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Re: Defining the parametric modulation regressor for load

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Fri, 7 Dec 2007 05:00:52 -0800

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 ```Dear Olga, Olga Prilipko wrote: >> Dear SPMers, >> >> After unsuccessfully searching the archives and the net I would very >> much appreciate your help on the following: >> I want to identify which regions of the brain show an activation >> pattern that follows 1) linear and 2) a quadratic trend with the >> increasing task load. >> I have 4 levels of load and was going to define the matrix with 1 >> condition (all onsets of all 4 loads) with a categorical parametric >> modulation. >> My questions are: >> >> 1) how should I enter the parametric modulation vector? Would 1,2,3,4 >> for the 4 loads be correct or should I orthogonize this and enter -1.5 >> -0.5 0.5 1.5(for linear) and 1 -1 -1 1 (for quadratic) instead? Just use [-1.5:1.5] for the linear trend and then choose a polynomial expansion of 2. SPM will the automatically generate the regressors for the quadratic trend and orthogonalizes it w.r.t. to the linear trend. You will end up with 3 regressors: 1. onsets 2. linear trend 3. quadratic trend (orthogonal to 2.) >> >> 2) should I specify 2 parametric modulations (with identical vectors) >> one being described as linear and the other as quadratic in order to >> tease apart regions that may follow different trends ? see above. >> >> 3) finally, when defining the contrast, should I specify 0 1 0 if I >> want to visualize the regions with the linear trend and 0 0 1 with >> quadratic trend (the 1st regressor beeing the condition)? Correct. 0 1 0 test the linear trend (increase), 0 -1 0 tests a linear decrease across load conditions, and 0 0 1 test for the quadratic trend. Good Luck, Jan >> >> Thank you very much, any help will be very much appreciated. >> >> Olga Prilipko > -- 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 California Blvd [log in to unmask] Pasadena, CA 91125 ```