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Hi Helmut, Sorry I haven't been too clear. My mistake. The contrast for when I use the hrf+time derivatives has got, in fact, more columns: To see only the effect of task A for example (which is repeated twice, as also tasks B and C) I used the contrast: [0 0 0 0 1 -1 1 -1 0 0 0 0] With "B C A A C B" I meant the order in which I acquired the fMRIs and therefore for consistency I put the data from those sections in the same order in the large final density matrix. I meant to say that I have got a large density matrix which includes 6 sessions, and therefore 6 small matrices. In the case of only hrf every small matrix has got just one column or regressor (apart from the constant always put at the end by SPM). In the case of hfr+time derivative the small matrix has got in fact two columns. I am not sure what you mean when you ask if I included the dispersion, I guess I didn't otherwise I would have known ... At least I hope. I must say that the change of perspective you mention is quite interesting. But it means I haven't got a clue of how brain really works. Does it really happen sometimes that it is more active at the "off-times" instead of at the "on-times", or is it just an indication that something has gone wrong in the subtraction between data on the on and off condition? Laura -----Original Message----- From: Helmut Laufs [mailto:[log in to unmask]] Sent: 01 July 2005 08:28 To: Mancini,Laura Cc: SPM (Statistical Parametric Mapping) Subject: Re: deactivation Hi Laura, could you please clarify what your regressors in your model are (you say you have 3 conditions and model either hrf or hrf+ time derivatives [incl. dispersion?], but it appears you always have 6 colums in your design). I cannot work it out, even should you have put one colum per block (B C A A C B), which gives six colums, including the derivatives should give you more... In principle: onbe [/another] way to look at 'deactivations' is this: change your perspective and see it as: the set of brain areas that 'deactivates' during your respective condition is in fact more active at the "off-times" of your condition. Maybe this is a start? Helmut ----- Original Message ----- From: "Laura Mancini" <[log in to unmask]> Sent: Thursday, June 30, 2005 6:52 PM Subject: deactivation > Dear SPMers, > > I wonder if you can help soving a puzzle. > > I have 6 fmri acquisition, two for conditions A versus rest, two for B > versus rest, two for C versus rest. The order of the acquisitions is: > B C A A C B A = only hand movement > B = only speech production > C = hand mov + speech prod > I did only t-tests, not F-tests. > If as basis set for the SPM analysis I consider only the hrf I have the > following. > When I consider the contrast [0 0 1 1 0 0] I see a huge activation in the > motor cortex, while with [0 0 -1 -1 0 0] there is little or no > (de)activation > When I consider [1 0 0 0 0 1] or [0 1 0 0 1 0], I see little activation in > some parts of the brain, while with [-1 0 0 0 0 -1] or [0 -1 0 0 -1 0] I > see > large (de)activation almost everywhere in the brain. > > If as basis set for the SPM analysis I consider only the hrf+time > derivatives I have the following. When I consider the contrast [0 0 1 > 1 0 0] I see much less activation in the > motor cortex, while with [0 0 -1 -1 0 0] there is maybe more > (de)activation > When I consider [1 0 0 0 0 1] or [0 1 0 0 1 0], I see much more activation > in some parts of the brain, and with [-1 0 0 0 0 -1] or [0 -1 0 0 -1 0] I > see less (de)activation. > > I am quite puzzled. Has anybody find something similar? > Is it more likely that there was a problem during the acquisition or > the analysis of the data, or that it is a real behaviour of the brain? > > Many thanks, > Laura > > > ********************************************************************** > This email is confidential and intended solely for the person or > entity to > whom it is addressed. If this email was not intended for you please > notify the UCLH Mail Administrator at [log in to unmask] > This footnote confirms that the email and attachments contained no viruses > when they left UCLH. > ********************************************************************** This email is confidential and intended solely for the person or entity to whom it is addressed. If this email was not intended for you please notify the UCLH Mail Administrator at [log in to unmask] This footnote confirms that the email and attachments contained no viruses when they left UCLH.