Dear SPM:
I have a question about psychophysiological interactions. In it's
initial formulation the interaction is defined as
Xi = Xk x Gp * Bi + [Xk Gp G] * BG +e
This is a good model if one wants to see how activity in one area
results in responses in another. But what if one wants the inverse,
i.e. one wants to know what areas contributed to the modulation, i.e.
one has the interaction and wants to work backwards. So if I see
activity in area A, I want to know what areas contributed to this
activity under the influence of the psychological variable. (Rather
than finding the area whose response is influenced by A)
Would it be valid instead to put in the model
Xi = Xk/Gp * Bi + [Xk Gp G] * BG + e. ???
and would this give the area (or set of areas) that contributed to
the psychophysiological interaction in Xk?
Thanks,
Darren
Darren R. Gitelman, M.D.
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