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Dear Swann, I am sure you know the answer to this question but I will state it explicitly :) One should always adjust p-values to control false positive or discovery rates. In the context of multiple ROI inferences; a Bonferroni correction for the number of ROIs would be the simplest adjustment procedure. This holds irrespective of how the ROIs were specified (i.e., using prior information or localizing contrasts). I hope this helps. With very best wishes, Karl PS If you found a reference suggesting that mulitple ROI analyses are exempt from the multiple comparison problem, I would be amused (in a dark way) to hear about it. At 11:45 10/12/2009, swann pichon wrote: >Dear Pr Friston, > >I took the liberty of forwarding to you a question that has been >addressed on the SPM list yesterday. > >This concerns the issue of chosing ROIs from the dataset which is >being tested, an issue you have commented in Friston et al 2006, and >the issue of multiple comparision in ROI analyses (Poldrack et al >2009), which is far from being a standard in the field. > >In adavnce, thank you very much for your helpful comments, > >Kindest regards, > >Swann > > >---------- Forwarded message ---------- >From: swann pichon <<mailto:[log in to unmask]>[log in to unmask]> >Date: 2009/12/9 >Subject: Reporting ROI analyses and correcting for multiple comparisons >To: <mailto:[log in to unmask]>[log in to unmask] > > >Dear all, > >It seems that a majority of papers publishing ROI analyses does not >apply correction for multiple comparisons. > >The rationale behind it, I suppose, being that ROI analyses >correspond to planned comparisons and therefore are exempt from >usual corrections applied to post-hoc tests. > >- Is there a relevant reference that supports that point ? > >- Can this logic also be applied to ROIs that are dependent of the >dataset tested ie when ROIs have been generated using main effects >of a factorial design rather than using a functional localiser ? > >Concerning this last question, I am considering Friston et al 2006's >remark (A critique of functional localiser) which states : >"The key thing to appreciate is that a contrast testing for a >particular effect can be used as a localiser for the remaining >[orthogonal] effects. In this sense, any factorial fMRI study has as >many functional localisers, embedded within it, as there are >orthogonal contrasts. A typical two-by-two design has three >orthogonal contrasts. The natural conclusion is that all fMRI >experiments are simply collections of functional localisers." > >Thanks a lot for helpful comments, > >-- >Swann Pichon, PhD >Laboratory for Behavioral Neurology and Imaging of Cognition >Department of Neuroscience, University Medical Center >1 rue Michel-Servet, 1211 GENEVA 4, Switzerland >Tel: +41 (0)22 379 5979 >Fax: +41 (0)22 379 5402 >Gsm: +33 (0)6 26 43 83 61 ><http://labnic.unige.ch/>http://labnic.unige.ch/ >