Dear Susana,
On Thu, 24 Aug 2006, Susana Novais Santos wrote:
> Thanks for your reply. What if we look at the hemodynamic response function?
> Won't a plot of each region's HRF provide us with this information? If there
> is a temporal shift between the plots, can't we say that one region peaks
> before the other?
Yes and no. First of all, you have to obtain an good estimate of the HRF -
which is what all event-related designs are about. As a second step, you
have to think about whether different onsets/peaks/widths of HRFs in
different regions are really caused by time shifts or whether they are
just due to different vascular effects (oxygenated blood reaching one
region faster than another etc.). If you have your good estimate and are
sure that the time differences are task-related, then you can go ahead
with your interpretation.
Volkmar
> On Aug 18, 2006, at 7:30 AM, Volkmar Glauche wrote:
>
> > Dear Susana,
> >
> > you will have a chance to obtain sub-TR resolution if you repeat your
> > trials using similar stimuli. For each repetition, you would start
> > stimulus presentation at a different time within your TR. Doing this
> > often and randomised is what is commonly called event-related fMRI.
> >
> > Volkmar
> >
> > On Fri, 18 Aug 2006, Susana Novais Santos wrote:
> >
> > > Dear Volkmar,
> > >
> > > Thanks so much for your quick reply. I am disappointed to learn there
> > > is no
> > > way to obtain the activation curve for a voxel for a given TR.
> > >
> > > Our problem is as follows: In our study, we observe several regions
> > > activating
> > > during a certain TR (3 seconds). We are trying to understand the
> > > sequence in
> > > which those regions activate (which region peaks first and so on), in
> > > order to
> > > establish a functional connectivity model with some temporal
> > > information. Do
> > > you have any other suggestions?
> > >
> > > Thanks very much,
> > > Susana
> > >
> > >
> > > On Aug 17, 2006, at 3:01 AM, Volkmar Glauche wrote:
> > >
> > > > Dear Susana,
> > > >
> > > > if I understand you correctly, there is no way to get a "temporal
> > > > series
> > > > of activation values" with a time resolution higher than TR.
> > > > Also, the
> > > > single value you got is not the "peak activation for that time
> > > > period",
> > > > either.
> > > > In a fMRI experiment one usually observe each voxel in a 3D data
> > > > set
> > > > exactly once per TR. Thus, you sample BOLD response in each voxel
> > > > at a
> > > > sampling frequency of 1/TR. One may get a higher time resolution
> > > > per
> > > > event type if you either shorten your TR (acquire less slices,
> > > > smaller
> > > > FOV...) or in an event-related design where one usually acquires
> > > > many
> > > > instances of BOLD responses for the same stimulus and the
> > > > stimulus onset
> > > > SO is somehow randomised with respect to TR. By re-ordering your
> > > > timepoints in terms of SO, you get a peri-stimulus time histogram
> > > > with a
> > > > higher time resolution than TR.
> > > >
> > > > Volkmar
> > > >
> > > > On Thu, 17 Aug 2006, Susana Santos wrote:
> > > >
> > > > > Dear All,
> > > > >
> > > > > I am trying to obtain the activation at a given voxel during
> > > > > a whole
> > > > > TR (3 seconds). I know how to
> > > > > extract the single value that represents the peak activation
> > > > > for that
> > > > > time period, but how can I obtain
> > > > > the temporal series of activation values for the whole
> > > > > duration of
> > > > > the TR?
> > > > >
> > > > > Thanks very much in advance for your help,
> > > > > Susana
> > > > >
> > > > >
> > > >
> > > > --
> > > > Volkmar Glauche
> > > > -
> > > > Department of Neurology
> > > > [log in to unmask]
> > > > Universitaetsklinikum Freiburg Phone 49(0)761-270-5331
> > > > Breisacher Str. 64 Fax 49(0)761-270-5416
> > > > 79106 Freiburg
> > >
> > >
> > >
> >
> > --
> > Volkmar Glauche
> > -
> > Department of Neurology [log in to unmask]
> > Universitaetsklinikum Freiburg Phone 49(0)761-270-5331
> > Breisacher Str. 64 Fax 49(0)761-270-5416
> > 79106 Freiburg
>
>
--
Volkmar Glauche
-
Department of Neurology [log in to unmask]
Universitaetsklinikum Freiburg Phone 49(0)761-270-5331
Breisacher Str. 64 Fax 49(0)761-270-5416
79106 Freiburg
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