----- Original Message -----
From: Daniel Y Kimberg <[log in to unmask]>
Date: Monday, March 14, 2005 10:25 am
Subject: Re: [SPM] Any Papers on Presenting fMRI Results?
<snipped out more general discussion here>
> Just to be a little more concrete, if I see a dozen low-powered
> studies with similar task contrasts, the thresholded maps make it easy
> for me to make generalizations such as "motor planning almost always
> activates the SMA, but never visual cortex."
I'm afraid I disagree strongly with this line of reasoning. Thresholded
maps don't allow any conclusions to be drawn about areas that were not
significantly activated, other than they failed to reach significance.
To conclude that an area is never activated because it doesn't show up
on thresholded maps is incorrect, and I would say is exactly the problem
with thresholded maps - they tempt people to affirm the NULL without
proper justification. Without at least a power analysis, based upon
reasonable interval predictions (x % signal change, although deciding on
x for fMRI studies is quite tricky), one is really in no position to
conclude anything at all about brain regions that show no significant
activation.
Tom J.
> Unthresholded maps,
> which contain more information, support an unmanageably large number
> of such comparisons. To the extent people can whittle this down
> informally by restricting their interest to local maxima or voxels
> that are especially hot (e.g., "there's always a peak in the IPL"), I
> think it remains true that thresholded maps support localization
> better, it's just that the readers are choosing the threshold rather
> than the author of the study. On the whole, I prefer to let the
> authors pre-filter the data for reliability, as long as they tell us
> how they did so. But I realize this is far from optimal, especially
> if you want to support meta-analysis.
>
> dan
>
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