Dear everyone,
Adding to Christian's comment, there seems to be some confusion about what the values stored in the c1, c2, c3 files represent. Are they probabilities or are they proportions? If they are probabilities then the values should still be probabilities afterwards, except that they are now in template space and adjusted to perserve regional amount or concentration of the probability values. But this doesn't turn them into volume or density values then (transforming a probability map for e.g. encountering elephants in different habitats doesn't manifest an elephant).
When looking at papers there's quite some variability when it comes to thresholds/additional restrictions, for example, Momenan et al. (2004, https://dx.doi.org/10.1016/j.neuroimage.2003.10.038 ) introduce the term "density volume", which is either 1 (if GM intensity is > 0) or 0 (if GM intensity is 0).
Even if it's trivial, smoothing is another aspect as it makes a difference whether you extract average "volume"/"density" values from smoothed data or unsmoothed files. Frequently, "volume" is extracted based on an unsmoothed ROI but smoothed data.
However, the terms and the distinction "volume"/"density" is common in literature, similar to talking about "activations" when looking at thresholded T maps.
Best
Helmut
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