Large-Scale Brain Area Functional Organization - Computational Explorations in Cognitive Neuroscience - page 218

still holds. We will explore this kind of attentional in-
teraction between spatial and object processing in chap-
ter 8.

call the exploration of presenting multiple digits to the
same network in chapter 3).
Relatedly, it is possible to compare a visible stimu-
lus with a stored representation of another stimulus by
just assessing how well the visible stimulus activity pat-
tern fits with the pattern of weight values that encode
the stored stimulus. The speed with which the network
settles, or the goodness (see chapter 3) of the resulting
activation state are measures that can be used to assess
the general fit of that stimulus with a stored one. As-
suming that there is one stored pattern that is obviously
closest to the visible one, then this measure will tell you
how close these two are.
Although it may initially seem unnecessarily redun-
dant, it is actually quite likely that the brain has mul-
tiple representations that could be used for comparison
purposes. For example, we think that the frontal cor-
tex has somewhat redundant representations of much
of the content of posterior cortex, because this infor-
mation must be represented in the specialized frontal
cortex for active maintenance purposes. Thus, frontal
cortex could hold the representation of one of the items,
and the other could be represented in the posterior sys-
tem (e.g., by viewing the item). The comparison would
then be based on overlap or some kind of goodness mea-
sure that indicates how consistent these two representa-
tions are. The idea that the frontal cortex is somehow
involved in comparisons is intriguingly consistent with
the existence of the same capacity limitations for both
relative judgments (comparisons) and active memory
(Miller, 1956). It is also possible that multiple, some-
what redundant representations within the posterior cor-
tex, each with a different emphasis but probably some
overlap, could be used in a similar fashion.
Thus, there are a number of possible solutions to this
problem, but we are not aware of implemented models
that demonstrate the actual viability of these ideas.

7.6.2 Representing Multiple Instances of the Same

Thing

An extreme case of the binding problem occurs with
multiple instances of the same item . Clearly, it is not
possible to to distinguish these instances on the basis of
object-centered features like color and shape. However,
there are at least two other ways that the actual num-
ber of items can be accurately represented. First, the
sequential application of an attentional mechanism di-
rected at each of the items in turn, when combined with
some kind of counting mechanism, could result in an
appropriate representation.
Alternatively, unless the items are presented in the
same location, spatial location representations can be
used to disambiguate the case where only one item is
present from the case with multiple instances of the
same item. Thus, it is important to take into account the
full range of possible representations, not just a single
canonical object-based representation, when consider-
ing whether there really is a problem.

7.6.3 Comparing Representations

Another problem (related to the representation of multi-
ple items) arises whenever you have to compare two dif-
ferent representations. Although one can use attention
to time-share the same representational space across
two different items, this does not work when you need
to actually compare the two items, because you must
have both of them represented at the same time. We
suggest two solutions that use a single representational
space without requiring that both representations be dis-
tinctly active in that space, and another that uses two
distinct sets of representations with a comparison be-
tween them.
One potential solution using a common representa-
tional space would be that the natural network dynamics
of inhibition and pattern overlap will result in a repre-
sentation of what the two items have in common, which
can be used as a basis for the comparison judgment (re-

7.6.4 Representing Hierarchical Relationships

Although we have emphasized the notion of a struc-
tural hierarchy of representations at increasing levels
of abstraction, there are many other forms of hierarchi-
cal structure in the world that need to be represented
in some systematic fashion. One problem is that most