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On Fri, Jun 8, 2012 at 12:35 PM, Alexa Morcom <[log in to unmask]> wrote: > Dear SPM colleagues > > I'm attempting to scale regressors in a design matrix so that their sum of > squares equals 1. The reason to do this is that I have subjects with > different HRF timings in my task and I want to be able to calculate > 'amplitude' images (or 'derivative boost') from canonical and temporal > derivative images using the formula in Vince Calhoun's 2004 paper, but this > formula makes assumptions about the scaling of canonical and derivative > regressors. > The key in their methods is that the HRFs are scaled correctly. If you scale the design matrix, then you will be biased by the number trials in each condition. > > I've looked at previous helpful postings by Donald McLaren > See http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=SPM;fa723f27.0908 (Aug > 17 2009) > ..as well as the 2010 Neuroimage paper by Jason Steffener and others which > highlights the scaling issue. > > My first question is a check on some basic GLM maths. I thought that T > values should not change with regressor scaling, even though beta values > do, as the variance estimate changes too by the right amount. Can someone > confirm whether this is correct, as I have different T results following my > scaling operation. If this is correct, something is not right. > If you scale the HRF regressors (not DM), then the statistics shouldn't change. However, if you scale each column of the DM, then the T-statistics can change as you are changing the relationships between different columns. > > IF something is not right, can someone advise on what is not working about > normalising columns of SPM.xX.X so that sum of squares of each column = 1? > This is how I understand the recommendations in Steffener et al 2010 and > this is what I've done (followed by re-estimation of the model & contrasts, > obviously). But I may well have misunderstood something in the method. > I'd email Jason about how to normalise the HRFs rather than the DM. > > Thanks! > > Alexa > > > > The University of Edinburgh is a charitable body, registered in > Scotland, with registration number SC005336. > >