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help > Performing gPPI on a subset of sessions/runs
May 15, 2014 10:05 PM | Christian Utzerath
Performing gPPI on a subset of sessions/runs
Dear Dr. McLaren,
First, I'd like you to thank you for your continued support of the gPPI toolbox. Since you last helped us out we have made substantial progress towards answering our research questions. My current question is whether is it possible to have gPPI only perform a PPI on specific runs. If so, how?
Our experiment is split into several runs. While each of the runs features the same conditions ('A' or 'B'), participants perform different tasks in each run (task 1, task 2, task 3). We include all of these task (3 of each task) in one large SPM.mat design, organized by task (1 1 1, 2 2 2, 3 3 3). A and B are the first two regressors within each run.
We have run gPPI over the entire design, so looking at A vs. B over all tasks. Our current goal is to estimate A vs. B during each task instead of collapsing across them. In the SPM GLM, we would then simply create a contrast vector that has non-zero values over one task at a time, thus modeling the contrast for three runs at a time while skipping all the others.
The help text of the gPPI function 'createVec.m' suggests that this can also be done in gPPI. Specifically, it states that the 'left cell string' (in P.Contrasts(n).left field) can be defined as vector if the contrast is known. Then 'right cell string' is supposed to be left empty in that case.
However, our attempts to define this vector manually have not been successful. We first simply defined a contrast vector over the entire design, with 0's for nuisance regressors and the other tasks that we to skip. After this didn't work, we added placeholder zeros for the three PPI regressors that gPPI adds, since we noted that without this, our vector was shorter than the number of betas in the design (SPM.mat Vbeta field). Again, the analysis aborts when SPM enters the contrast manager. While SPM.mat seems mostly complete, SPM.xCon remains empty.
We also tried isolating specific sessions by trying looking for our conditions only with the prefixes of these sessions*, but that led to the estimation of all 9 sessions again. Can you tell us how to perform a gPPI on specific (or isolated) sessions?
All the best,
Chris
* Specifically, we entered
P.Contrasts(n).left = {'A'};
P.Contrasts(n).right = {'B'};
Prefix.Left = {'Sn(1)', 'Sn(2)', 'Sn(3)'};
Prefix.Right = {'Sn(1)', 'Sn(2)', 'Sn(3)'};
First, I'd like you to thank you for your continued support of the gPPI toolbox. Since you last helped us out we have made substantial progress towards answering our research questions. My current question is whether is it possible to have gPPI only perform a PPI on specific runs. If so, how?
Our experiment is split into several runs. While each of the runs features the same conditions ('A' or 'B'), participants perform different tasks in each run (task 1, task 2, task 3). We include all of these task (3 of each task) in one large SPM.mat design, organized by task (1 1 1, 2 2 2, 3 3 3). A and B are the first two regressors within each run.
We have run gPPI over the entire design, so looking at A vs. B over all tasks. Our current goal is to estimate A vs. B during each task instead of collapsing across them. In the SPM GLM, we would then simply create a contrast vector that has non-zero values over one task at a time, thus modeling the contrast for three runs at a time while skipping all the others.
The help text of the gPPI function 'createVec.m' suggests that this can also be done in gPPI. Specifically, it states that the 'left cell string' (in P.Contrasts(n).left field) can be defined as vector if the contrast is known. Then 'right cell string' is supposed to be left empty in that case.
However, our attempts to define this vector manually have not been successful. We first simply defined a contrast vector over the entire design, with 0's for nuisance regressors and the other tasks that we to skip. After this didn't work, we added placeholder zeros for the three PPI regressors that gPPI adds, since we noted that without this, our vector was shorter than the number of betas in the design (SPM.mat Vbeta field). Again, the analysis aborts when SPM enters the contrast manager. While SPM.mat seems mostly complete, SPM.xCon remains empty.
We also tried isolating specific sessions by trying looking for our conditions only with the prefixes of these sessions*, but that led to the estimation of all 9 sessions again. Can you tell us how to perform a gPPI on specific (or isolated) sessions?
All the best,
Chris
* Specifically, we entered
P.Contrasts(n).left = {'A'};
P.Contrasts(n).right = {'B'};
Prefix.Left = {'Sn(1)', 'Sn(2)', 'Sn(3)'};
Prefix.Right = {'Sn(1)', 'Sn(2)', 'Sn(3)'};
Threaded View
Title | Author | Date |
---|---|---|
Christian Utzerath | May 15, 2014 | |
Donald McLaren | May 16, 2014 | |
Christian Utzerath | May 29, 2014 | |