Hi, Donald.
Great point about the correlation. I might be able to determine this,
but I was always under the assumption that the BOLD (HRF) and rCBF
followed similar curves (rCBF being a bit lower in amplitude), while
rCBV followed a coincident curve similar to the dispersion derivative of
the informed basis set. If this is the case (which I think is right),
then perfusion and BOLD should be positively correlated. However, the
rub comes when you are contrasting between two task states (e.g., low
effort vs. high effort, etc.) in the same region.
If we assume that BOLD and perfusion are positively correlated, then
BOLD should be amplified in situations where rCBF is high. But, I
wonder if BOLD is amplified for both the low and high effort task
states, thus rendering the contrast lower in amplitude. This hypothesis
might not hold if BOLD amplitude has no limit, as the low and high
effort states should scale proportionally relative to rCBF. However, I
suspect BOLD has an amplitude limit, which reinforces the possibility
that the difference between states will be lower in an environment of
high baseline perfusion.
We don't have resting state data. We have three runs of BOLD and one
run of perfusion (simply an average over a task run identical to those
used with the BOLD). As such, our perfusion maps reflect (on average)
what should be happening in the brain during the BOLD runs. I
vascillated back-and-forth about a simple resting perfusion baseline
versus this task-related perfusion map, but opted for the latter because
it better accounts for perfusion measurement in regions showing up in
the BOLD data. I didn't see how using "default mode" perfusion as a
covariate would help account for task-related BOLD differences
pre-/post-intervention.
J
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
On Behalf Of MCLAREN, Donald
Sent: Monday, May 04, 2009 11:02 PM
To: [log in to unmask]
Subject: Re: [SPM] Influence of Global Perfusion Change on BOLD
Contrasts?
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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