<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: Times New Roman; font-size: 12pt; color: #000000'><P>Hi Inna,</P>
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<P>You're welcome. The offline compensation method suggested (ft_regressconfound) is based on general linear modeling, and not on the signal space separation method (Taulu et al., 2005, MaxFilter, Neuromag - not implemented in FieldTrip) you are referring to. </P>
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<P><SPAN style="FONT-FAMILY: 'Times New Roman','serif'; FONT-SIZE: 12pt; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: AR-SA; mso-bidi-language: AR-SA">Head movements blur the topography of the neuronal sources of the MEG signal, increase localization errors, and reduce statistical sensitivity. </SPAN>This GLM-based method is geared to improve statistical sensitivity rather than topographical blurring and localization bias. More specifically, it does not interpolate/extrapolate the magnetic field distribution measured by the sensor array to a magnetic field distribution that would have been measured had the sensors been at a desired location. Instead, it removes head movement related trial-by-trial variance from the data, by incorporating head position time-series into a general linear model. A recently performed study (*) showed that this method improves statictical sensitivity without changing the pattern of activity. This is useful when you're testing an hypothesis, derived from a cognitive research question, because it reduces the risk of false negatives. </P>
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<P>With respect to your question; in case you are particularly interested in reducing localization bias, I could recommend you to have a look at Uutela et al., NeuroImage, 2001. The forward calculation correction method suggested there incorporates head position information into the source reconstruction procedure. This method is implemented as ft_headmovement in FieldTrip. I cannot give you any numbers except from the ones reported there, but for all offline methods it holds that you need to be able to measure the magnetic field distribution in order to know (and interpolate) it. </P>
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<P>Hope this answered your question a bit. Feel free to ask more details.</P>
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<P>Yours,<BR>arjen</P>
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<P>(*) Stolk, Todorovic, Schoffelen & Oostenveld. Online and offline tools for head movement compensation in MEG. NeuroImage, accepted pending revisions.</P>
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<BLOCKQUOTE style="BORDER-LEFT: rgb(16,16,255) 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px"><B>Van: </B>"Inna McGowin" <mcgoiv0@wfu.edu><BR><B>Aan: </B>"FieldTrip discussion list" <fieldtrip@science.ru.nl><BR><B>Verzonden: </B>Woensdag 14 november 2012 14:31:15<BR><B>Onderwerp: </B>Re: [FieldTrip] correction for movement tool<BR><BR>Hello Arjen,<BR>Thanks for the links on the head motion correction. <BR>Could you please point a source or a reference on which the FieldTrip head motion correction method is based? Is it expansion of magnetic scalar potential in spherical harmonics or some other method? How accurate is the correction and what are the limitations for its application; in other words does it correct for motion up to 3 cm or beyond? <BR><BR>What is the minimum motion that does not need to be corrected, for example we allow 5 mm?<BR><BR>Thanks,<BR>Inna <BR><BR><BR><BR>
<DIV class=gmail_quote>On Wed, Nov 14, 2012 at 5:07 AM, Stolk, A. <SPAN dir=ltr><<A href="mailto:a.stolk@fcdonders.ru.nl" target=_blank>a.stolk@fcdonders.ru.nl</A>></SPAN> wrote:<BR>
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<P>Hi Nenad,</P>
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<P>There are a few (online and offline) tools implemented in FieldTrip to deal with head movements in MEG.</P>
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<P>For instance, have a look here:</P>
<P><A href="http://fieldtrip.fcdonders.nl/example/how_to_incorporate_head_movements_in_meg_analysis" target=_blank>http://fieldtrip.fcdonders.nl/example/how_to_incorporate_head_movements_in_meg_analysis</A></P>
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<P>ft_regressconfound is particularly useful for removing trial-by-trial variance from the data that can be explained by head movements. We found it to have a significant impact on statistical sensitivity.</P>
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<P>At our institute, we use a real-time head localizer to reposition the subject between and within sessions (prevent/minimalize different head positions). </P>
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<P><A href="http://fieldtrip.fcdonders.nl/faq/how_can_i_monitor_a_subject_s_head_position_during_a_meg_session" target=_blank>http://fieldtrip.fcdonders.nl/faq/how_can_i_monitor_a_subject_s_head_position_during_a_meg_session</A></P>
<P><BR>Currently, it is implemented only for CTF systems (with the continous head localization upgrade) but we are working on extending its implementation to other systems. </P>
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<P>Yours,</P>
<P>Arjen<BR><BR></P>
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<BLOCKQUOTE style="BORDER-LEFT: rgb(16,16,255) 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px"><B>Van: </B>"Nenad Polomac" <<A href="mailto:polomacnenad@gmail.com" target=_blank>polomacnenad@gmail.com</A>><BR><B>Aan: </B><A href="mailto:fieldtrip@science.ru.nl" target=_blank>fieldtrip@science.ru.nl</A><BR><B>Verzonden: </B>Woensdag 14 november 2012 10:35:10<BR><B>Onderwerp: </B>[FieldTrip] correction for movement tool
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<DIV class=h5><BR><BR>Dear all,<BR><BR>Do you have some tool which can correct data for the movements during MEG measurement? Our data were obtained on the CTF MEG 275 system.<BR><BR>All the best!<BR><BR>Nenad <BR></DIV></DIV>_______________________________________________<BR>fieldtrip mailing list<BR><A href="mailto:fieldtrip@donders.ru.nl" target=_blank>fieldtrip@donders.ru.nl</A><BR><A href="http://mailman.science.ru.nl/mailman/listinfo/fieldtrip" target=_blank>http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</A></BLOCKQUOTE><BR></DIV></DIV><BR>_______________________________________________<BR>fieldtrip mailing list<BR><A href="mailto:fieldtrip@donders.ru.nl" target=_blank>fieldtrip@donders.ru.nl</A><BR><A href="http://mailman.science.ru.nl/mailman/listinfo/fieldtrip" target=_blank>http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</A><BR></BLOCKQUOTE></DIV><BR><BR clear=all><BR>-- <BR>Inna McGowin<BR><BR><BR>_______________________________________________<BR>fieldtrip mailing list<BR>fieldtrip@donders.ru.nl<BR>http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</BLOCKQUOTE><BR></div></body></html>