Kyle,
>
> 1. How is the zero-point on the y-axis calculated for the data
> output? My
> understanding from previous postings on this topic is that the y-axis
> is
> essentially a measure of percent signal change. My question is,
> percent
> signal change relative to what? Might this be relative to the overall
> average MR signal in the ROI? Is the method of calculating the
> zero-point
> related to the options chosen on the “compute ROI data” menu?
>
the zero point is the estimate from the finite impulse response model
for the timepoint at which the stimulus is presented. this is relative
to the baseline signal, which is basically the constant in the full
statistical model. SPM99 sets the global whole-brain mean to 100, but
across voxels this can vary, so the signal change values in the roi
toolbox are reported as a proportion of the mean MR signal of the
voxels in the ROI. I hesitate to call it % signal change directly,
because there are other steps (like the removal of other condition
effects, and high pass filtering) that will make it not be exactly the
same as a direct measure of % signal change in the raw data.
> 2. I have a hunch that my second question might somehow be related to
> the
> answer for the first question, but maybe not. I’ve noticed that in
> most of
> the graphs, all of the lines tend to pinch-down at the beginning and
> end of
> the graph (i.e., at both ends of the x-axis). Is this attenuation a
> result
> of how the signal intensity for the roi is calculated? Alternatively,
> is
> it normal to see an attenuation of the MR signal at the start and
> finish of
> each block?
>
if I understand you correctly - this is a feature of the hemodynamic
response. when we average we start averaging before the stimulus has
been presented (and thus when there should be no response) and we
average until after the stimulus has been presented (and thus the
resposne should have faded away).
cheers
russ
> I'd greatly appreciate any help with these questions. I think the roi
> toolbox is a wonderful addition to SPM and I appreciate Dr. Poldrack's
> work
> to make it available to the neuroimaging community.
>
> Sincerely,
> Kyle Simmons
>
>
---
Russell A. Poldrack, Ph.d.
Assistant Professor
UCLA Department of Psychology
Franz Hall, Box 951563
Los Angeles, CA 90095-1563
phone: 310-794-1224
fax: 310-206-5895
email: [log in to unmask]
web: www.poldracklab.org
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