JiscMail Logo
Email discussion lists for the UK Education and Research communities

Help for SPM Archives


SPM Archives

SPM Archives


SPM@JISCMAIL.AC.UK


View:

Message:

[

First

|

Previous

|

Next

|

Last

]

By Topic:

[

First

|

Previous

|

Next

|

Last

]

By Author:

[

First

|

Previous

|

Next

|

Last

]

Font:

Proportional Font

LISTSERV Archives

LISTSERV Archives

SPM Home

SPM Home

SPM  May 2009

SPM May 2009

Options

Subscribe or Unsubscribe

Subscribe or Unsubscribe

Log In

Log In

Get Password

Get Password

Subject:

Re: DCM: fixed vs. random effects BMS; direction of connectivity change

From:

Eric Zarahn <[log in to unmask]>

Reply-To:

[log in to unmask]

Date:

Sun, 24 May 2009 15:33:13 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (142 lines)

Dear All,

Is there a most relevant paper for model selection in the context of  
DCM as summarized so elegantly in this thread? Thanks in advance.

Eric

Quoting Klaas Enno Stephan <[log in to unmask]>:

> Dear Darren,
>
> The choice between fixed effects (FFX) and random effects (RFX)   
> analyses in the context of BMS is no different from choosing between  
>  FFX and RFX in the context of any other statistical analysis (like   
> SPM):  if one believes that the effect of interest (here: model   
> structure) is a fixed property of the population studied, one should  
>  use a FFX analysis.  If, however, if one believes that the effect  
> of  interest is a random variable in the population studied, a RFX   
> analysis is preferable.
>
> FFX analyses are appropriate, for example, when studying low-level   
> physiological phenomena where it can (relatively safely) be assumed   
> that these phenomena exist as fixed properties of the population and  
>  that variability across subjects is due to measurement noise alone.  
>   FFX BMS requires summing of the log evidences across subjects and   
> then comparing this across models (equivalently: multiplication of   
> Bayes factors).
>
> RFX analyses should be preferred when studying cognitive processes   
> (due to potential inter-subject variability in strategy and, for   
> systems with degeneracy, the possibility that networks are used   
> differently across subjects to implement task demands) or patients   
> (due to potential variability in pathophysiology or in the degree in  
>  which brain function has been compromised by the disease).  RFX BMS  
>  uses the new Variational Bayes method in SPM8.
>
> Best wishes,
> Klaas
>
>
>
>
>
> ________________________________
> Von: Darren Gitelman <[log in to unmask]>
> An: [log in to unmask]
> Gesendet: Freitag, den 22. Mai 2009, 16:40:54 Uhr
> Betreff: Re: [SPM] DCM: fixed vs. random effects BMS; direction of   
> connectivity change
>
> Narender
>
> Thanks for this update. I have some questions about it.
>
>
> On Fri, May 22, 2009 at 3:23 AM, Narender Ramnani   
> <[log in to unmask]> wrote:
>> -------------------------------------------
>> Dear Narender,
>>
>>> We are investigating connectivity between two areas using DCM. Our
>>> anatomical model simply consisted of
>>> forward and backward connections between them.
>> ...[Details deleted for brevity]...
>>
>>> We used random effects Bayesian model selection to distinguish between a
>>> number of
>>> models (varying the modulating influence of our experimental effect on
>>> each connection, and varying the location of the input).
>> ...[Details deleted for brevity]...
>>
>>>
>>> (1) Is our model comparison approach appropriate?
>>
>> Yes - it is compelling and the model space is conceptually nice.
>> The only point I would make is that you appear to have used a random-effects
>> inference over subjects. This means that a priori, you expect each subject
>> could have a different architecture. Usually, in straightforward systems
>> neuroscience studies, one assumes that all subjects have the same
>> basic architecture (but different parameters). This means the fixed-effect
>> pooling of log evidence is more appropriate and usually gives more
>> significant results.
>
> If I understand correctly, this suggests it would be reasonable to do
> a fixed effects analysis to compare models across subjects, as long as
> one thinks that "all subjects have the same basic architecture". This
> seems to apply best to groups of normal subjects (or homogeneous
> groups). However, in the case of abnormal subjects (e.g.,
> neurodegenerative disease) the assumption of having the same basic
> architecture might no longer apply, and in that case a random effects
> model would more properly apply?
>
>>
>>> (2) We are interested in whether or not we can say that modulations
>>> represent increased or decreased connectivity. Are we able to make
>>> inferences about this from the contents of matrix B (e.g. some values in
>>> some subjects are negative)?
>>
>> Yes, exactly. The best way to report these is to report the % increase or
>> decrease in the fixed connectivity (A) implied by the condition specific
>> effect  (B). This means commuting (100*DCM.Ep.B{i}(j,k)/DCM.EpA(j,k))
>> for each connection (j,k) and condition (i). (this is for fMRI, in M/EEG the
>> parameters are already gain parameters). For group results you can
>> use the group average,where each subject's estimate is weighted
>> by its precision
>
> Does this take the place of doing direct t-tests on the B matrix
> parameters as has been done in the past? Would it be best to analyze
> the "normalized" (B/A) matrix parameters for each condition instead?
>
> Thanks
> Darren
>
>
>> I hope this helps,
>>
>> Karl
>>
>>
>>
>>
>>
>> --
>> Narender Ramnani
>> Reader in Cognitive Neuroscience
>>
>> Cognitive Neuroscience Laboratory
>> Department of Psychology
>> Royal Holloway University of London
>> Egham, Surrey TW20 0EX
>>
>> Tel: 01784 443519 (Direct)
>> Fax: 01784 434347 (Departmental)
>> email: [log in to unmask]
>>
>> www.pc.rhul.ac.uk/staff/n.ramnani
>>
>
>
>
>

Top of Message | Previous Page | Permalink

JiscMail Tools


RSS Feeds and Sharing


Advanced Options


Archives

June 2019
May 2019
April 2019
March 2019
February 2019
January 2019
December 2018
November 2018
October 2018
September 2018
August 2018
July 2018
June 2018
May 2018
April 2018
March 2018
February 2018
January 2018
December 2017
November 2017
October 2017
September 2017
August 2017
July 2017
June 2017
May 2017
April 2017
March 2017
February 2017
January 2017
December 2016
November 2016
October 2016
September 2016
August 2016
July 2016
June 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
July 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
October 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
August 2013
July 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
March 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
March 2011
February 2011
January 2011
December 2010
November 2010
October 2010
September 2010
August 2010
July 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
August 2009
July 2009
June 2009
May 2009
April 2009
March 2009
February 2009
January 2009
December 2008
November 2008
October 2008
September 2008
August 2008
July 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
August 2007
July 2007
June 2007
May 2007
April 2007
March 2007
February 2007
January 2007
2006
2005
2004
2003
2002
2001
2000
1999
1998


JiscMail is a Jisc service.

View our service policies at https://www.jiscmail.ac.uk/policyandsecurity/ and Jisc's privacy policy at https://www.jisc.ac.uk/website/privacy-notice

Secured by F-Secure Anti-Virus CataList Email List Search Powered by the LISTSERV Email List Manager