Thanks for the reply- I am familiar with O'Reilly's page; however the first
level in the case of my experiment (different from the examples given there)
is the *session* and not the *subject*, hence my question below- regarding
whether or not it would be valid to use such few time points to estimate
correlations between brain areas; or whether it would be best to run the PPI
at the second (subject) level instead (more time points, greater reliability).
Thanks,
Regina
On Wed, 29 Jul 2009 01:37:27 -0400, David V. Smith <[log in to unmask]>
wrote:
>Hi,
>
>Take a look at Jill O'Reilly's page on PPI.
>http://www.fmrib.ox.ac.uk/Members/joreilly/what-is-ppi
>
>In general, you'll need to model the PPI at the 1st level using the time
>course of your seed region (derived from your pre-processed 4D data file)
>and your task regressor.
>
>Cheers,
>David
>
>
>-----Original Message-----
>From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
>Of Regina
>Sent: Wednesday, July 29, 2009 1:22 AM
>To: [log in to unmask]
>Subject: [FSL] PPI setup: session vs. subject level?
>
>Dear all,
>
>I am currently analyzing a study that I have modeled using the three-level
>approach fsl recommends (session, subject, group) - where a session in this
>case is simply a run of the same fMRI experiment- and now I would like to
>run a PPI analyzes in it using a seed obtained from a group effect. My
>question regards at what level one should set up the PPI in this case.
>
>It seems to me that, for PPI analyzes purposes, one should ideally run it at
>the subject level (2nd), rather than at the session level (1st): given one
>is interested in detecting correlations in activity between brain areas
>dependent on task context, those correlations would tend to be less
>reliable if they were estimated based on shorter time epochs (i.e.,
>run/session) than if they were estimated based on a longer time epoch (i.e.,
>the entire fMRI session), correct?
>
>If that is indeed the case (where the PPI setup should be done at the
>subject level rather than at the session level, despite the fact the
>experiment was originally modeled beginning at the session level)- which
>'filtered_func_data' file should I use to extract the time course (using the
>group effect seed) from? Currently, having adopted a fixed-effects model in
>the intermediate level for each subject, I don't have a 'filtered_func_data'
>for each subject as a result- and have instead one 'filtered_func_data' for
>each cope of interest (feat directory) within each subject's .gfeat
>directory. . . Would that (the 'filtered_func_data' from the cope of
>interest for a given subject) be the one to use. . . ?
>
>Thanks much for your feedback,
>
>Regina
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