Dear spm,
I have a question concerning the specification of contrasts in an
event-related analysis.
The baseline of my experiment consists of a blocked design, alternating
between the conditions A(active) and B(rest).
During condition A, several short changes in the (visual) stimulus
occur.
I have implemted these in SPM97 as single events (hrf-function) and an
impulsetrain (condition A).
I'm interested in the single events per se, i.e. those regions that are
extra activated or deactivated compared to the
activationlevel during condition A.
I see three possible solutions:
1. I look at the SPMF for one event-type, having implemented all
other events and the block as CNI
2. I define the block as CNI and all event types as COI; I define the
contrasts(for three event types) [1 0 0; 0 1 0; 0 0 1]
to look at each event type individually.
3. all event types and the blocked covariate are implemented as COI
(e1 e2 e3 block).
Then I define the contrasts [ 1 0 0 -1;
-1 0 0 1;
0 1 0 -1;
0 -1 0 1;
0 0 1 -1;
0 0 -1 1]
to look at the activations or deactivations compared to the
blocked respons.
My questions:
1) What could be the benefit of using a contrast [1 0 0] instead of the
SPMF?
2) As I see it, the implementation of the blocked covariate as COI or
CNI wouldn't influence the beta's, but would
change the z-maps, because of its influence on the effective degrees of
freedom. So, if I'm only interested in the the
differential respons in between event types, would it be better to
implement the blocked covariate as CNI? In general,
what's the influence of an extra COI on the SPMZ's?
3) If one is interested in the extra activation caused by a single event
during an active condition A, should one look at the
F-map (with the block as CNI) or is the contrast between the event type
and the block a more appropiate, valid way?
4) Is there another solution to this problem?
Thanks in advance,
Erik
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Erik Beatse
Department of Radiology, KUL MR Research Centre
University Hospital Gasthuisberg, K.U.Leuven
Herestraat 49, B-3000 Leuven (Belgium)
Phone: +32 16 347753, Fax : +32 16 343765
mailto:[log in to unmask]
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