Hi Marco,
Thanks for sharing your observations. My reply is more an attempt to keep this thread active, until an expert decides to answer, but here are a couple of thoughts.
I suppose it is feasible that the torso of a subject may have moved (rotated), even in the theoretical case of no head movement between runs. That would lead to a different ECG projection on the sensors, and thus a smearing out of the average SSP vector
relative to the ones optimised for each run separately.
Could the explanation for the blinks have to do with the fact that the eyes are close to the "white zone" (SSS no-mans-land, between inner and outer convergence zones), and thus signals from there not being totally accurately coregistered to the initial
position? In a similar vein, perhaps the blink-related field is spatially under-sampled, and thus not accurately represented in the SS basis. (Does the blink-field induce high spatial frequency components in the sensors at the edge of the helmet that don't
get dealt with?)
In any case, thanks for pointing this out. Very interesting and relevant even if my logic fails :)
/Chris
--
Christopher Bailey, MSc
MEG Engineer, MINDLab Core Experimental Facility
Center of Functionally Integrative Neuroscience (CFIN)
Aarhus University, Denmark
tel. cell: +45-2674-9927
tel. office: +45-7846-9942
Dear all,
I'd be happy to have some feedback on my question, which I think is
quite a general issue. If however you think it's trivial, I'd be happy
to know the obvious solution :-)
Thanks,
Best,
Marco
On 17 January 2014 15:47, Marco Buiatti <[log in to unmask]> wrote:
Dear Neuromag users,
I have a question concerning what is the best strategy to compute SSP
projectors for blink/cardiac artifact removal in Neuromag data.
Typical datasets from my lab contain 4 to 8 separate runs from each
subjects, and co-registration across runs is typically computed with
Maxfilter (Maxmove), based on the head position measurement performed
at the beginning of each run.
In principle, since co-registration has already been done, it should
make sense to compute SSPs for all runs together. However, from my
experience, results are not always as good as by computing and
applying SSPs to each run separately.
To test this, I have compared the two approaches on one of my datasets:
1) Local: Compute SSPs for both blinks and cardiac artifacts
separately on each run (here the first run of my dataset)
2) Global: Compute SSPs for both blinks and cardiac artifacts on all
runs together.
My dataset consists in 8 runs of 6 minutes each, already co-registered
by MaxMove.
I did this test by using Brainstorm (actually, I have posted a similar
message on the Brainstorm forum a few weeks ago, but I did not get any
feedback from the Neuromag users).
Here I attach:
* The blink and cardiac artifact for MAG (top row) and GRAD (bottom
row, topography is the norm of the grads), first run.
* The SSP projectors for MAG and GRAD for blinks (top row) and cardiac
(bottom row) for approach 1) (local) and 2) (global). They look very
similar, though blink topographies are more localized around the eyes
for the 'local' approach, and cardiac topographies look also slightly
cleaner.
* Comparison of residual blink for local (left side) vs global (right
side) approach.
* Comparison of residual cardiac for local (left side) vs global
(right side) approach.
As you see, though most of the artifact is removed with both
approaches (note the change in scale of the amplitude), the local
approach is more successful.
Of course, it is just an example, I cannot generalize.
Is this due to an inefficient co-registration, or is it possible that
blinks/cardiac artifacts change during the recordings?
What is your typical procedure?
Thanks in advance for your feedback,
Best,
Marco
--
Marco Buiatti, PhD
CEA/DSV/I2BM / NeuroSpin
INSERM U992 - Cognitive Neuroimaging Unit
Bāt 145 - Point Courrier 156
Gif sur Yvette F-91191 FRANCE
Ph: +33(0)169.08.65.21
Fax: +33(0)169.08.79.73
E-mail: [log in to unmask]
http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
***********************************************
--
Marco Buiatti, PhD
CEA/DSV/I2BM / NeuroSpin
INSERM U992 - Cognitive Neuroimaging Unit
Bāt 145 - Point Courrier 156
Gif sur Yvette F-91191 FRANCE
Ph: +33(0)169.08.65.21
Fax: +33(0)169.08.79.73
E-mail: [log in to unmask]
http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
***********************************************
