 |
 |
Thanks for your response, Jan. I am aware that a parametric modulator that does not change will not explain additional variance in the data. For some of my subjects this parametric value changes and for some it does not. I could modify my scripts to omit the non-changing modulator, but technically SPM should be capable of tossing out this modulator automatically without crashing which is what the original code in spm_orth is intended to do by using the any() function. spm_orth should not work successfully for some vectors and fail on others. But, in the meantime I'll use your solution to work around the problem. Thanks, Ryan --- On Wed, 5/13/09, Jan Gläscher <[log in to unmask]> wrote: > From: Jan Gläscher <[log in to unmask]> > Subject: Re: [SPM] Possible bug in spm_orth > To: "Ryan Haynes" <[log in to unmask]> > Cc: [log in to unmask] > Date: Wednesday, May 13, 2009, 2:08 PM > Hi Ryan, > > I came across a similar problem before and admittedly > didn't spend as much > time as you on figuring out a solution for spm_orth, > because in the end it > doesn't make sense to include a parametric modulator > that only has the same > value for all events, not even a single one. It doesn't > explain any > additional variance in the data that is not captured by the > primary onset > regressors. Therefore, when coding up some batch scripts I > always check if > my modulator has at least 2 differing values in it and if > not, I don't > include it in the design matrix. > > Cheers, > Jan > > > Ryan Haynes wrote: > > Hi, > > > > I am trying to determine if an error I'm getting > in spm_fMRI_design is coming from a possible round-off error > in spm_orth. This bug is extremely rare as it only occurs in > certain cases of parametric modulators and only in the > situation in which the modulator for every event is > identical such that the two vectors for an event are > linearly dependent. For example, consider the following > matrix "u" inside spm_get_ons: > > > > 1 40 > > 1 40 > > 1 40 > > 1 40 > > 1 40 > > 1 40 > > 1 40 > > > > spm_orth on this matrix returns a vector of 1's > and a vector of 7.10e-15. Mathematically this should be a > vector of 1's and a vector of 0's of course and for > most practical purposes 7.10e-15 is essentially 0. But, the > problem that occurs is that if you run spm_orth on this > matrix again you get a vector of 1's and a vector of > 0's. This throws and error in matlab in spm_fMRI_design > because of the following: > > > > for i = 1:length(Fc) > > X(:,Fc(i).i) = spm_orth(X(:,Fc(i).i)); > > end > > > > Matlab expects the return of spm_orth(X(:,Fc(i).i)) to > have the same number of dimensions as X(:,Fc(i).i) which is > doesn't since spm_orth drops vectors that contain only > 0's. What should have happened is that spm_orth should > have dropped the vector containing 7.10e-15 originally. > Instead of the following code in spm_orth > > > > if any(D) > > x = [x D]; > > end > > > > maybe this code should be used: > > > > epsilon = 1e-10; > > if ~isempty( find(D) > epsilon ) > > x = [x D]; > > end > > > > The reason this bug is rare is that it only occurs for > certain numbers and for certain lengths of vectors. For > example, the following vector of length 6 > > > > 1 40 > > 1 40 > > 1 40 > > 1 40 > > 1 40 > > 1 40 > > > > does not cause this error while the one of length 7 > does. Likewise, if you replace 40 with 50 in the > 7-dimensional vector it also does not cause the error. > Please let me know if this actually is a bug, or if not, > please let me know what I'm doing wrong. Thanks. > > > > Ryan Haynes > > > > -- > Jan Gläscher, Ph.D. Div. Humanities & Social > Sciences > +1 (626) 395-3898 (office) Caltech, Broad Center, M/C > 114-96 > +1 (626) 395-2000 (fax) 1200 E. California Blvd > [log in to unmask] Pasadena, CA 91125