Hi all. I posted this message
last week, but I did not get a response. So I thought I would re-post it in
hopes of some feedback this time around.
Our lab is performing a box car
experiment involving a hand grip task. Basically, the subject squeezes a bladder
to 35% of their maximal contraction strength, and holds this for 30 seconds. The
subject performs this exercise 3 times each session with a minute of rest
between each squeeze.
When it comes time to design a
matrix for this study, I've been adding the force applied to the bladder (which
is recorded when the subject is performing the task in the
scanner) as a regressor. However, I'm wondering if I should be offsetting
the regressor by 5 sec so that the hrf function of the regressor will line up
with the BOLD response ellicited by the hand grip task (i.e. add
5 sec of data at the beginning and take away 5 sec of data at the end)? If
this is the case, would it be even better if I add 5 sec of data before each of
the three hand grips and take away 5 sec of data following each hand
grip?
The reason I'm asking this
is because when I stipulate what scans I want to look at in the design matrix
(eg. set up the design to look at the 10
scans starting
at scan #13) they don't line up with the regressor, and I'm guessing that's
because when you stipulate what scans you want to look at, SPM
automatically shifts that window period so that the hrf curve lines up with the
5 sec delay of the BOLD response, whereas, inputing a regressor does not take
that delay into consideration.
Anyways, any help would be much
appreciated.