Dear SPM'ers,
The activity measured is a weighted sum of the present activity and the
activity of that voxel from the recent past (done by the HRF convolving of
the contrast). This HRF is based on the haemodynamic delay (HD) (let's say
8 sec) and I think also on the TR (let's say 3.2 sec). The kernel used to
form the HRF must thus be 8/3.2, because the HD is in seconds, while our
data is in image-units (1 such unit = TR) and the convolution takes place
on "something" in image-units. Is this reasoning correct ?
One single scan thus lasts for TR seconds, resulting in a difference in
time between the first and the last voxel of that scan recorded. Does SPM
correct for this fact ?
If yes, does it then presume an ordering in the scan direction (inferior
to superior, posterior to anterior, ...) and adapts it thus the HRF to the
spatial location of the voxel. We use sometimes interleaved scanning
techniques (all slices are not recorded sequentially from inferior to
superior), resulting in hashed images, which need to be re-hashed to get
correct images in SPM, meanwhile losing all the temporal information,
which will certainly result in bad interpretations, conclusions.
If not, will the used -uncorrected thus- strategy not lead to wrong
conclusions ? I can imagine that some voxels, having a nearly significant
significance level, are abusively marked as significant or insignificant,
resulting in smaller or larger brain regions than it should be in reality,
due to this neglected difference in time.
Or doesn't SPM count for it, but is it anyway negligible ?
Any help will be nice
Best regards,
Patrick
Hendrik
Computational Neuroscience
Laboratory of Neurophysiology
K.U.Leuven Faculty of Medicine
Campus Gasthuisberg O & N
Herestraat, 49
B-3000 Leuven
Belgium
Tel : +32 16 34.59.61
Fax : +32 16 34.59.93
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http://simone.neuro.kuleuven.ac.be
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