(1) SPM has the requirement that the voxels be suprathreshold. If they
are not, then it jumps to the nearest voxel that is suprathreshold.
and (2) The solution is to set the p-value to be .99 so you sample the
same voxel in each participant.
Furthermore, the shift raises the point that single voxels might not
be ideal for PPI. When the lab I work in conducts PPI analysis, we
generally use a sphere. By setting the threshold to .99, then you can
almost always sample the same voxels across subjects and account for
variable by using the sphere.
On Thu, Apr 30, 2009 at 9:20 AM, Jeffrey West <[log in to unmask]> wrote:
> hello
>
> i am attempting to conduct a ppi analysis on a group of 10 subjects. from my
> prior second level analysis (one sample t-test) i have selected a VOI at the
> coordinates x = -10, y = 40, and z = 40. so this is where i want to plant my
> seed. in order to conduct the ppi on the group i need to get the con images
> for the each individual's ppi and enter these images into a second level
> analysis.
>
> the problem arises when i try to select the coordinates -10,40, 40 for the
> voi being conducted on the first individual. when i enter them in the gui
> and click on the eigenvariate button, these coordiantes are switched to
> -6,46, 44. both coordinates -10 40 40 which i want to select and -6 46 44
> are not activated in this person. i could understand the switch if the new
> coordinates were active for the person, but i dont understand why i cannot
> enter my coordinates if the automatically entered are also not activated.
>
> so my question is 1) why is spm automatically switching the coordiantes and
> 2) how do i enter my coordinates into the voi to plant the seed to get con
> images that show correlations between -10 40 40 and other regions. i am
> using spm5.
>
> thank you kindly for any help you could offer on this matter.
>
> jef
>
--
Best Regards, Donald McLaren
=====================
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
=====================
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