 |
 |
Dear Ben, the problems with the new design matrix may arise from the fact that your new regressors are highly correlated. If you are interested in testing your question, i.e. whether a specific connection depends on the level of force used, the following design may be easier to use: 1. a regressor encoding the presence of all trial (all stimuli) 2. a parametric modulation of this regressor that represents the force level of each trial. This could either be ordinal [1,2,3,4] or encode the actual force used [10,20,30,70]. The latter is probably more elegant. In either case, these two regressors will be orthogonal and should not cause you any worries. Best wishes, Klaas At 17:23 04/01/2007, you wrote: >Hi, Klaas, > >Sorry to ask you for help again. I still have the problems with the >design for DCM analysis (see below). Would you help me out? > >Thanks much! > >Ben > >-----Original Message----- >From: Xu, Ben (NIH/NINDS) [E] >Sent: Thursday, December 28, 2006 10:22 AM >To: 'Klaas Enno Stephan' >Subject: help with design matrix for DCM analysis > >Dear Klaas, > >I'm having trouble building a GLM design matrix for a DCM analysis. I >posted the problem sometime ago but have not received any response. So, >here I am, troubling you again for help. > >We have constructed a conventional design matrix with SPM2 for a set of >event-related fMRI data that involves motor tasks and got consistent and >significant action across task conditions. Now, we are reconstructing a >design matrix (with as an implicit baseline as in the conventional >design, i.e., the resting period between trials) to be used for DCM >analysis, but running into problems. The main problem is that if a >condition is included in the "input" regressor, it no longer shows any >significant action (or any activation at all) when modeled again as a >separate regressor (needed as modulating factor for DCM). Here's what I >did: > >The data set consists of 4 motor conditions. Conditions 1-3 require >similar movement force. Condition 4 requires much stronger movement >force than Conditions 1-3. The conventional SPM analysis (with implicit >"Rest" as a baseline control) showed consistent action in the expected >brain regions for all four conditions. In addition, Condition 4 showed >more action in the same and other regions. > >Our interest in the DCM analysis is to see how the increase in motor >force modulates two specific connections among 4 active regions. Here is >the GLM design matrix for the DCM analysis: > >Input regressor: includes all 4 conditions > >Movement regressor: includes Conditions 1, 2, and 4 > >Force regressor: includes Condition 4. > >(Condition 3 was not modeled as a separate regressor, and "Rest" is >implicit) > > When I looked at the activation in "Results," it did not show any >significant voxels (either with FWE or FDR correction at .05) for >"Movement." The activation for "Force" was mostly in a few unexpected >regions. Only the "Input" regressor showed relatively similar action >patterns as seen with the conventional SPM analysis. > >Why do activation results change so dramatically with the new design >matrix? Why are the separate regressors no longer showing significant >voxels? Is implicit baseline a problem? I can't draw VOIs without >significant voxels. I assume, without significant voxels, DCM analysis >will not work either, is that correct? > >Your help is greatly appreciated. > >Ben