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Hi Jeff, I have been having the same problem. It can be difficult to interpret empty room recordings. We make one every day, after tuning the machine, and we have a Matlab script that will give noise, spectral plots and covariance matrixes for all channels, but it is not easy to get useful information from those. Basically you have two main sources of noise: external noise and noise that's coming from the sensors and the electronics themselves. The internal noise is minimised by tuning, but we have less control over external noise. I wouldn't know how to tell them apart in the recording, except that perhaps external noise should be more affected by SSS, so should be removed to a greater extend by Maxfilter. As a test I double grounded our MSR a few weeks ago. This should seriously increase external noise, as the MSR is then much less effective. But I really couldn't see much difference between the normal empty room recordings and the ones made with double grounding. This surprised me, I have to admit, but there might be more analyses I can still do on the data. Any suggestions are very welcome. Maarten From: Announcement for the Neuro MEG list [mailto:[log in to unmask]] On Behalf Of Jeff Alstott Sent: 08 March 2011 19:39 To: [log in to unmask] Subject: Re: SQUID behavior in an empty room I should restate, for simplicity, that my question really is: What does the MEG signal "mean" in an empty room setting? In a setting with zero change in magnetic flux? What would be some appropriate resources to draw upon to examine this more? The answers could relate to the behavior of the SQUIDs themselves, or to all the other hardware downstream of them. I'm just trying to get a fuller picture of what is happening during such null cases. On Fri, Mar 4, 2011 at 6:15 PM, Jeff Alstott <[log in to unmask]<mailto:[log in to unmask]>> wrote: Hello all, I am analyzing data from a Neuromag Vectorview system for evidence of critical phenomena, and empty rooms keep puzzling me. I am still quite new to MEG, but my analysis of the "signal" seen from an empty room, with proper shielding, shows signatures of a system at criticality. This is surprising to me. While I have no experience with SQUIDs, an initial literature search seemed to show that SQUIDs have critical dynamics when there is no external magnetic flux. Is this correct? What would be some further resources to understand what the MEG signal "means" in an empty room setting? Thank you for your help! Jeff Alstott Cambridge, BCNI US NIH, NIMH