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
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Jeffrey N. Browndyke, Ph.D.
Duke University Medical Center
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