[FieldTrip] LCMV and SAM
jedmeltzer at yahoo.com
Thu Jul 21 23:21:21 CEST 2011
Without getting into software implementation questions (basically fieldtrip has LCMV, and CTF software has SAM) I think the main difference is that LCMV is a "vector" beamformer. At any given location, it estimates a virtual signal in all three spatial dimensions (or only two if you constrain it to tangential sources, which makes sense for MEG but not for EEG). So you have three virtual signals at each point, and how you combine them is up to you - take the biggest one, or the magnitude of the whole vector, or any other combination. I'm not sure what the default for power mapping is in fieldtrip (I have mainly used SAM for beamforming so far, but I use fieldtrip for other purposes).
SAM has an extra step involved where it "optimizes" a dipole orientation at each location to maximize the signal, so you only get one signal at each location. In this sense it's nonlinear. The calculation is more complex due to the optimization, but the result is simpler to deal with. This is called a "scalar" beamformer. For pros and cons, you might look up papers on vector vs. scalar beamformers in general. Here's one recent one that I saw that compared them and has further references:
Quraan, M. A., S. N. Moses, et al. (2011). "Detection and localization of hippocampal activity using beamformers with MEG: a detailed investigation using simulations and empirical data." Hum Brain Mapp 32(5): 812-827.
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