Dear Tamara,
For your analysis, it should be best to use individual normalised GM files, if this is not achievable, the average of your individual normalised GM files might do it, and it this is not achievable, it should be a pre-existing file which is reasonably representative for the GM distribution in your data. To give an example, assume you rely on a low-dimensional registration technique, leading to a rather inaccurate between-subject registration of your data, then a GM file generated with a more accurate registration technique is misleading even if it's the "same" MNI space. Another example, the data entered into your analysis was smoothed with a certain smoothing kernel, but the GM file is an unsmoothed one. This would also be not ideal.
Apart from that, the CAT12 files are no priors, although they do represent a certain GM distribution of course. But when going with the six-tissue-type TPM.nii as included in SPM12, it would be the well-established tissue priors used for SPM12 segmentation (which does not automatically imply it is meaningful for your analysis, see above). When going with Template_1_IXI555_MNI152_GS.nii, it would be the GM map for the second out of five registration steps, which is quite arbitrary (why the second and not e.g. the forth), leaving aside that they are based on a certain high-dimensional diffeomorphic registration technique with a certain accuracy that one might not achieve otherwise.
Concerning spaces, the TPM.nii in SPM12 matches an asymmetric variant of the MNI ICBM 152 non-linear 6th Gen space (this is *not* identical to the symmetric one available here http://nist.mni.mcgill.ca/?p=858 ), although there are slight differences, as the tissue priors were generated from different subjects (the IXI data set) with different registration techniques. The CAT12 templates were generated from the IXI data set via Geodesic Shooting aka Shoot, and "finally transformed to MNI space" as stated in a CAT12 help text. Although not explicitly mentioned it seems to be the MNI ICBM 152 non-linear 6th Gen asymmetric space, and not the (somewhat larger) MNI ICBM 2009 space. Again, due to different subjects there would be slight differences to the original atlas.
Thus, if you were to convert between spaces, it would be best to warp the exact files, be it TPM or a CAT12 file, onto the target template. But as stated, I'm not sure whether this is really that useful.
Hope this helps
Helmut
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