Dear spm folk,
I need your help with setting up a design matrix for a sparse fMRI study
designed to look at responses in auditory cortex.
Subjects heard 10 secs of auditory stimuli with the EPI off. Then the EPI was
started and collected 3 seconds of info. A 10 second gap followed and the next
stimulus was given; i.e: Acquisition time (TA) was 3secs with an interscan
interval (TR) of 20 secs. There were four conditions with 10 scans each = 40
scans in all. I'm assuming that this is like an event-related fMRI study.
The problems is that I cannot seem to get spm99 to model for the part of the
BOLD signal I'm looking for. Because the stimuli started 10sec before the EPI,
there is no initial dip and then rise in my signal (i.e. no 'boxcar'), simply
the (flattish) peak of the BOLD signal.
At present I am specifying the model in the following way:
Interscan interval > 20
etc...
Stochastic design > no
SOA > variable
vector of onset > scan number (for each condition (1-40))
variable durations > no
parametric modulation > none
are these trials > events
select basis set > hrf (I suspect that this may be where I've gone wrong)
interactions among trials > 0
user specified regressors > 0
At no point have I specified the TA (other than in slice timing).
The matrix looks like it is expecting a boxcar signal.
Thanks in advance for your help.
Alex.
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