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
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