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Thank you for your answers. Donald, in answer to your point, I used a contrast showing the difference in activation between the two sessions (Post-Pre). The two sessions were modelled in the 1st level design. One further question if I may, to clarify this. Would it also constitute double dipping if one extracts the data from around a peak of activation for the main effect (size of pre-post difference) and correlate this with other variables, such as psychophysics? In other words, is any such selection procedure always circular, or is it so only if one uses as a selection criterion the same variable one is testing (in this case correlation with performance)? Thank you. Regards, Mario Gatica On 29 November 2011 22:31, MCLAREN, Donald <[log in to unmask]> wrote: > A couple of points: > > (1) It is unclear what went into the second level analysis, pre or post > images, the difference in pre or post, or one of the images. > > (2) It is very circular, you have searched the data to extract, rather > than pick an area a priori to extract the data. > > Best Regards, Donald McLaren > ================= > D.G. McLaren, Ph.D. > Postdoctoral Research Fellow, GRECC, Bedford VA > Research Fellow, Department of Neurology, Massachusetts General Hospital > and > Harvard Medical School > Office: (773) 406-2464 > ===================== > This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED > HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is > intended only for the use of the individual or entity named above. If the > reader of the e-mail is not the intended recipient or the employee or > agent > responsible for delivering it to the intended recipient, you are hereby > notified that you are in possession of confidential and privileged > information. Any unauthorized use, disclosure, copying or the taking of > any > action in reliance on the contents of this information is strictly > prohibited and may be unlawful. If you have received this e-mail > unintentionally, please immediately notify the sender via telephone at > (773) > 406-2464 or email. > > > > > On Tue, Nov 29, 2011 at 4:13 PM, Mario Gatica < > [log in to unmask]> wrote: > >> Dear SPMers, >> >> I would like to ask for an opinion concerning the statistical validity of >> an analysis I did. >> >> I have pre/post experiment where subjects are scanned before and after an >> intervention, in order to compare changes in activation. During the >> scanning sessions subjects were just passively stimulated. This main effect >> was very robust. >> >> In addition to fMRI I also did psychophysical testing (outside the >> scanner) and I am trying to find correlations between changes in >> performance and changes in activation. >> >> I followed the straighforward approach of doing a 2nd level design with >> the changes in performance entered as a covariate (1 value per subject) and >> then the contrast "0 1" or "0 -1", as the case may be, limiting the >> analysis to the postcentral gyrus with a mask (about 1000 voxels). With >> this design, no voxels survived multiple comparisons correction, so I tried >> progressively more relaxed thresholds until activation appeared (usually >> with an uncorrected p of 0.05). I then made a spherical mask around this >> coordinate and extracted the mean raw data from the individual conn* images >> used in the group analysis (around 30 voxels), and used these to do a >> Pearsons correlation test outside of SPM (with the psychophysical data), >> which was positive. >> >> My question is, is this a valid thing to do at all or is the analysis >> biased? On the one hand, the correlation done outside of SPM was strongly >> significant and made sense theoretically. On the hand, this does smell like >> biasing the analysis. Of course such a thing would not be valid when >> testing the main effect of interest. Does that also hold for correlation >> with an external variable (such as behavioural performance)? >> >> Many thanks in advance. >> >> Mario Gatica >> >> -- >> Mario Gatica >> Institut für Neuroinformatik >> Ruhr-Universität Bochum >> >> > -- Mario Gatica TELEFON: 01520 830 5011 Institut für Neuroinformatik RUB