By default, optseq2 assumes that you want to perform an FIR analysis
where you get an average for each post-stimulus time point so that you
can create a waveform. This is in contrast to assuming the shape to the
hemodyn respnse where you only estimate a single value (the amplitude).
By default, the time between FIR waveform points is the TR, but you can
perform sub-TR estimation by setting the dPSD to less than the TR. The
problem is that every estimate you make reduces the efficienecy. So,
assuming a shape is more efficient than an FIR because it only has one
estimate. An FIR with dPDS=TR is more efficient than dPSD=TR/2 because
there are half as many estimates. Probably when you go to analyze the
data you will use an assumed shape, and then you will get the efficiency
back.
On 2/21/2020 9:43 AM, Gergely Darnai wrote:
External Email - Use Caution
Dear Developers,
I am planning to run PVT (psychomotor vigilance task) in fMRI using
event related design. This is an extremely simple reaction time task:
participant has to respond to the appeared geometric shape as quickly
as possible. The key feature of this paradigm is that we will have
fluctuating and quite rapid event presentation times (ranging between
300 and 500 msec). Another important information is that we will use
FSL FEAT for the evaluation. We decided to use opseq2 to optimize the
design with the following parameters:
optseq2 --ntp 150 --tr 2 --psdwin 0 20 --ev evt1 1 45 --nkeep 3 --o
exp --nsearch 1 --tnullmin 3 --tnullmax 11 --repvar 10
Although with this design I get quite satisfying efficiency and VRF
scores I do not understand that if I decrease dPSD why does it have
significant negative effect on efficiency and VRF. Could you explain
this? If I understand well, this is the only option to shift the onset
of the event from the scanning points, and I would assume that if
there is fluctuation in time between scanning points and stimuli
presentation, it would help to "catch" the hemodynamic response easier
(if the stimulus onset always goes together with the scans, we can
always catch the same timepoints of the HRF). Did I misunderstand
something? If I use FSL that is based on HRF estimation (and not on
FIR), do these parameters (dPSD) and scores (efficiency & VRF) have
meaning and function at all? My last question is related to event
duration. Although I have fluctuating and short events, as you can see
I chose 1 sec (because it has to be the integer multiple of the dPSD).
Is it problematic?
Thank you for your suggestions,
Gergely
-
Gergely Darnai PhD
Department of Behavioural Sciences
Medical School, University of Pécs
Phone: +36/72/536-256
Fax: +36/72/536-257
H-7624 Szigeti u. 12, Pécs, Hungary
___
Freesurfer mailing list
Freesurfer@nmr.mgh.harvard.edu
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
___
Freesurfer mailing list
Freesurfer@nmr.mgh.harvard.edu
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
