Estimation of FWHM on surface.
Dear Ladies and Gentlemen most likely Dr. Sereno,
I would like to estimate the FWHM of cortical thickness of single subjects that have been brought into fs_average space using spherical alignment. I have crated my own matlab routine to do so but some problems remain. According to your article “Smoothing and cluster thresholding for cortical surface-based group analysis of fMRI data” it is possible to obtain FWHM using following formula.
FWHMsurf = dv*sqrt(-2ln2/(ln(1-var(ds)/var(s))) where dv is the average inter-neighbor distance, var(ds) is the variance inter-neighbor differences, and var(s) is the overall variance of the values at each vertex. There are some things in your paper that id do not understand. According to your paper FWHM can only be estimated correctly when the smoothwm surface is used. However the data of my single subjects are in FS_average space.
Question 1
The smoothwm surface is present in the FSaverage map. Is this indeed the correct surface to use when cortical thickness of individual subjects has been brought into spherical aligned space and if not what other surface should be used?
Question 2
What is the average inter neighbor distance dv of the alternative mesh? Is the distance of the unit mesh really 1 or something else?
Question 3
According to the formula given above var(ds) is the variance of the inter neighbor differences. My question is the following. Suppose you have two vertices A and B with thickness 2 and 4. From the perspective of vertex A the difference is 2 while from the perspective of vertex B the difference is -2. Should I estimate ds only from positive values (count relation between A and B only once) or should I take both values into account (count relation between A and B twice). The results are of course very different.
Question 4
I would like to extract data from a very large number of POIS. It is possible to draw a circle on a mesh with a function from surfstat. Again what is the correct mesh if data of the underlying subjects has been brought into spherical aligned space? Can I use the smoothwm mesh or should I use some standard sphere? And which standard sphere should that be?
Jan Willem Koten (van Grevenbicht)
Zollernstrasse 57, 52070 Aachen, Germany
Telephone: (0049) (0)241 212 44
Zollernstrasse 57, 52070 Aachen, Germany
Telephone: (0049) (0)241 212 44
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