Dear All,
We have just carried out an fMRI experiment which involved 9 subjects
carrying out two functional runs. Each run was a block design, with 20
blocks each containing 5 scans ( to make a total of 100 volumes per
subject, per condition)
run1: ACACACACACACACACACAC
run2: BCBCBCBCBCBCBCBCBCBC
Where C represents a low-level baseline (i.e. not of interest)
We have just completed a first-level analysis looking at the group
averaged differential activation between A and B. The contrast we
specified (in SPM99b) was:
-1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 1
Where the first nine columns represented the A runs for each subject,
and the next nine represented the B runs for each subject.
We already knew from looking at the group A versus C and B versus C
contrasts that the activation patterns were very similiar and sure
enough the differential contrast shown above picked out only one
cluster:
P(corrected)=0.04, extent= 1 voxel, Zscore=4.64
So my question (apologies for the long winded way of getting here) is
whether a single voxel cluster is deemed to be worth reporting or would
most SPM users impose a cluster size constraint? It obviously makes the
P<0.05 threshold, and I know its small size has been taken into account
during the calculation of the corrected P values.
Note that we have no a-priori hypothesis about where to look for the A
versus B differential activation, so from what I have been reading on
this mailbase, a small volume correction is not appropriate.
Many thanks,
krish
--
Dr Krish Singh, ([log in to unmask])
Magnetic Resonance and Image Analysis Research Centre, Liverpool
University
Pembroke Place, Liverpool, L69 3BX, UK. Tel 0151 7945645. Fax 0151 794
5635
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