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Well you are right - that's what we see all the time. Even if you do subtraction of incorrect and correct response to get the ERN - which works well for a subset of EEG sensors in fronto-central locations - in other regions not involved in the error processing you get patterns difficult to interpret.<div><br></div><div>But condition comparisons or subtractions in source space must not necessarily be more accurate - in case of minimum norm you "smear" the currents following a mathematical minimization criterion - "the minimum norm". Who says that the brain does work that way in a particular task - minimizing the current. So it is difficult to say what works best in terms of "best physiological model".</div><div><br></div><div>At that point - I usually don't care that much about the "most accurate source localization" any more and look what is the best measure to differentiate (e.g. conditions, groups) - once you've ruled out all the trivial effects you've got a good chance to see correlates of physiological differences. And if you can replicate that I would call it "a reliable, valuable correlate", which does not necessarily mean "true generator". It is rather a reasonable solution from the infinite amount of inverse solutions, which is probably all what we can expect in psychophysiology anyway. </div><div><br></div><div>Christian</div><div><div><div><br><div><div>On Sep 29, 2008, at 6:09 PM, Padraig Kitterick wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">If one cannot assume similar source configurations in the two conditions to be compared, presuming that any noise is constant and equivalent, is it safe to say that subtraction would result in distorted data ('distorted' not the best choice of words but my vocabulary has failed me today!), or at least data which would be difficult to interpret?</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Thanks,</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">P.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Christian Wienbruch wrote:</div> <blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Not necessarily - the sensors pick up signals from multiple generators<span class="Apple-converted-space"> </span>following the<span class="Apple-converted-space"> </span>superposition principle. This basically is the sum of the products leadfield L_ij times dipole moment p_j plus noise forming the magnetic induction B_i at sensor i</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">B_i = L_ij *p_j + n</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">If you subtract two field B1 and B2 and they share a common generator - this one is excluded in sensor space. In theory this works perfectly well in absence of noise.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Practically speaking<span class="Apple-converted-space"> </span>this concept works pretty good<span class="Apple-converted-space"> </span>for certain EEG components like the ERN which is the difference between incorrect and correct responses.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">"Distortions" usually come from the noise term and generators not being active in both conditions, or differently active in both conditions. Additionally you have source model related distortions e.g. regularisation.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">So it depends mostly on your signal to noise ratio if it is wise to subtract fields in sensor space or not.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">It also depends on how accurate you can position the MEG helmet in headframe coordinates across conditions, subjects, ...</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">If that can not be done reliably don't even think about subtraction in sensor space. You just add additional variance to your data.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">-Christian</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">On Sep 29, 2008, at 5:19 PM, Padraig Kitterick wrote:</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div> <blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Because you are distorting the dipolar topographies when you do a subtraction at the sensor level. The resulting data is likely contain field patterns which do not relate to the leadfields of the actual sources that gave rise to the data. Thus, any source reconstruction which relies on lead field models, i.e. minimum norm, will give spurious results.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">-Padraig</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Yury Petrov wrote:</div> <blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Why not first subtract one average response from the other and then<span class="Apple-converted-space"> </span>localize?</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">On Sep 29, 2008, at Sep 29, 2008 | 6:28 AM, Alex Clarke wrote:</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><p style="margin: 0.0px 0.0px 0.0px 0.0px; min-height: 14.0px"><span class="Apple-converted-space"> </span><br class="khtml-block-placeholder"></p> <blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Hi there,</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">I have a question regarding how best to statistically compare two<span class="Apple-converted-space"> </span>conditions. So far I have only being comparing between 2 conditions<span class="Apple-converted-space"> </span>using ROIs and comparing current estimates over time. However, I'd<span class="Apple-converted-space"> </span>also like to see the difference between two conditions across the<span class="Apple-converted-space"> </span>whole brain. I was wondering what the best approach to this was<span class="Apple-converted-space"> </span>(Ideally ending up with a dSPM map of condition1 - conditon2).</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Any help on this would be appreciated</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Thanks,</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Alex Clarke</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">_______________________________________________</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Mne_analysis mailing list</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Mne_analysis@nmr.mgh.harvard.edu <<a href="mailto:Mne_analysis@nmr.mgh.harvard.edu">mailto:Mne_analysis@nmr.mgh.harvard.edu</a>></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis">https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis</a></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><p style="margin: 0.0px 0.0px 0.0px 0.0px; min-height: 14.0px"><span class="Apple-converted-space"> </span><br class="khtml-block-placeholder"></p> </blockquote><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">_______________________________________________</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Mne_analysis mailing list</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Mne_analysis@nmr.mgh.harvard.edu <<a href="mailto:Mne_analysis@nmr.mgh.harvard.edu">mailto:Mne_analysis@nmr.mgh.harvard.edu</a>></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis">https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis</a></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><p style="margin: 0.0px 0.0px 0.0px 0.0px; min-height: 14.0px"><span class="Apple-converted-space"> </span><br class="khtml-block-placeholder"></p> </blockquote><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">--<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Pádraig Kitterick</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Graduate Student</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Department of Psychology</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">University of York</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Heslington</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">York YO10 5DD</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">UK</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Tel: +44 (0) 1904 43 2883</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Email: p.kitterick@psych.york.ac.uk <<a href="mailto:p.kitterick@psych.york.ac.uk">mailto:p.kitterick@psych.york.ac.uk</a>></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">_______________________________________________</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Mne_analysis mailing list</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Mne_analysis@nmr.mgh.harvard.edu <<a href="mailto:Mne_analysis@nmr.mgh.harvard.edu">mailto:Mne_analysis@nmr.mgh.harvard.edu</a>></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis">https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis</a></div> </blockquote><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Christian Wienbruch</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">University of Konstanz</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Clinical Psychology</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Fach D27</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">78457 Konstanz</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Christian.Wienbruch@uni-konstanz.de <<a href="mailto:Christian.Wienbruch@uni-konstanz.de">mailto:Christian.Wienbruch@uni-konstanz.de</a>></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div> </blockquote><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">--<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Pádraig Kitterick</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Graduate Student</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Department of Psychology</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">University of York</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Heslington</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">York YO10 5DD</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">UK</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Tel: +44 (0) 1904 43 2883</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Email: <a href="mailto:p.kitterick@psych.york.ac.uk">p.kitterick@psych.york.ac.uk</a></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div> </blockquote></div><br><div> <span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><div>Christian Wienbruch</div><div><br class="khtml-block-placeholder"></div><div>University of Konstanz</div><div>Clinical Psychology</div><div>Fach D27</div><div>78457 Konstanz</div><div><br class="khtml-block-placeholder"></div><div><a href="mailto:Christian.Wienbruch@uni-konstanz.de">Christian.Wienbruch@uni-konstanz.de</a></div><div><br class="khtml-block-placeholder"></div><br class="Apple-interchange-newline"></span> </div><br></div></div></div></body></html>