[FieldTrip] error when using ctf2grad

Lozano Soldevilla, D. (Diego) d.lozanosoldevilla at fcdonders.ru.nl
Fri May 31 15:49:03 CEST 2013

Hi Ye, Are you using a recent fieldtrip version? The ctf2grad function has recently updated improving its robustness. I don't have any clue why G3AR but it basically means that your CTF system has a specific gradiometer balancing. By the way, which CTF system are you using: CTF64, CTF151, CTF275? In any case, I could have a look if you file a bug and send to me the *.res4 file and we'll figure out what's happening (but please try with a recent Fieldtrip version and tell me) Regarding your gradiometer question, I asked Robert and this is the answer we can provide (please correct me if I'm wrong here), the CTF system sensors consist in first-order axial gradiometer that measure the magnetic field gradient change in the radial direction (orthogonal to the scalp). The following is taken from the CTF documentation "MEG File Formats - release 5.2.1" appendix C. --- Higher-order gradiometer formation is a noise-cancellation technique exclusive to the CTF MEG System. It permits the MEG detectors to be sensitive to the weak signals of the brain, yet impervious to the much stronger sources from the environment. This process, which is carried out in real time, allows the system to be run without the expense of magnetic shielding, or in combination with a standard shielded room for enhanced noise reduction. In order to calculate the forward solution, the same procedure must be performed on any field data calculated from a simulated dipole. The procedure consists of calculating the field at each coil in the sensor as well as each coil in a set of reference channels. The field values at the reference channels are then multiplied by the appropriate weight and subtracted from the MEG sensor channels. --- The data set on disk contains in the header the parameters that are used to convert the data between the original (raw) representation and the higher-order gradiometer representation. This conversion can be done in real-time during acquisition, but can also be done (and undone) afterwards using CTF DataEditor, or using ft_denoise_synthetic.m function. The grad structure included in the FieldTrip representation (i.e. hdr.grad or data.grad) should be consistent with the data, hence it contains grad.balance.current grad.balance.G1BR grad.balance.G2BR grad.balance.G3BR and potentially other "balancing" schemes (your "G3AR " for instance). The grad.balance.current describes which balancing was applied to the data and to the specification of the sensor array (grad.tra) for the forward computation. In order to provide a more intuitive description, I've made a wiki FAQ to explain How does the CTF higher-order gradiometer work: http://fieldtrip.fcdonders.nl/faq/how_does_the_ctf_higher-order_gradiometer_work All the best, Diego ----- Original Message -----
> From: "Ye Mei" <frank.ye.mei at gmail.com> > To: fieldtrip at science.ru.nl > Sent: Thursday, 30 May, 2013 9:33:53 PM > Subject: [FieldTrip] error when using ctf2grad > Hi All, > > I have some problem using ctf2grad. > > The program will set grad.balance.current = 'G3AR';, > and then do getfield(grad.balance, grad.balance.current) > but > grad.balance = G1BR: [1x1 struct] > G2BR: [1x1 struct] > G3BR: [1x1 struct] > > and has no G3AR in it. So an error will pop up. > > Can anyone help me with that? > > Also, What do G1BR, G2BR, G3BR, G3AR stand for? > > > thanks, > Ye > _______________________________________________ > fieldtrip mailing list > fieldtrip at donders.ru.nl > http://mailman.science.ru.nl/mailman/listinfo/fieldtrip -- PhD Student Neuronal Oscillations Group Donders Institute for Brain, Cognition and Behaviour Centre for Cognitive Neuroimaging Trigon, room 0.83 Kapittelweg 29 Radboud University Nijmegen NL-6525 EN Nijmegen The Netherlands E-Mail: d.lozanosoldevilla at fcdonders.ru.nl Tel: +31-(0)24-36-66274 Web: http://www.neuosc.com/
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