[Eeglablist] CTF MEG data import

Sarang S. Dalal sarang at hurricane.ucsf.edu
Wed Dec 14 22:29:57 PST 2005


CTF datasets may be stored to disk with or without the correction  
applied; you will have to check your data. I just wrote a colleague  
about this:

This can be a confusing situation and it is important to know exactly  
what is saved in your file. In CTF's DataEditor program, if you go to  
"Analyse -> Processing Parameters" you will see "Noise Reduction of  
Saved Data" -- this is the data that ctf_read will pull off the disk,  
regardless of what "Current Noise Reduction" says. However, you can  
use "Save As" (or the newDs command line tool) and it will apply the  
gradient correction setting when saving the new dataset.

When using ctf_read, the values returned by  
ctf.sensor.info.grad_order_no will indicate whether or not the data  
is corrected ("0" means no correction, "3" means synthetic third  
gradient correction).

Currently, there is no way to apply or remove the corrections with  

Also, if you are doing any source localization (dipoles, beamforming,  
minimum norm, etc.), it is important to note that the correction  
changes the lead field (forward solution).


On Dec 14, 2005, at 8:34 AM, zhdanova at post.tau.ac.il wrote:

> Hi everybody.
> I am working on MEG data from CTF machine recorded at another  
> facility (we do
> not have our own MEG :-(. I am not familiar with all the details of  
> acquisition process, but as far as I know there are a couple of  
> external
> sensors located throughout the MEG room that record environment  
> noise for the
> purpose of noise cancellation in the MEG signal.
> I use CTF_READ to read the MEG data. My question is whether the  
> values returned
> by CTF_READ are already corrected for the environment noise  
> measured by these
> external sensors? Does this have anything to do with the COEFS  
> parameter?
> Thanks,
> Andrey Zhdanov,
> Research Student,
> Wohl Advanced Imaging Institute,
> Tel Aviv Sourasky Medical Center
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