Consider performing a 
Principles of random effect analyses
These analyses are also called second level analyses, and concentrate on the variation of activation between subjects in deciding the significance levels for SPMs.
For a comprehensive description of this problem in SPM, see the random effects chapter of the Human Brain Function book - second edition.
See also the RFX section of the FIL/SPM bibliography.
There is a random effects discussion on the Wisconsin site
Get a fuller picture in this tutorial or this one.