LLN, le 16/09/05
Dear Nici,
I asked the same question about concatenated runs filtering some time
ago but did not get any answer. I thus cannot help you with this.
You should however be aware that concatenation is not recommended. If
the scanner stopped and restarted between runs, yet you modelled your
conditions as a single session, then your data no longer represent a
continuous timeseries, and several aspects of SPM will be disrupted,
eg.:
- highpass filtering,
- temporal autocorrelation estimation,
- grand-mean scaling,
- session-meaning.
(I quote Rik Henson, from this list).
Note that I once compared "concatenated runs + constant" to
"separated sessions" and the results were quite similar, though not
identical.
Hope this helps,
Mauro
>Dear SPM experts,
>
>I’ve checked the spm archives but I couldn’t find an answer to the
>following question:
>
>I’m using an event-related design, in which some events depend on
>the behavior of the subjects such that there are sometimes only 2
>per run. Since I can not acquire the data in one long run, I would
>like to concatenate 5 runs (lets say each contains 200 images) to
>one long containing then images 1 to 1000. Right now I’m adding a
>constant for each run to correct for session specific effects as
>well as one extra covariate modeling the transition from one run to
>the next (not sure whether this is really necessary). I would like
>to highpass-filter the data and use the default SPM2 value of 128 s.
>If I apply this filter to the concatenated run, the “session
>specific-constants” are filtered too. This is clearly visible in the
>design matrix. My first question is: is this a problem??????
>
>In the meanwhile, I’ve changed the spm code a little bit, such that
>the filtering is applied for each original run, i.e the filter is
>applied to image 1-200 of the concatenated data, then to image
>201-400, then to 401-600 and so on. If I check the design matrix the
>constants look fine. Now question number 2: is this correct or do I
>overlook something important here?
>
>Thanks a lot,
>Nici
>
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Mauro PESENTI
Research Associate, National Fund for Scientific Research (Belgium)
Unite de Neurosciences Cognitives
Departement de Psychologie
Universite Catholique de Louvain
Place Cardinal Mercier, 10
B-1348 Louvain-la-Neuve
tel.: +32 (0)10 47 88 22
fax: +32 (0)10 47 37 74
E-mail: [log in to unmask]
http://www.nesc.ucl.ac.be
http://www.nesc.ucl.ac.be/mp/pesentiHomepage.htm
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