Dear Jack,
1) Is this strictly unidirectional (say area A -> B but independent from B
-> A) or could it be that there is no directionality or no independance ?
An additional point to Christian's answer. PPIs are concerned with context
or condition-specific changes in the contribution one brain area makes to
another. This means that there is another "direction" to consider in
interpreting PPIs, i.e. in relation to your psychological factor. Take two
regions, A and B and a psychological variable, say attention, identified by
a PPI with the "source" voxel in A. One interpretation of the PPI is that
attention modulates the contribution of A to B. An alternative
interpretation is that A modulates attention-related responses in B. So, if
attention is identified with activity in another area, C, then A can be
considered to be modulating the contribution of C to B (this would be
converting the PPI into a physiological interaction).
Karl's paper goes into details:
Friston KJ Buchel C Fink GR Morris J Rolls E Dolan RJ Psychophysiological
and modulatory interactions in neuroimaging NeuroImage 1997;6:218-229
best wishes,
John
************************************************
John Morris
Research Fellow,
Wellcome Department of Cognitive Neurology,
12, Queen Square,
London WC1N 3BG
Tel: 0171 833 7487
Fax:0171 813 1420
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