI button for one session (e.g. 1st session) with
adjustment for effects of interest (null space X0 comprises only the
session constants)
- PPI setup: (1) 'psychophysiological interaction'
(2) include only one regressor (modelling a block of
pain stimulation)
(3) weight for this regressor: 1
The warning mesage that I get when setting up the PPI is the following:
>> Warning: Matrix is singular to working precision.
(Type "warning off MATLAB:singularMatrix" to suppress this warning.)
> In /apps/public/spm02/spm_peb_ppi.m at line 204
Looking at line 204 in spm_peb_ppi.m,
Yc = Y - X0*inv(X0'*X0)*X0'*Y;
I realize that the warning can be traced to the null space X0, because
the inverse of X0 is not defined. Consequently, PPI.Y is a vector of
NaN's.
X0 is created by spm_regions.m when I press the VOI-button and
comprises in my situation the session-specific block partition of the
design matrix (xX.iB) and high-pass filter (xX.K). Thus, in the
3 session case the first part of X0 has three column, the first with
1's and the second and third with 0's. Because these are perfectly
collinear the inverse of X0'*X0 in line 204 is not defined.
Here are my questions:
(1) Am I doing something wrong when I adjust for the effects of
interest when I extract the data with spm_regions?
(2) Is this a problem at all for the PPI computations?
(3) Conceptually speaking, shouldn't the null space X0 only comprise
the "real" session constant (the column of 1's) and not the entire
block partition of the design matrix (i.e., leave out the columns
of 0's)? This would allieviate the problem with perfect
collinearity of some of the columns in X0.
Thank you for your comments in input.
Jan
--
Jan Gläscher NeuroImage Nord
+49-40-42803-7890 (office) Dept. of Systems Neuroscience, Bldg S10
+49-40-42803-9955 (fax) University Medical Center Hamburg-Eppendorf
[log in to unmask] Martinistr. 52
20246 Hamburg
Germany
http://www.uke.uni-hamburg.de/kliniken/neurologie/index_16969.php
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