LLN, le 9/10/07
Dear Maite,
>better to have a more equilibrated design between different
>conditions or there shouldn't be problems with this design.
It is usually better to have exactly the same number of volumes for
all the to-be-compared conditions, although there isn't anything
wrong _per se_ to have an unequal number of volumes. The disadvantage
with unequal block durations is one of statistical efficiency. It's
the same issue as with a classic 2-sample t-test: if one sample is
much larger than the other, the efficiency is lower than if the two
samples are of the same size (all things being equal, viz, the total
sample size (sum of the two samples) being the same). (I quote a
message posted on 4 July 05 by Stephen J. Fromm, from the list for
this argument).
>Another question I have is what is presented when I write the contrast [1 0]?
[1 0] tests for positive differences between condition A and
"Anything that is not declared explicitely in your vectors". Note
that comparing a condition with the average of "Anything that is not
declared" is statistically valid, but it will of course make sense
only if the design matrix is appropriate, i.e., if what is left
implicit can reasonnably be used as a reference. For example, if you
have a resting/fixation condition and you are not interrested in
this condition per se, you can leave it implicit and a contrast such
as [1 0] will mean "Activation minus Rest/fixation". (And it will be
totally similar to declaring Rest/fixation explicitely and use a
contrast such as [1 -1].) If you don't have anything left implicit or
if what is left is not a good reference condition, then you shouldn't
use a [1 0] contrast.
>I wanted to see what brain areas are activated only with the task condition.
It will always be RELATIVE TO something.
Hope this helps,
Mauro.
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Mauro PESENTI
Research Associate, National Fund for Scientific Research (Belgium)
Unite de Neurosciences Cognitives
Departement de Psychologie
Universite Catholique de Louvain
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