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I have a question regarding how to graph results that came from a vertex wise analysis using the command mri_glmfit and mri_glmfit-sim. I was interested in investigating an interaction effect between groups and my variable of interest (continuous) while co-varying for three nuisance variables. After running mri_glmfit and mri_glmfit-sim to correct for multiple comparisons. I visualized the results and found significant clusters. I'm interested in graphing these results. Based on archived questions to this mailing list the individual values for each cluster can be found within the ocn.dat file and the cluster information can be found in the summary file. My analysis looked at volume, thickness and surface area. Since mean volume and area is difficult to interpret I want to convert the values to a total measure. It has been suggested in the past this can be done by multiplying each individuals values within the ocn.dat file by the number of vertices the cluster has. However, from my understanding this could be done by altering the mri_segstats command (that mri_glmfit-sim automatically runs) to include the --accumulate option. When I do these methods to convert mean area and volume to total area and volume the results are different. My first question is 1) Shouldn't these values be identical? The values from multiplying mean volume by number of vertices are roughly around 3500. Whereas using the --accumulate in mri_segstats are around 2500. What could be causing this discrepancy? 2) If my cluster has a size of 1500 mm^2 (in a model for area) does it make sense that every individual's values after extraction and conversion to total area are larger than the cluster size? 3) ocn.dat files are the input values meaning they're raw and would need to be corrected in a statistically (in a similar way that I modeled it in freesurfer) before graphing right?
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