Hi Eric,
the dynamic expectation maximization is used for inversion of
stochastic DCM models- I think that when there's no direct input, then
the spm_dcm_estimate script will use this inversion scheme. This leads
to much better fits, but it's using noise rather than the paradigm to
explain a lot of the data. I guess your B-values are very small? That
also explains the drop in variance explained when you define direct
inputs, as the estimate-script will use the deterministic scheme
instead.
We previously also experienced problems with flat-line fits using
deterministic DCM on an event-related paradigm - Peter came with some
helpfull suggestions and 1) increasing the number of conditions used as
direct input (even though our question was related to the modulatory
influence of one of the conditions) and 2) using the 2-state version of
DCM actually did a difference- the average % variance explained
increased from ca 5 % to 17%.
So these two things may be worthwhile to try?
Best, Brian
On 2014-12-04 18:22, Eric Holst wrote:
> Dear Peter, dear SPM-experts,
>
> Leaving the hypotheses of my work aside, I have spent some time
> testing my VOIs under different conditions by reducing my models to
> two regions and systematically varying the input of all my factors to
> regions and/or connections. Let's say my two regions (VOIs) are left
> and right PPC as mentioned before (I have tested different VOIs as
> well) with reciprocal intrinsic connections. This is what I've found
> out:
>
> 1.) Whenever I *only* define one ore more factors as modulatory input
> to one or more connections (without defining the same or a different
> factor as driving input to any of both regions) the explained
> variance
> of my model is ranging from 38-40%. This is independent of the
> factors
> or the contrasts I use for extracting my VOIs. The model takes way
> longer to estimate using a different algorhythm ("Dynamic Expectation
> Maximisation"). To my knowledge this is not useful, as I have not
> defined a driving input as pertubation to my system.
> In other words: it makes no sense setting up a DCM without driving
> input. But on the other hand I have a high percentage of explained
> variance.
>
> 2.) This high percentage of explained variance drastically drops to
> 0-3% (mostly 0%) soon as I *additionally* define any factor as
> driving
> input to one or more regions. This is again independent of the
> factors
> and contrasts I use. But as far as I know this is the right method
> for
> setting up DCMs.
>
> Maybe you or someone else can explain this finding? I am a little bit
> confused about it.
>
> I am not sure that concatenating my sessions will make a big
> difference, but I am willing to give it a try. A script for
> concatenating sessions would be great. Thank you very much Peter for
> your help!
>
> Best,
> Eric
>
>> Dear Eric,
>> Regarding your DCMs flat-lining. First, you are correct that
>> extracting the ROIs with SPM12 won't make a difference. And I'll
>> assume that your contrasts and effects of interest contrast are
>> correctly specified - always worth checking. (You get a good level of
>> explained variance in your ROIs, which is a good sign.)
>>
>> Model fitting is more challenging with fast event-related designs,
>> and you do not have many trials per condition. I think concatenating
>> your sessions could be very important. I appreciate the technical
>> challenge in doing this - I am planning a feature for SPM to do this
>> automatically, but it won't be ready for a while. In the meantime, I
>> think Donald McLaren (CC'd) may have a concatenation script that may
>> help. Otherwise, you'll need to do this manually with your onsets. As
>> you say, you'll need to be very careful with dummy scans etc.
>>
>> Good luck,
>> Peter.
>>
>> -----Original Message-----
>> From: SPM (Statistical Parametric Mapping)
>> [mailto:[log in to unmask]] On Behalf Of Eric Holst
>> Sent: 03 December 2014 13:54
>> To: [log in to unmask]
>> Subject: Re: [SPM] DCM: Problem with percentage of explained
>> variance
>>
>> Dear Peter,
>>
>> Thank you very much for helping me again.
>>
>> ad 1. I have already estimated the same DCM analysis in SPM12,
>> unfortunately yielding the same results (explained variance of 0% in
>> spm_dcm_fmri_check). But I have not tried to extract my VOIs with
>> SPM12 for the DCM. I am not sure if this has any impact on the priors.
>>
>> ad 2. Your assumption is right, I have modeled separate DCMs per
>> session. Reading the older posts about concatenating sessions worries
>> me a little bit, as I am not good at MATLAB coding and there are many
>> pitfalls for calculating my concatenated onsets (dummy scans at the
>> beginning of each scan, etc.). This is why I am quite sure, I won't be
>> able to do it manually. Is there an easy way to do it (maybe using
>> gPPI)? I am very willing to give it a try, if there is an 'easy' way
>> to do it.
>>
>> You are presuming that the problem may result from a small number of
>> trials. Each of my sessions consists of 40 trials in randomized order
>> (20 trials for condition 1 and 20 trials for condition 2). Wrong or
>> missing answers were not further analysed. My subjects had an average
>> percentage of right answers of about 70-90%, so for each condition I
>> have 13+ trials. I am not sure, if this is sufficient.
>>
>> Please see attached file "DCM_inputs" as a DCM example for one
>> session of one subject (condition 1). The first input "photic" is a
>> driving input to bilateral V1 regions, the second input "form" is a
>> modulatory input to bilateral V1->V4 connections. I am varying the
>> third input "att_1HEM" as modulatory input to regions/connections in
>> my models (representing attentional modulation under condition 1).
>> Note that there is no peak in "att_1HEM" for trials under condition 2.
>> For these conditions I have a second modulatory input "att_2HEM" with
>> different models, whereas the other inputs "photic" and "form" are
>> kept unchanged.
