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Subject:

Re: first level ancova

From:

Jesper Andersson <[log in to unmask]>

Reply-To:

FSL - FMRIB's Software Library <[log in to unmask]>

Date:

Tue, 30 Jun 2009 15:40:53 +0100

Content-Type:

text/plain

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text/plain (48 lines)

Dear Vitaly,

> i have an analysis approach question:
>
> event -related design with behavior measure (e.g. reaction time)
>
> what do you think about analyzing the data on the single subject  
> level as an
> ancova, where 1 EV has 1's for all events, zeros elsewhere, and  
> another EV
> has the demeaned behavior measure at each event, zeros elsewhere
>
> versus
>
> binning events into fast events or slow events, separate EVs, and then
> performing a contrast at the first level to pass up.
>
> so option A is the discrete approach, option B is a binary approach.

I would suggest a third approach. If you look at http://www.fmrib.ox.ac.uk/fsl/feat5/detail.html#stats 
  you'll find a description of the "3 column format" for specifying  
event-related designs. The third column can be used to put in a  
parametric modulatory effect. You would then make one EV that model  
the average effect of these events and another one that models the  
modulatory effect.

Let us say you have 3 events that ocurred a 5 30 and 50 seconds. Let  
us further say reaction times were 2 1 and 3 seconds respectively. For  
your modulatory EV you would then specify

5   0.1  0
30 0.1 -1
50 0.1  1

where the second column gives you "eventy events" and the third column  
is the mean-corrected reaction times.

You can then use the contrast [0 1] (assuming that the first column is  
the vanilla events) to take the dependence on reaction time to the 2nd  
level.

Note that this assumes that the reaction times have a effect on the  
amplitude of the response, and not on the duration. In principle one  
could equally well assume the reverse, and unfortunately it is very  
hard to test since a changes in amplitude and duration will result in  
very similar models for event related designs.

Good luck Jesper

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