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2.4 The Usefulness of Co-activation Graphs
With brain imaging data in this format, it becomes possible to formulate some very
simple questions and use some well-understood methods to answer them. For in-
stance, a long-standing project in our lab has been adjudicating between functional
topographies of the brain based on the principle of localization and those based
on the principle of redeployment. Localization-based approaches to functional to-
pography, insofar as they typically expect brain regions to be dedicated to a small
and domain-restricted set of cognitive functions, would be committed to the notion
that differences in cognitive domains would be reflected primarily in differences in
which brain regions support tasks in the domain. In contrast, redeployment-based
approaches, being based on the idea that most brain regions are used in many dif-
ferent tasks across cognitive domains, would expect very little difference in which
brain regions were used in each domain. However, because redeployment neverthe-
less expects brain regions to have fixed low-level functions [3-5], it is committed
to the notion that differences in functions and domains must instead be the result of
differences in the ways in which the areas cooperate in supporting different tasks. To
put this in more concrete visual terms, imagine a simplified brain with six regions
that together support two different cognitive domains. If one supports a localization-
based (or a classical modular) organization for the brain, one would expect the re-
gional cooperation patterns to look like those in the diagram on the left. In contrast,
redeployment predicts an organization that looks something more like that shown in
the diagram on the right (Fig. 2.2).
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3
1
5
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2
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2
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Localization-based organization
Redeployment-based organization
Fig. 2.2: Two different possibilities for the functional organization or the cortex.
Figure shows an imagined brain with six regions supporting two cognitive domains.
Localization predicts that domain 1 (
blue
) and domain 2 (
black
) will utilize different
brain areas, while redeployment predicts that the domains will utilize many of the
same brain areas, cooperating in different patterns.
There is an obvious analog for these features in our co-activation graphs: compar-
ing the graphs from different domains, node overlaps indicate Brodmann areas that
support tasks in both domains, whereas edge overlaps would indicate a similar pat-
tern of cooperation between Brodmann areas. Thus, localization predicts little node