>> The aim of my study is to find out if the best model under BMS is
>> same or different for condition 1 vs. condition 2. The duration of
>> input "form" is only 100 ms, the other durations are always 6+
>> seconds. I thought that the problem could arise from the short
>> duration of "form"
>> in comparison to the other inputs, but when I estimate models
>> without "form" the explained variance goes up to 1% in some (rare)
>> cases - which is still not good enough.
>>
>> Is there anything more I could try or test? Maybe there is a problem
>> with my contrasts?
>>
>> Best,
>> Eric
>>
>>
>>
>>> Hi Eric,
>>> Your procedure for picking ROIs sounds perfect. Two things come to
>>> mind:
>>>
>>> 1. You mention in your original email that you are using SPM8.
>>> Could you try exactly the same DCM analysis in SPM12? The priors were
>>> changed to reduce the chance of flat-lining.
>>>
>>> 2. You have 6 sessions, and it doesn't look like you've
>>> concatenated them into a single session, thus I assume you're
>>> creating separate models per session? This means in each model, you
>>> may not have many trials? Try SPM12 first, otherwise I suggest
>>> concatenating the sessions into one long session, with extra
>>> regressors to model session effects. See many previous posts on how
>>> to do this.
>>>
>>> Best,
>>> Peter.
>>>
>>> -----Original Message-----
>>> From: SPM (Statistical Parametric Mapping)
>>> [mailto:[log in to unmask]]
>>> On Behalf Of Eric Holst
>>> Sent: 03 December 2014 00:25
>>> To: [log in to unmask]
>>> Subject: Re: [SPM] DCM: Problem with percentage of explained
>>> variance
>>>
>>> Dear Peter, dear SPM-experts,
>>>
>>> This may serve as an example to demonstrate how I have defined and
>>> extracted my VOIs.
>>>
>>> First, I have created masks for my ROIs with the SPM Anatomy
>>> Toolbox (version 18) in MNI-space.
>>> In my example, this is for posterior parietal cortex of the left
>>> hemisphere (PPC_L), consisting of Brodman area 5+7.
>>>
>>> As mentioned before, I have calculated one SPM per subject
>>> containing sessions 1-6, so for each subject...
>>> - SPM => Results
>>> - t-contrast for "attention"
>>> - masking (inclusive) => image: ROI_PPC_L (as created with SPM
>>> Anatomy
>>> Toolbox)
>>> - p value adjustment: none
>>> - threshold: p = 0.05
>>> - extended threshold voxels: 0
>>> - goto: global maximum (which is inside my predefined mask)
>>> - eigenvariate
>>> - adjust for: effects of interest (EOI)
>>> - session: 1-6
>>> - sphere radius: 6 mm
>>>
>>> In this way I have extracted my VOIs for all subjects and sessions
>>> for PPC_L. For PPC of the right hemisphere and the other regions (V1
>>> and V4) I have used different masks and/or contrasts, resulting in 36
>>> VOIs per subject (6 regions x 6 sessions). By definition these VOIs
>>> were based on activations in my SPM analysis, subjects without
>>> activation in any of these ROIs were not further analysed.
>>>
>>> Based on these VOIs I have set up my DCMs. The basic setup of each
>>> DCM
>>> is similar to the one used for the „attention to motion“ paper,
>>> including 6 regions with reciprocal intrahemispheric connections
>>> between
>>> V1-V4 and V4-PPC and reciprocal interhemispheric connections
>>> between
>>> V4-V4 and PPC-PPC. Driving input is allocated to both V1 regions.
>>> On this basis I have set up multiple DCMs with alternating modulatory
>>> input of factor "attention" to either regions or connections to
>>> subsequently perform a BMS analysis.
>>>
>>>
>>> Peter, I hope I have answered your questions. If you have more
>>> questions, please let me know. If the extraction of my VOIs is
>>> correct, where else could I dig for my mistake?
>>>
>>>
>>> Thank you for your time!
>>>
>>> Best,
>>> Eric
>>>
>>>
>>>
>>>> Hi Eric,
>>>> Sorry you're still having problems with your models. Simplifying
>>>> the models to 3 regions is a good start. Could you tell us more
>>>> about how you choose your ROIs? Are they based on activations in
>>>> your SPM analysis, in the same experimental conditions as you're
>>>> modelling with the DCM?
>>>>
>>>> Best,
>>>> Peter
>>>>
>>>> -----Original Message-----
>>>> From: SPM (Statistical Parametric Mapping)
>>>> [mailto:[log in to unmask]] On Behalf Of Eric Holst
>>>> Sent: 01 December 2014 13:55
>>>> To: [log in to unmask]
>>>> Subject: [SPM] DCM: Problem with percentage of explained variance
>>>>
>>>> Dear Peter, dear SPM-experts,
>>>>
>>>> some time ago I posted a question about very low percentage of
>>>> explained variance (0%) when checking my DCMs with
>>>> spm_dcm_fmri_check.
>>>>
>>>> (see
>>>>
>>>> https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind1411&L=spm&F=&S=&X=
>>>> 4
>>>> C60EE6C21C6F1C1B5&Y=eric.holst%40web.de&P=334445)
>>>>
>>>> As this could be due to complexity of my DCMs I have reduced my
>>>> basic model from 6 to 3 regions, but explained variance is still at
>>>> 0%.
>>>>
>>>> I have read about increasing the area of the VOIs (I am using a 6
>>>> mm sphere), but most of my VOIs already consist of 30+ voxels.
>>>> [Btw: I smoothed my data in preprocessing.]
>>>>
>>>> Now I am wondering about the right way to proceed. What should I
>>>> do now to improve my models?
>>>>
>>>> Any help is appreciated and thanks in advance,
>>>>
>>>> Eric Holst
>>>>
>>>> Department of Neurology
>>>> Charité, Berlin, Germany
