MRI help

Are there any useful rules of thumb about what the critical cluster size should be in reporting results. I have a results with a cluster size of 1 that reaches the 2.3 threshold. (i) this cant be a real ROI - can it? (ii) why is the critical z val set at 2.3 anyway? Any help gratefully received. Thanks Philip. ******************************************************************** Philip Quinlan E-Mail: ptq1(a)york.ac.uk Department of Psychology FAX: (01904) 323181 The University of York Tel: (01904) 320000 Ext. 3135 Heslington Direct : (01904) 323135 York YO10 5DD U.K. ********************************************************************