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.
>
>
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>
>
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>
--
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Dr Richard Henson
MRC Cognition & Brain Sciences Unit
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Cambridge
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