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SPM  November 2009

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Subject:

Re: Comparing conditions with different duration.

From:

Rik Henson <[log in to unmask]>

Reply-To:

Rik Henson <[log in to unmask]>

Date:

Mon, 16 Nov 2009 16:42:41 +0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (51 lines)

Christopher -

Yes, what I said was under the assumption that the neuro-vascular 
mapping (eg shape of HRF) is not affected by trial duration - ie linear 
convolution assumptions hold. You are correct to point out that there 
are many factors (neural habituation, blood flow, volume, oxygenation 
etc) that might also change with trial duration, and violate such 
assumptions.

Rik

Christopher Benjamin wrote:
> ... as clarification/elaboration, Glover 1999 found that the predicted blood flow response 
> was closer to the actual blood flow response at some event durations as compared to 
> others, with the predicted diverging from the actual duration quite a bit with longer 
> durations.
>
>
>
> Hi Rik, Dorian,
>
> I've been looking into this a bit lately because i'd like to set up a design where events 
> won't have the same duration, and is there a bit of a problem because the blood flow 
> model created by SPM will be better for some durations than others?  I.e., particularly 
> when the durations are very different, you might get distorted results?  E.g. Glover 1999 
> found marked discrepancy between blood flow response as predicted from a brief event, 
> and actual blood flow (though maybe this no longer applies), and Zwart et al 2009 note 
> that in this setting 
> <If the GLM design matrix used describes the response to one stimulus type less 
> accurately than the response to another, the residual after &#64257;t will be larger. Since the 
> standard deviation of this residual is used to compute signi&#64257;cance of the &#64257;tted amplitude, 
> the computed t-score will be reduced. This not only reduces detection power, but will 
> often be erroneously interpreted as decreased activation.> Zwart et al., 2009. p1656
>   

-- 

-------------------------------------------------------
                 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
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