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
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.
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?