> 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
-------------------------------------------------------
