Geraint,
If one has 4 sessions and 1 trial type in each session,
then is the contrast 1 1 1 1 the appropriate one?
Cathy
-------------------------
Catherine L. Reed, Ph.D.
Assistant Professor of Psychology
University of Denver
2155 S. Race St. Denver, CO 80208
Phone: 303-871-4622 FAX: 303-871-4747
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On Sun, 17 Oct 1999, Geraint Rees wrote:
> Dear Cathy,
>
> |1. VARIABLE EPOCH LENGTHS
> |
> |I acquired 80 images with a block design of 2 conditions
> |times 5 block repeats. After discarding the first 3 images
> |in the first block, I am left with 77 images: 5 A, 8 B,
> |8 A, 8 B, 8 A, 8 B, 8 A, 8 B.
> |
> |I am having difficulty with defining the parameters for
> |the design matrix of the fMRI analyes:
> |Assuming A is the condition of interest, I have to define
> |variable SOAs ([0 13:16:77]) as well as variable epoch lengths
> |(5 8 8 8 8). The epoch length specification doesn't seem
> |to work. The only work around I have found is to erase all
> |the images of the first block. Is there another way?
>
> My understanding is that in SPM99 each epoch-related response is defined
> in terms of basis functions that define the length of that epoch type. So
> within any one epoch type, the epoch length must be the same throughout.
> The solution here is therefore to model epochs of different lengths as
> different conditions and estimate the average effect using an appropriate
> contrast. This will cost you a small penalty in terms of degrees of
> freedom for modelling the extra 'short' epoch as a third condition.
>
> |2. COMBINING DATA FROM MULTIPLE SUBJECTS
> |
> |I have questions regarding how to combine the data from
> |multiple subjects and several sessions in a single statistical analysis.
> |As far as I know, there is no option on the fMRI stats model set up
> |to indicate multiple subjects-- only multiple sessions.
> |
> |Let's say I have 2 subjects that have 2 sessions each. Both sessions
> |alternate the same 2 conditions, but the order of session 1 is ABAB and
> |the order of session 2 is BABA. Using the fMRI stats model set up in
> |SPM99b, when asked to input the number of sessions I have specified 2.
> |When asked for the scans for session 1, I included session1 data from
> |Subject 1 and session 1 data from subject 2. Is this correct? Is
> |there a better way of doing this (e.g., specifying 4 sessions and
> |putting each subject's images from each session into separate sessions)?
>
> There's no distinction (in a fixed effects model) between subjects and
> sessions in SPM99b. The second way you suggest (specifying four sessions)
> would be better. If you combine two sessions from different subjects in
> one 'megasession' then you will no longer be correctly modelling the mean
> differences in activity between subjects as this is estimated on a
> session-specific basis (the effects of no interest in the design matrix).
> As between-subject variability in fMRI is often high compared to
> within-subject variability, this is not a good idea.
>
> More generally, if you enter four separate sessions, and then estimate the
> average effect using a contrast across all four sessions, you will be
> using a fixed effect model over two subjects. Whether this is most
> appropriate, or whether you would prefer to use a random effects model
> (which would require more subjects for reasonable power) depends on the
> experimental question you're interested in- see Karl's paper in NeuroImage
> I 10:1-5
>
> best wishes,
>
> Geraint
>
> -----------------------------------------------------------------
> Dr. Geraint Rees
> Wellcome Advanced Fellow Lecturer
> California Institute of Technology Institute of Neurology
> Division of Biology 139-74 University College London
> Pasadena 12 Queen Square
> California 91125 London WC1N 3BG
>
> voice (626) 395-2880 voice (171) 833-7472
> fax (626) 796-8876 fax (171) 813-1420
>
> http://www.klab.caltech.edu/~geraint [log in to unmask]
> -----------------------------------------------------------------
>
>
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