Hi Mike,
thanks for the quick response. I had the same idea (using the
pseudo-inverse instead of the inverse), but were equally unsure. Maybe
someone else can comment on this?
Cheers,
Jan
On 2005-06-17 (Fri) at 08:41:38 -0500, Mike Angstadt <[log in to unmask]> wrote:
> I had the same problem initially. I found a post from someone awhile back
> that suggested changing the inv function to a pinv, which gave me actual
> numbers for my PPI.Y instead of NaNs. However, I asked on here but never
> received any feedback about whether this was a valid fix or not.
>
> -Mike
>
> At 01:57 PM 6/17/2005 +0200, you wrote:
> >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
> >-------------------------------------------------------------------------
> >GnuPG/PGP key id: FEC4B55C
> >fingerprint: 5A36 1EF6 8472 117E 805A F240 3146 A410 FEC4 B55C
> >-------------------------------------------------------------------------
>
--
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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