Dear Erik,
> 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
> impulse train (condition A). I'm interested in the single events per
> se, i.e. those regions that are extra activated or deactivated compared
> to the activation level 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.
In fact simply identifying event-related responses (having accounted
for set or block-related responses) would involve a contrast like [1 0
0 0]. Your contrasts are testing for event-related responses that are
greater that basline shifts.
> My questions:
>
> 1) What could be the benefit of using a contrast [1 0 0] instead of the
> SPMF?
The ensuing SPM{T} would distinguish between activations and deactivations
(in this instance the SPM{F} = SPM{T} squared).
> 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 responses 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?
If you use contrasts and SPM{T} the d.f. and parameter estimates are
all identical so you it does not matter.
> 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?
A simple contrast [1 0 0 0] is appropriate.
> 4) Is there another solution to this problem?
I am sure there are many solutions but the approach you outline (with
my qualifications seems very good. Be careful that the event-related
repsonses are not correlated too much with the block covariate or you
will find it difficult to detect either.
With best wishes - Karl
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