LLN, le 28/02/05
Dear All,
My 2 cents to this very interesting discussion. I really have mixed
feelings about continuous effect size maps.
On the one hand, Matthew's suggestion to make more use of such maps
somehow relates to my own suggestion of making it more explicit when
different thresholds are use in the analyses and the figures (where,
usually, less severe thresholds are used to make small clusters a bit
more visible and good-looking...). I also have much sympathy with the
idea that we should know more about sub-threshold
differences/similarities, especially when claims are made over
regions (e.g., hemispheric asymetries, etc.).
On the other hand, this sounds like paying very little attention to
statistics, doesn't it? Transposing this approach to behavioral data,
it sounds to me like simply looking at histograms and saying "Well,
guys, this bar seems a little bit different/similar from that one, so
let's say that there is/isn't a difference". Concerning Matthew's
point below...
>how many papers make this clear? -"area A was significantly
>activated, but of course that isn't to say the whole brain wasn't
>activated about the same amount, who knows?"
Right. We should certainly all keep in mind that brain imaging
techniques do not tell us "what is necessary to realize a function"
and unfortunately not even "all that contribute to a function". But
I'm not sure that the primary goal of our statitical tests is to say
"area X is more activated that the rest of the brain". Most of the
time, we look at differences across conditions and what we (want to)
conclude is "area X is more activated in condition A than in
condition B". Setting a threshold is of course arbitrary, but, well,
that's the best statisticians have found so far...
Not sure this really 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
Place Cardinal Mercier, 10
B-1348 Louvain-la-Neuve
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