Dear Dominic,
> If, however, you actually want to be able to distinguish between
> brain areas which respond to a continuous mental 'set' during block
> A, and other brain areas which respond to the specific events, then
> this is more difficult. You would need the events to be quite
> sparse, so that the sum of a1 and a2 DOESN'T look much like a box-car
> for block A. Then you could orthogonalize the 'event' regressors
> with respect to the box-car, and you might still have some
> statistical power (I think that there is a Chawla & Friston paper in
> which they did something a bit like this).
I agree that distinguishing set from event-related activity when the two are
potentially confounded is very hard. The paper Richard is referring to is:
Chawla D, Rees G & Friston KJ. The physiological basis of attentional
modulation in extrastriate visual areas Nature Neuroscience 2, 671-676
(1999)
You can download a copy from my home page. What Dave tried to do in this
paper was disentangle attentional 'set' during a block from the attentional
modulation of evoked event-related responses to stimuli within that block.
Minimizing the collinearity of the set and event-related responses was
complicated, but essentially involved inserting relatively long ISIs (up to
~36s with a distribution of ISIs between ~1-36s!) into the event train.
There may be good psychological reasons in your own paradigm why this would
be unacceptable, but as Richard says you need to minimise the collinearity
between a1, a2 and the boxcar to allow efficient (i.e. correct) estimation
of parameters.
Best wishes,
Geraint
http://www.klab.caltech.edu/~geraint
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