Hi AFNI experts,

I've just read the following part of the afni_proc documentation:

Quote
-volreg_align_to POSN : specify the base position for volume reg

e.g. -volreg_align_to last
default: third

This option takes 'first', 'third' or 'last' as a parameter.
It specifies whether the EPI volumes are registered to the first
or third volume (of the first run) or the last volume (of the last
run). The choice of 'first' or 'third' should correspond to when
the anatomy was acquired before the EPI data. The choice of 'last'
should correspond to when the anatomy was acquired after the EPI
data.

The default of 'third' was chosen to go a little farther into the
steady state data.

Note that this is done after removing any volumes in the initial
tcat operation.

Please see '3dvolreg -help' for more information.
See also -tcat_remove_first_trs, -volreg_base_ind and
-volreg_base_dset.

Could you explain a bit more why only options "first, third and last" are available? What are the physical issues re. to when the anatomical was acquired? One could unluckily catch motion in the first, third or last volume. One would register then all the volumes without motion to a volume with motion artefacts. Therefore, I thought to use a mean volume across time for the registration base with the option "-volreg_base_dset". But obviously you guys must have chosen this as standard for good reason. Any thought on that? Do I miss some points here?

Best wishes

Michael H