I think your design is OK. I can't see any real issues with it. Looks like you have 2 conditions, and have modeled context and response as single events regardless of conditions, so they do not totally overlap with the events of interest. I do think it is possible that the context and response events may steal some of the variance from your conditions of interest, but they may also do a good job of parsing our variance related to conditions of non-interest (context and button press), so I think there is no way to really know for sure if including them in the model will help of hurt you, which may not be consistent across voxels anyways. I tend to follow this sort of philosophy of being more inclusive in modeling events so I think your choice is reasonable.
Your results looks like it was done correctly as well. As this is single subject results, I would pay almost zero attention to the pmod effects. It is important to remember that t-statistics from first level models have no influence on the second level analysis, only the beta values carry through. At the first level the residuals are from data in the BOLD trace not well fit by the model, but in the second level analyses the variance is driven by the beta values for all participants in that voxel (not the single-subject variance at that voxel), thus it is not uncommon to have strong second level effects which do not produce reliable activity in first-level single subjects) analysis.
If you want a sanity check on your single subject data, do a main effect of a task and see that it activates regions like the visual cortex. Or if responses activate motor.
So long story short, don't worry too much if subject 1 doesn't show the pattern expected based on group results in other papers. That is just how it goes in fMRI.
Best,
Colin Hawco, PhD
Neuranalysis Consulting
Neuroimaging analysis and consultation
www.neuranalysis.com
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-----Original Message-----
From: Alexandre Obert [mailto:[log in to unmask]]
Sent: November-21-15 3:55 AM
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Re: [SPM] parametric modulations - again
Dear Colin, Dear all,
In attachments, you can see the protocol with duration and sequence of each event.
I also attached a matrix, contrast manager display and results (poor activations, I did not expected strong ones but it's very very tiny
here) from a 1st level (I renamed some of the columns for convenience of
explanations)
Thank you for your reply !
Alexandre
Le 20/11/2015 23:34, [log in to unmask] a écrit :
> It may be helpful for clarity if you paste an image of your design matrix. Also some explanation of event timings may be a bit helpful in this context. For example you are modelling responses and fixation. Are they very close in time with your events of interest? If you model two nearly overlapping events (e.g. less than 2 seconds apart) this can cause your effect size to be split across the two 'conditions' in an HRF analysis.
>
> My understanding is that your contrast would be correct (main effect of the pmod), and bring the con files into the second level. But it would likewise be helpful to paste an image of the contrast manager output from the SPM results window along with the design matrix. If you are less experienced with SPM and the design matrices it is easy to make a mistake at the contrast specification scale.
>
> Cheers,
>
> Colin Hawco, PhD
> Neuranalysis Consulting
> Neuroimaging analysis and consultation www.neuranalysis.com
> [log in to unmask]
>
>
> -----Original Message-----
> From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
> On Behalf Of Alexandre Obert
> Sent: November-20-15 4:55 PM
> To: [log in to unmask]
> Subject: [SPM] parametric modulations - again
>
> Dear all,
>
> I hope someone will be able to help me...
> I'm trying to make a parametric modulation analysis but I've got very strange results.
> It remains possible that I incorrectly set my design - a technical mistake rather than a theoretical one...
> Thus, maybe someone could help me and say to me if I there is an error in my design ?
>
> First, my experiment :
> My experiment contains 2 experimental conditions (let's name them A
> and
> B) each of these conditions contains sentences stimuli preceded by a Context.
> I want to test the linear effect of 2 parametric modulators for each condition, Beside them, there is some variables from my experiment that I modeled (I think they could have an effect but it doesn't interest me) :
> fixation crosses and the response of the participants (I think it could generate 'motor' activations).
>
> I set my design as follow:
> I modeled the onsets of the Context, then the condition A and the 2 parametric modulators and in the same way the condition B and its 2 PM ( the PM are correlated for each condition but since SPM8 orthogonalizes the PM at 1st level, it's not a problem to model them in the same design, right ?).
> I also modeled the onsets of the fixation crosses and responses of the participants.
>
> Finally, in order to test the linear effects, I set contrasts such as
> [0
> 0 1] with "1" on my parametric modulators at first level and use the
> con* files into a one sample 2nd level design for each PM.
>
> The results for at least one PM are very strange and unexpected. It's possible that it comes from an unexpected effect of the PM but I would like to be sure that I did not make any error in my design....
>
> I'm ready for any question and advice ;)
>
> Regards,
>
> Alexandre
>
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