Dear All,
I got a problem with the
relative intensities of some regressors
in my design matrix. I"m modelling an
event-related study with events of
different length in spm99. I'm using an
hrf+dt basis set for a discrete event,
and an hrf alone for an event of
variable length (specified by using the
"variable durations" option). The model
looks fine, apart for one feature. The
scale of the basis functions ranges from
-2 to 2.5 for the discrete event (as
seen in the "regressors" plot visualized
by "Explore design"), and from -20 to 80
for the event of variable length.
However, the range displayed in the
"basis set and peristimulus sampling"
plot is approximately the same for both
types of events (i.e., from -2 to 2.5
for the discrete event and from 0 to 2.5
for the event of variable length). The
same difference is present in the actual
design matrix, of course. This scaling
discrepancy is then reflected, I guess,
in the plotting of the estimated
responses. When I plot both event types
together, I got a flat line for the
event of variable length, and a
pronounced response for the discrete
event, even if the voxel is arising from
a t-contrast asking for differences
between the former event and the latter
one.
I was a bit surprised to find such a
difference in the average values of the
regressors of the design matrix, since
in the past these were detrended (at
least some partitions of it). However, I
couldn't find trace of any such
operation in the new code, but maybe I
missed it.
My questions are:
i) shouldn't different columns of the
design matrix have a comparable mean, so
that they are modelling only different
slopes and not different intercepts, too
?
ii) am I right in assuming that the
different relative intensities of the
responses, as assessed in the plot
section, are due to the different
relative intensities of the regressors,
in the design matrix ?
Thanks in advance,
--
Ivan Toni Ph.D.
Institut für Medizin
Forschungszentrum Jülich
D-52425 Jülich
Deutschland
tel. 49 2461 61 2483
fax. 49 2461 61 2820
email: [log in to unmask]
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