Mind, Body, World- Foundations of Cognitive Science, 2013a

data reveals internal structures that are local in nature has been named the locality
assumption (Farah, 1994).
However, Farah (1994) hypothesized that the locality assumption may be
un warranted for two reasons. First, its validity depends upon the additional
assumption that the brain is organized into a set of functionally distinct modules
(Fodor, 1983). Farah argued that the modularity of the brain is an unresolved empirical
issue. Second, Farah noted that it is possible for nonlocal or distributed architectures,
such as parallel distributed processing (PDP) networks, to produce single
or double dissociations when lesioned. As the interactive nature of PDP networks is
“directly incompatible with the locality assumption” (p. 46), the locality assumption
may not be an indispensable tool for cognitive neuroscientists.
Farah (1994) reviewed three areas in which neuropsychological dissociations
had been used previously to make inferences about the underlying local structure.
For each she provided an alternative architecture—a PDP network. Each of these
networks, when locally damaged, produced (local) behavioural deficits analogous to
the neuropsychological dissociations of interest. These results led Farah to conclude
that one cannot infer that a specific behavioural deficit is associated with the loss of
a local function, because the prevailing view is that PDP networks are, by definition,
distributed and therefore nonlocal in structure.
However, one study challenged Farah’s (1994) argument both logically and
empirically (Medler, Dawson, & Kingstone, 2005). Medler, Dawson, and Kingstone
(2005) noted that Farah’s whole argument was based on the assumption that connectionist
networks exhibit universally distributed internal structure. However, this
assumption needs to be empirically supported; Medler and colleagues argued that
this could only be done by interpreting the internal structure of a network and by
relating behavioural deficits to interpretations of ablated components. They noted
that it was perfectly possible for PDP networks to adopt internal representations
that were more local in nature, and that single and double dissociations in lesioned
networks may be the result of damaging local representations.
Medler, Dawson, and Kingstone (2005) supported their position by training a
network on a logic problem and interpreting the internal structure of the network,
acquiring evidence about how local or how nonlocal the function of each hidden
unit was. They then created different versions of the network by lesioning one of
its 16 hidden units, assessing behavioural deficits in each lesioned network. They
found that the more local a hidden unit was the more profound and specific was
the behavioural deficit that resulted when the unit was lesioned. “For a double dissociation
to occur within a computational model, the model must have some form
of functional localization” (p. 149).
We saw earlier that one of the key goals of connectionist cognitive science was
to develop models that were biologically plausible. Clearly one aspect of this is
342 Chapter 7