[FieldTrip] why are grid points located outside the brain?

[Maris Skujevskis]

Dear Fieldtrip users,

Can someone please explain the reason why in the tutorial
http://www.fieldtriptoolbox.org/tutorial/sourcemodel (or in general, for
that matter) the outer layer of grid points of the source model lie outside
the brain though they are defined as being inside?

This is what I am refering to:
http://www.fieldtriptoolbox.org/_detail/example/mnispace/mni_fig1a.png?id=tutorial%3Asourcemodel
http://www.fieldtriptoolbox.org/_detail/example/mnispace/mni_fig2b.png?id=tutorial%3Asourcemodel

According to my naive understanding, this means that in this model the
potential neural sources may also lie outside the brain, which makes no
sense. Clearly, I am missing something here...

Thanks!

Maris
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