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Great, it did the trick. The bottom right plot is now correct. The bottom left, instead, is still HRF inverted. I don't know if it makes sense correcting also that, but it's still good enough to know the way it works. Thank you very much Rik. Dorian 2009/2/11 Rik Henson <[log in to unmask]>: > >> Therefore it is right to conclude that whatever activation comes with >> contrast "-1" its always a decrease? > > A -1 contrast on a parametric modulator (of a canonical HRF) will always > show voxels in which the parameter estimate for the canonical HRF (and > therefore effectively the fitted HRF peak around 5-6s) decreases with > increasing values of your modulator. This could be an increasingly negative > deactivation. > >> But there is no way to have the correct plot with SPM or some other tools? >> > > One can reconstruct the fitted responses fairly easily from the parameter > estimates, basis functions and original modulation values (see line 550 > onwards of spm_graph), but perhaps the simplest fix in this specific case is > to change line 605 of spm_graph to > > [j i] = max(abs(mean(Y,2))); > > Rik > >> Best Regards. >> Dorian. >> >> 2009/2/11 Rik Henson <[log in to unmask]>: >> >>> >>> It is because the parametric plot in the bottom right of plotarea2 refers >>> to >>> the signal at 15.8s, which is normal undershoot of the HRF (not its peak >>> like in plotarea1). But because this region shows a "negative" HRF vs >>> baseline, the parametric function is reversed - ie the region in >>> plotarea2 >>> does show a decreasing (more negative) response with increasing parameter >>> values, as in the bottom left panel, but the plot on the bottom right >>> reverses this. I agree this is very confusing, and generally ignore the >>> plot >>> on the bottom right. The contrast and results pertain to the parameter >>> estimate for your HRF, and are consistent. >>> >>> Rik >>> >>> Dorian P wrote: >>> >>>> >>>> Dear all, >>>> >>>> I am running an analysis with parametric modulations. The contrast "1" >>>> is supposed to reveal areas with >>>> increasing signal while "-1" with decreasing signal >>>> (https://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind05&L=SPM&P=R168802). >>>> >>>> However, when I plot the contrast "-1", I am getting some areas with >>>> increasing >>>> activation and other areas decreasing. You can see the effect at the >>>> attached pictures. How is this possible? >>>> >>>> I have observed that the attached examples are one for for response at >>>> 5.1 seconds and the other at 15.8 seconds. This is >>>> how SPM plots that, I just used the plot button. >>>> >>>> Any clue on what kind of modulations I am getting? >>>> >>>> Dorian. >>>> PhD Student >>>> Ruhr University Bochum. >>>> >>>> ------------------------------------------------------------------------ >>>> >>>> >>>> ------------------------------------------------------------------------ >>>> >>>> >>> >>> -- >>> >>> ------------------------------------------------------- >>> Dr Richard Henson >>> MRC Cognition & Brain Sciences Unit >>> 15 Chaucer Road >>> Cambridge >>> CB2 7EF, UK >>> >>> Office: +44 (0)1223 355 294 x522 >>> Mob: +44 (0)794 1377 345 >>> Fax: +44 (0)1223 359 062 >>> >>> http://www.mrc-cbu.cam.ac.uk/people/rik.henson/personal >>> ------------------------------------------------------- >>> >>> >>> >>> > > -- > > ------------------------------------------------------- > Dr Richard Henson > MRC Cognition & Brain Sciences Unit > 15 Chaucer Road > Cambridge > CB2 7EF, UK > > Office: +44 (0)1223 355 294 x522 > Mob: +44 (0)794 1377 345 > Fax: +44 (0)1223 359 062 > > http://www.mrc-cbu.cam.ac.uk/people/rik.henson/personal > ------------------------------------------------------- > > >