[FieldTrip] MEG resting state covariance matrix estimate without empty room recording?

[Peter Goodin]

Hi Fieldtrip list, 

I'm having a bit of a quandary at the moment regarding resting state data. In order to generate the covariance matrix all the papers I've seen estimate it from an empty room recording on the day of testing which makes sense. The problem I have is that while I have 5+ minutes of resting state data for each participant, there's no empty room recordings to go along with it. 

So I've been doing some thinking about the "least wrong" method of estimating the covariance (between a "trial by trial" method where covariance is estimated from epoched data) vs. estimation from the entire recording. My conclusions have been less than stellar with the idea that the trial by trial method is a really stupid one due to the non-timelocked nature of resting state analysis while estimation from the entire recording is fraught with problems due to the shifting nature of resting state data leading to a bad estimation of noise to begin with. 

To further complicate the issue I'm using a neuromag system which removes noise from outside the head sphere as a required method, but I'm not sure if this would be a positive or negative influence on the covariance matrix. 

Has anyone had to deal with a similar problem / can anyone recommend any literature on the topic? 

Thanks for any assistance, 

Peter
_____________________
Peter Goodin, 
BSc (Hons), Ph.D Candidate (submitted).

Brain and Psychological Sciences Research Centre (BPsych)
Swinburne University, 
Hawthorn, Vic, 3122
http://www.swinburne.edu.au/swinburneresearchers/index.php?fuseaction=profile&pid=4149

Monash Alfred Psychiatry Research Centre (MAPrc)
Level 4, 607 St Kilda Road, 
Melbourne 3004