Hi,
Thanks for your reply.
I don't mean the shape of the HRF with its upshoot and undershoot, but
the shape of a boxcar in SPM 8.
My professor told me that instead of assuming 1 big event with an on and
offset (a proper boxcar), SPM8 assumes a block filled with a number of
events which are spaced in time in a certain way. I wondered whether
that was true and if so, how SPM determines the number and timing of
these events.
Cheers,
Stella
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
On Behalf Of MCLAREN, Donald
Sent: dinsdag 4 januari 2011 17:29
To: [log in to unmask]
Subject: Re: [SPM] HRF modelling in block-design in SPM8
It is a boxcar that is convolved with the canonical HRF. This leads to a
higher peak at the beginning and then comes back to a steady state.
This is due to the undershoot pulling the response down. Thus it's not a
purely horizontal line.
Hope this answers your question.
On Tuesday, January 4, 2011, Wit, Stella de <[log in to unmask]> wrote:
>
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> Hi SPMers,
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> We have a question about the HRF modelling in a block/epoch design in
SPM8.
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> We wondered whether anybody knows how SPM8 exactly models the block
design boxcar x HRF, for instance in a design in which 4 conditions are
presented in blocks of 20 trials, each block lasting ~56 seconds, each
block presented 3 times. So in this model 4 conditions are modelled with
3 boxcars lasting 56 seconds.
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> We heard that SPM8 does not just combines a boxcar x canonical HRF,
but treats this boxcar as if at certain timepoints events occur, so the
plateau phase of this 56 seconds boxcar is not exactly a straight
horizontal line.
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> Does anybody know whether this is true, and if so, how SPM8 ascertains
what the timing of these events within the boxcar is?
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> Cheers,
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> Stella de Wit
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> Stella de Wit
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> Department of Psychiatry
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> VU university Medical Center, Medical Faculty room C461
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> van der Boechorststraat 7, 1007 MB, Amsterdam
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> the Netherlands
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> Email: [log in to unmask]
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--
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA Research Fellow,
Department of Neurology, Massachusetts General Hospital and Harvard
Medical School
Office: (773) 406-2464
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