Dear SPMers,
I have a question concerning regressors in an epoch design.
My experiment involves 3 conditions:Rest,Visual presentation and
a Motor task condition,repeated X9 per subject.I model them with
hrf-convolved box-car functions.Each condition lasts 13 volumes,
so I have 351 volumes for each subject.During the motor condition I
am continuously sampling a motor parameter,and therefore have a
different value of that parameter for each volume acquired.When I
want to look for parametric modulation of the BOLD signal related
to that variable,and go through the epoch design options in
SPM_99,I am only given the option to enter 9 values:I therefore
made a user defined regressor (let's call it V) containing all of my
values.Then I tried convolving it with the hemodynamic response
function using something like Vc=conv(V,spm_hrf(3))(my TR is 3
s).Obviously the convolution process produces a longer vector than
my initial regressor.The question is:how do I go about scaling that
vector to a length that is equal to the number of scans?Does
anyone have a different suggestion concerning the implementation
of this design?
Thanks !
Charis.
Charidimos Tzagarakis MD
Brain Sciences Center and University of Minnesota.
Address:
Brain Sciences Center (11B)
Minneapolis VA Medical Center
One Veterans Drive
55417 Minneapolis MN
USA
Tel:612-725-2000 ext 1756
email: [log in to unmask]
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