Hi Jillian,
By dividing the volume by the number of voxels per resel you'll get
the number of resels in your search space (ie the brain mask used in
the 3rd level analysis). If you then use
ptoz 0.05 -g RESELS_COUNT
you'll get the voxel-wise height threshold necessary for a p-value of
0.05 in a GRF of the volume (in resels) of your data set. In other
words, any voxel with a z-score equal to or higher than the output of
that command reaches a corrected level of significance in your
experiment. There is no additional extent threshold. So yes, it is
considerably more conservative than an uncorrected p<0.05. On the
other hand, it appropriately takes into account the number of
independent comparisons being made.
Hope this helps.
Joe
------------------
Joseph T. Devlin, Ph. D.
Dept. of Psychology,
UCL
Gower Street
London, WC1E 6BT, UK
email: [log in to unmask]
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