Hi Tom,
Thank you very much for your suggestions. I think it is helpful to me.
Xiaoyun
On Fri, 11 May 2007 09:45:17 -0500, Tom Johnstone <[log in to unmask]>
wrote:
>Some other possible options are:
>
>i) instead of dividing by baseline voxelwise, compute the mean
>baseline value across the whole brain (which would be 10000 if using
>FEAT) and use that as the denominator in % signal change calculations.
>This is more or less the approach used by SPM I believe, and will give
>more reasonable values in areas of low signal. The problem is that it
>is not really a correct measure of % signal change, so I tend not to
>like this option.
>
>ii) use the mean baseline value for a small region of voxels
>surrounding the voxel of interest in calculations of % signal change.
>This is a compromise between the voxelwise approach and i) above.
>Because most areas of low signal are in areas showing quite high
>signal gradients, this approach will tend to give a denominator in %
>signal change that is somewhat higher than some of the individual
>voxels, while still representing the local baseline signal to a
>certain degree.
>
>iii) use z-scores instead of % signal change. This changes the
>interpretation somewhat, since z-scores are a measure of contrast to
>noise, rather than contrast to baseline signal. But z-score values
>will be more consistent across brain regions, and in most cases in my
>experience give highly similar statistical results as % signal change.
>
>Tom
>
>--
>
>Tom Johnstone
>Waisman Laboratory for Brain Imaging and Behavior
>Waisman Center
>University of Wisconsin-Madison
>Tel. +1 608 263 2743 Fax. +1 608 265 8737
>[log in to unmask]
>http://brainimaging.waisman.wisc.edu/~tjohnstone
>=========================================================================
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