Dear Neuromeg list.
We have two questions about the centre of the spherical expansions in
Maxfilter:
1. Occasionally MaxFilter bails out with the error:
"ERROR: sphere fitted to isotrak extends outside of the sensors! (r0
= (14 3 36) mm, rad = 11 cm)"
However, when we run the Neuromag utiltity:
/neuro/bin/util/fit_sphere_to_points
(or our own sphere-fitting code), we seem to get sensible fits, eg
Centre = [1.347 7.374 42.871] (Radius=98.608).
The MaxFilter manual implies that the MaxFilter sphere-fitting is more
sophisticated/robust than simple methods, so should we not trust the
"fit_sphere_to_points" results above? Or is the error actually with
MaxFilter? If the latter, should we just "bypass" this MaxFilter bug by
passing "-frame head -origin 1.347 7.374 42.871" (in this example)?
2. Related to question 1 above, we have noticed that the sphere fit by
MaxFilter and the sphere fit by fit_sphere_to_points tend to differ,
sometimes vastly so, with the origins differing by 10mm in one or
another dimension. (When we independently fit a sphere using SPM
functions in matlab, the origin is consistently very close (within 1 mm)
to the fit_sphere_to_points result). Here are some examples:
Subject 1:
-0.7770 5.3440 35.9980 fit_sphere_to_points origin
-0.9496 5.1591 36.2188 SPM origin
2.2 6.2 38.6 MaxFilter Inside Origin
Subject 2:
-9.5500 0.8510 39.8750
-9.5891 1.4581 39.3087
-10.1 5.5 36.9
Subject 3:
-0.6850 11.9050 51.6610
-0.5834 11.6485 51.4186
-10.4 12.5 46.6
So should we ALWAYS use independent sphere fitting, rather than relying
on MaxFilter? (we expect not, but just wanted to get your attention! ;-)
Or are we misunderstanding what MaxFilter actually does?
Many thanks
Rik Henson, Jason Taylor, Danny Mitchell
--
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Dr Richard Henson
MRC Cognition & Brain Sciences Unit
15 Chaucer Road
Cambridge
CB2 7EF, UK
Tel: +44 (0)1223 355 294 x522
Fax: +44 (0)1223 359 062
http://www.mrc-cbu.cam.ac.uk/~rik.henson
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