The "Vertex Analysis: requires you to prepare two files: the individual segmented structures combined into one file, and a design matrix to indicate which file belongs to which group.
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The files will be ordered alphabetically; knowing the order is required for the later steps (specifically, creating the design matrix).
Design matrix and contrast files
The design matrix is common to FSL statistical routines, and is the same as created in SPM. However, for FIRST there are some differences (see "Create a design matrix" in guide). Specifically, the group should be in one rather than two columns, and F tests should be used.
Note: Create this file using the FSL "Glm" GUI, as other important files are created at the same time.
Click to example of two-group, 96 subject design matrix file.
Create this file in the same directory as everything else. Either start from Follow these steps:
1) Open Glm
Either start from the FSL GUI, or from a terminal "cd" to the directory, and type Glm (case sensitive).
2)
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Enter the total number of subjects, and the number in the first group (if using a two-group design)
Select "Higher-level / non-timeseries design" from the pull-down menu, enter the number of subjects (# inputs), click Wizard, select "two groups, unpaired", enter the number of subjects in group 1, click Process, then Save. When saving, navigate to something like \mnt\hgfs\LinuxShare\temp, and choose a file name like design_2group.mat."
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3) Adjust the design to have one column for group
Go the to General Linear Model window, and set to 1 EV; the Group B column will disappear.
4) Change the second group EV value from 0 to "-1"
Scroll down to the second group (where values change from 1 to 0 in EV1 column); change all the "0"'s to "-1" (tedious).
5) Set one T contrast and one F contrast
From the top of the General Linear Model window select the "Contrasts & F-tests" tab, and set to one Contrast, one F-test, and select the C1 contrast for the F1 (click on the small square).
6) Save the file
Under the GLM Setup window click Save, navigate to the directory with the files, and choose a descriptive name ("des_2sample" for example).
This will save a number of files, as in the example below (where "des" was chosen as the name). These files are used as input to several of the following procedures.
Background on design matrix
The design matrix is a text file with header information and numbers in columns. By tradition, the file is saved with a name like "design.mat". Each row represents a subject, in the order stored in the concatenated bvars file. The simplest design matrix (for a two group dataset) is a two columns, with the first +1 for rows in group 1 and 0 otherwise, and similarly +1 in the second column for rows in the second group. There is also header information which includes the number of columns (I think), the number of subjects, and the maximum value of each column. For example, here is a two-group, 5 subjects/group design matrix:
Glm from FSL GUI
Type "fsl" to start the FSL GUI, click "Misc " then "GLM Setup".
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