Hello,
I am just wondering what the relationship is between
the PPI button and the Hemodynamics button (in the
results section) are in SPM2? I want to first make
sure that I understand what they do in the general
sense, please correct me if I am wrong.
PPI -> To deconvolve out the STANDARD HRF from the 1st
eigenvariate of the Volume of Interest. This gives an
estimate of the neuronal activity. This could then be
used as a design matrix to find regions that behave
the same way, that interact.
Hemodynamics -> To take the eigenvariate from a Volume
of Interest and the design matrix to estimate the HRF,
the state variables and the parameters of a math model
using priors.
Does it make sense to estimate the underlying HRF
using the Hemodynamics button and use this in the
deconvolution procedure in PPI? Would this give a
better estimate of the underlying neuronal activity?
If so, how?
Can the estimate of the of the underlying neuronal
activity, estimated from the PPI button, be used to
interpret the neuronal activity at that location? For
instance to infer how the neuronal response is related
to the stimulus.
Also, after the two procedures are run how should the
results be interpretted? Has anyone out there used
these procedures and published the work? Can the PPI
results be used to show different regions react
differently at the neuronal level to a stimuls? Or
even within region across subjects/groups?
Can the results from the Hemodynamics be compared
across regions/subjects/groups? For instance can the
estimated parameters be used as dependent measures?
Also what do the vertical lines represent in the
Hemodynamics output mean? I have tried the procedure
with visual data and my HRF estimate has no
undershoot, the vertical lines are to the left of the
confidence regions (the gray shaded sections) for the
hemodynamic parameters. And my efficacy value is 0.1.
I guess I am trying to start a discussion on these
useful procedures.
Jason.
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