Why are my 3D+time datasets so huge when I resample them to Talairach coordinates?
This happens because the default resampling size in AFNI is 1 mm. This value was chosen because it is about the voxel size we use for T1-weighted anatomical background images, so resampling from +orig to +tlrc coordinates will look good on these images. However, FMRI time series data are usually gathered at much coarser resolutions; typically, 3 mm on a voxel edge. In this case, resampling on the +tlrc grid at 1 mm resolution will expand the number of voxels by about a factor of 27 (only “about”, because the total volume of the +tlrc grid may be less than or greater than the total volume of the +orig grid). In this case, you don’t really need to resample your data to such a fine grid, since it doesn’t have such high resolution information in it anyway. You can change the resampling grid size (always cubical; cf. Q28) using a control on the Define Datamode control panel.
