Thanks for the detailed description, now I understand clearly what you mean.
When you defined the head model, you decided to use a source space with 15765 points sampling the brain volume.
At each point, we use 3 dipoles to represent all the possible orientations in space (see the Source estimation tutorial for help).
If you look in your source files (right-click > File > View file contents), you have 47295 source time series.
=> ImagingKernel: [47295x128 double]
These values are reinterpolated over a much denser grid of values to display it in the MRI viewer and overlay it with the subject’s MRI.
From a cloud of points, we create a full volume [197x233x189] (total of 8675289 values).
The grid of source points is only defined inside the brain volume, not in at all the points of the MRI. All the voxels of the MRI volume that are too far from a source are set to zero. This is why you have a lot of values at zero.
Double-click on a PNAI file, it opens the MRI viewer, at the top-right corner of the window you can see the value at the current voxel (if you don’t see this value, update Brainstorm, I added this recently - menu Help>Update)
The Cube variable in the .mat file has smaller dimensions ([74×92×69]) because you selected two options in the export process: “Volume downsample factor” and “Cut empty slices”
Hope this helps.
Cheers,
Francois