Jason -
Not much, but it depends on your TR.
A length of 1 means one scan, which is TR seconds.
In order to create the covariates, SPM splits each scan
into fMRI_T timebins (default = 16). That is, each timebin
is TR/16 seconds (if you do not change the default).
An epoch of length one is equivalent to setting the values
of those 16 timebins to 1.
An event however is modelled by setting only one of
those bins to value 1.
Once convolved with the HRF, the epoch response
(still in this high resolution time space) will look basically
like a taller version of the event response. In other words,
for durations up to ~4s, increasing the duration of the
assumed underlying neural activity changes mainly the height
of the HRF. For epochs longer than 4s, the epoch response
also starts to flatten out (i.e., its shape changes appreciably
as well). This is why I said it depends on your TR (ie the
length of your epoch in seconds).
Once these responses are downsampled every TR to
give your covariates (see earlier email list discussion
of the reference bin - fMRI_T0), there may be
less difference. Moreover, the significance of the
parameter estimates for the resulting covariates may
change little (again in the domain of typical TRs < 4s).
This is because the absolute scaling of the covariates
has little effect on the significance of the parameter
estimates. (The only situation where the scaling might
matter would be if the different epochs within an
epoch-type (covariate) were of different durations.)
Rik
Jason Steffener wrote:
> Hello,
> What would be the difference between the following when
> specifying a design matrix:
>
> 1)a box-car epoch of length one, convolved with the hrf
> 2)an event modelled with only the hrf
>
> Thank you,
> Jason.
>
> Jason R. Steffener, RTSV
> Department of Psychiatry, University Hospital, UMDNJ
> Office: (973) 972-1604
> http://www.umdnj.edu/~steffejr/jason.html
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