Jeff,
I've often thought about the same question and about how to address
it, the impact of perfusion on the BOLD response, and how to interpret
it. Unfortunately, the data that I am working with does not have task
data and perfusion; however, we are collecting such a data set.
As for your hypothesis/interpretation, let me turn the question around
on a single timepoint... Pre-intervention: does perfusion inversely
correlate with the BOLD signal? In answering this question, you are
determining whether effects are related to perfusion as a whole or if
its specific to the change in perfusion coupled with the change in
BOLD.
Do you happen to resting state data in these individuals?
On Mon, May 4, 2009 at 5:49 PM, Jeff Browndyke <[log in to unmask]> wrote:
> Fellow SPMers,
>
> I hope I can tap the collective expertise of those here with a BOLD and
> cerebral perfusion question.
>
> Typically, would the BOLD amplitude increase or decrease in a task-task
> contrast (e.g., 2back > 0back) where the global baseline perfusion was
> high? I ask because we're seeing baseline perfusion increases in some
> of our patients post-intervention, which appears to be coincident with
> lower SPM BOLD contrast effects. My intuition is telling me that the
> SPM BOLD effects are lower because physiologically increased perfusion
> (rCBF/rCBV) doesn't allow for much amplitude difference between
> oxygenated/deoxygenated states. The N-back task is being conducted
> properly by the patients and there are no data/motion/etc. artifacts to
> explain the SPM BOLD reductions relative to pre-interventional baseline
> SPM BOLD (which was coincident with lower perfusion).
>
> In our data pre-intervention scans are associated with perfusion 10-15
> mL/100mg/min lower than post-intervention. Individual contrast maps at
> each time point look fine (i.e., typical working memory network loci),
> but when we take contrasts to second-level, paired-t test there is a
> strong preference for pre-intervention > post-intervention in the BOLD
> data maps. However, if I run an ANCOVA through a separate process
> controlling for perfusion levels, the reverse is found (i.e.,
> pre-intervention < post-intervention). If we covary out perfusion, the
> SPM results are an almost opposite of what is found without the
> covariate. The only way I can think to explain this is that the
> perfusion differences between pre- and post-intervention are significant
> enough to alter the BOLD amplitudes, thus giving an illusory effect that
> pre-intervention is better than post-intervention. But, in reality,
> once one accounts for the perfusion difference, the expected is true
> (i.e., post-intervention better than pre-intervention).
>
> I just want to make sure that I'm interpreting this correctly. Thanks
> in advance to any and all for the time and expertise.
>
> Regards,
> Jeff
>
> _______________________________
>
> Jeffrey N. Browndyke, Ph.D.
> Duke University Medical Center
>
--
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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