Matthew and others, hi. I feel like I should try to argue a few
related points, although perhaps this thread has lost its relevance,
especially since I've already conceded I'm happy to live with the
outline map. That said...
Matthew Brett wrote:
> Stimulating though this argument has been, I have to disagree strongly
> with this:
>
> > I was agreeing that thresholded maps are nearly useless for
> > localization, but I don't feel on the whole they're any better or
> > worse than unthresholded maps.
>
> I disagree for all the reasons stated elsewhere in the thread, most
> recently be Federico. It comes down very simply to the point that Tom
> Johnstone made well:
Although I didn't reply to Federico, I felt his second point
("non-thresholded maps are the only available mean of attempting
dissociation inference") was potentially very harmful. You can't make
inferences about dissociations from single unthresholded maps either.
In my view, a better restatement of this second point would be: direct
statistical comparison is the only available means of testing a
dissociation between regions; single maps are useful only in
generating hypotheses. If we're talking about multiple maps (i.e.,
across studies), then clearly unthresholded maps contain more
information, although the statement is still false. Unthresholded
maps contain more information about localization than thresholded
maps, it's not all or none. At the same time, they facilitate a few
orders of magnitude more inter-region comparisons.
> > Without at least a power analysis ... one is really in no position to
> > conclude anything at all about brain regions that show no significant
> > activation.
>
> I don't think there's any way round this one for the single subject
> analysis; the thresholded map is useless for drawing conclusions about
> differences between brain areas.
As I'm sure you know by now, I agree with this. What I don't
understand is why you think unthresholded maps are any different in
this respect. For a specific inter-region comparison, single maps
contains one observation per region, so it's not generally possible to
test the reliability of any observed difference. The sentence of
Tom's that you quoted is equally applicable to unthresholded maps.
Just for reference, the message you're quoting was in the context of a
rebuttal of something I posted. I believe Tom interpreted something I
wrote to suggest that I thought a null hypothesis of no difference
could be affirmed in sub-threshold regions. In case this clears
anything up, of course I don't believe so. If it helps, I would be
happy to go on record as stating that anyone who holds such a belief
is completely unqualified to do research in this area. I'm not just
saying this to word my point strongly, I really don't want to see
reporting requirements shaped by the need to cater to people who don't
have basic familiarity with the field. (Besides which, obviously if
you have someone who's inclined to misinterpret data, throwing more
data at them is not generally helpful.)
> The argument for comparison across studies is simply an extension of
> the single subject case; if you have almost no evidence for
> localisation in one study, you merely have very little evidence for
> localization across 10 studies. If you have the continuous map for
> 10 studies, and they all show zero or negative activation for area A
> and strong activation for area B, you have a much stronger case for
> "A is more involved than B" than you could possibly have from 10
> thresholded maps, which have simply thrown away this information.
Of course there are some purposes for which unthresholded maps would
be better, maybe even most purposes. Unthresholded maps obviously
contain more information than thresholded maps. The reason I didn't
originally think unthresholded maps should be administratively
required in favor of thresholded maps in all cases is not because I
think they are always worse. It's because I think they have both
advantages and disadvantages, and that their inclusion should be a
matter of judgment and not one of policy. The most serious
disadvantage in my view is that they encourage reading deep meaning
into noise, especially in smoothed data. A second disadvantage is
that in the cases we're talking about (in which there's no direct
inter-region comparison reported), it's extraneous to the purpose of
the report. In my view, a figure should be included in an article if
it supports some argument that the authors want to make (or that the
reviewers think they should have wanted to make).
My defense of the thresholded map's role in localization, remember,
was just a response to your asking if thresholded maps contain
localizing information. I thought you were arguing that they contain
no such information, so I thought it would be helpful to provide a
thought experiment that proves otherwise. This is of course not the
same thing as claiming that localizing inferences can be drawn from
single maps. In any case, my argument about why I thought thresholded
maps might be more useful was mostly academic. In practical terms,
when it comes to localization, I think we're talking about two levels
of useless. To the extent the argument for unthresholded maps depends
on their utility for localization, I think it would be a harmful
practice.
dan
