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 The Tools -> Sphere/Register Surface menu item
 starts a four-part background process to create
 the spherical left and right hemisphere cortical
 surfaces and then registers them to an average
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(1a) Sphere Right Hemisphere Surface
(1b) Sphere Left Hemisphere Surface

The output files written by this procedure are:

 surface: $SUBJECTS_DIR/$name/surf/rh.sphere
 surface: $SUBJECTS_DIR/$name/surf/lh.sphere

(2a) Register Right Hemisphere Surface
(2b) Register Sphere Left Hemisphere Surface

 surface: $SUBJECTS_DIR/$name/surf/rh.sphere.reg
 surface: $SUBJECTS_DIR/$name/surf/lh.sphere.reg

Morph Targets

Two of the several sets of average spherical
target surfaces in the standard MGH distribution
directory:

  $FSURF_DIR/average

are accessible from the csurf Expert Preferences
interface (SphereReg tab).  Note that these two
targets are *not* aligned with each other (!) and
should never be mixed within one analysis:

  rh.average.tif
  lh.average.tif

  rh.average.curvature.filled.buckner40.tif
  lh.average.curvature.filled.buckner40.tif

The first set was used in older MGH FreeSurfer,
FreeSurfer0.8, and csurf0.8, and is still
accessible for back compatibility.

The 'buckner40' average, used in the default
recon-all pathway, is now (starting Feb 2015) the
default in csurf Expert Preferences.

Parameters (Expert Preferences -> SphereReg)

The main adjustable parameter is the coefficient
on the geometry-preserving term (-dist), which
has the strongest effect on the output.

The coefficient on the area-preserving term
(-parea) can also be adjusted, but even when
high, it allows 'taffy-like' deformations.

Finally, the coefficient on the sulcus error
(-corr), can be adjusted independently, but is
usually kept at default=1.0 for ease of parameter
comparison.

The defaults are:

 align to group targ: dist=0.1, parea=0.2
 align to indiv targ: dist=0.5, parea=0.1  (stiffer)

In summary, to get a less deformed morph (but one
that as a whole will less closely match the
target sulc), turn up dist.
