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SPM  March 2009

SPM March 2009

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Subject:

Re: mixed event-block SPM5

From:

Esther Fujiwara <[log in to unmask]>

Reply-To:

Esther Fujiwara <[log in to unmask]>

Date:

Tue, 24 Mar 2009 15:35:00 -0600

Content-Type:

text/plain

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text/plain (116 lines)

Hi Michael,
Thank you for taking the time to read and answer my questions, and sorry 
for any confusing wording.

I would not have thought of modeling post-event time points separately 
if this hadn't been mentioned in mixed-design papers like Visscher's. 
That being said, I am not sure how SPM5's hemodynamic lag is different 
(or the same) as modeling post-event times explicitly. If it is indeed 
the same as including event+1, event+2, event+3 etc as additional 
regressors, that would be an easier solution than what I think I need to 
do...

My baseline blocks consist of an active baseline (odd-number detection). 
Again based on papers using mixed design and modeling their (fixation 
cross) baselines separately (see also Mitchell et al.,  Psychol Sci. 
2007 Apr;18(4):292-7.), I was wondering why one would want to do that if 
SPM does it implicitly.


Thanks a lot,
Esther





Michael T Rubens wrote:
> Esther,
> 
> 
> On Tue, Mar 24, 2009 at 12:52 PM, Esther Fujiwara <[log in to unmask] 
> <mailto:[log in to unmask]>> wrote:
> 
>     Hi (sorry for any double posting),
>     I have a few questions about analysing a mixed block/event-related
>     design in
>     the same model. If I understand Visscher et al. correctly (NeuroImage 19
>     (2003) 1694–1708), blocks and events are modeled with different
>     functions
>     (gamma function vs. separate delta regressors for each time point
>     following
>     the event, covering 14-20 s post-event).
> 
>     My design has three blocks (ABC) and 2 event-types (1,2), presented
>     in each
>     block, i.e., a 2x3 design like this: 1A,1B,1C,2A,2B,2C. Task blocks
>     are 60
>     seconds long, with 30s baseline blocks. Events (within task blocks) are
>     mixed with null events.
> 
>     My questions:
>     1) If I want to model events and blocks in the same model using
>     SPM5, do I
>     need to use different functions for events and blocks? How/where do I do
>     this in SPM5?
> 
>  
> The difference between the event and block (epoch) is the duration that 
> you specify. For your events, specify the duration as 0. For the blocks 
> specify the duration as the actual duration of the block.
>  
> 
> 
>     2) How do I model the post-event time frames explicitly?
> 
> 
> Not sure I quite understand you here, but the convolution of your 
> boxcar/stick with your hrf accounts for hemodynamic lag.
>  
> 
> 
>     3) What is the difference between modeling a 'condition' and
>     'regressor'?
> 
> 
> Again, not crystal on your question here. A regressor is one column in 
> your design matrix. If you have multiple sessions of the same condition, 
> you may choose to model them with separate regressors in order for your 
> intercept term to properly account for baseline differences, then you 
> can combine the resulting parameter estimates in later analyses.
>  
> 
> 
>     And do I need to model baseline blocks and/or null trials explicitly?
> 
> 
> What does your baseline block consist of? In short, if you do not model 
> these, which you probably shouldn't, then neural activity during these 
> time periods will essentially be accounted for by the intercept term 
> (intrinsic baseline). Later if you do a contrast of [1] for the 
> condition, it will subtract the intercept from your condition specific 
> betas, along with the rest of the unaccounted variance.
> 
> Cheers,
> Michael
> 
> 
> 
> -- 
> Research Associate
> Gazzaley Lab
> Department of Neurology
> University of California, San Francisco

-- 
Esther Fujiwara, Ph.D.
Assistant Professor
Department of Psychiatry
University of Alberta
3087 Research Transition Facility
Edmonton, AB
Canada T6G 2V2

Phone: 780 492-6524
Fax: 780 492-6841

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