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:
>
>
>
>
>
>
>
>
>
>
>
> Hi SPMers,
>
>
> We have a question about the HRF modelling in a block/epoch design in SPM8.
>
>
> 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.
>
> 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.
>
> Does anybody know whether this is true, and if so, how SPM8 ascertains what the timing of these events within the boxcar is?
>
> Cheers,
>
>
> Stella de Wit
>
>
> Stella de Wit
>
>
> Department of Psychiatry
>
>
> VU university Medical Center, Medical Faculty room C461
>
>
> van der Boechorststraat 7, 1007 MB, Amsterdam
>
>
> the Netherlands
>
>
> Email: [log in to unmask]
>
>
>
>
--
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
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
intended only for the use of the individual or entity named above. If the
reader of the e-mail is not the intended recipient or the employee or agent
responsible for delivering it to the intended recipient, you are hereby
notified that you are in possession of confidential and privileged
information. Any unauthorized use, disclosure, copying or the taking of any
action in reliance on the contents of this information is strictly
prohibited and may be unlawful. If you have received this e-mail
unintentionally, please immediately notify the sender via telephone at (773)
406-2464 or email.
|