Dear SPMers,
The experimental contrasts (a difference between two conditions A and B)
is interpreted in usual SPM analysis as indicating that condition A shows
a larger response than condition B. However, difference measures,
as conceptualized in SPM could have three potential ways in which
conditions may differ: positive activity in A may be greater than positive
activity in B, positive activity in A may be greater than negative activity
in B, and finally, negative activity in B may be greater than negative
activity in A. All lead to a positive difference between conditions.
Is there a way of teasing out these three differences?
Also, how does one get a contrast for areas which are getting inhibited by
a particular task - does one just reverse the contrast for the active
versus control condition in a block-design experiment or is there a more
sophisticated way for doing this?
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