distributed representations

[David Plaut]

>But where's the evidence that brain damage degrades cognition gracefully? That
>is, the person just gets a little bit worse at a lot of things? Very commonly,
>exactly the opposite happens - the person remains normal at almost all kinds
>of cognitive processing, but some specific cognitive task suffers catastroph-
>ically. No graceful degradation here.

I think the issue here is a matter of scale.  "Graceful degredation" refers to
the gradual loss of function with increasing severity of damage - it says
nothing about how specific or general that function is.  Connectionist models
can be modular at a global scale, but use distributed representations and show
graceful degredation *within* modules.  I think you would agree that, within a
particular domain, this is a reasonable characterization of the behavior of
many types of patient (to the degree that we understand the modular
organization of certain aspects of cognition and the nature of individual
patients' damage).  Of course, severe damage to a module might still produce
catestrophic loss of its function, perhaps leaving the remaining functions
relatively intact.

On the other hand, the *degree* of specificity of impairment certainly places
constraints on the modular organization and the nature of the representations
within each module (although I think connectionist modeling illustrates the
danger of the "specific impairment implies separate module" logic).  Only
specific modeling work can demonstrate whether connectionist architectures and
representations can account for the behavior of specific patients in an
informative way.

-Dave
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David Plaut				dcp+ at cs.cmu.edu
School of Computer Science		412/268-8102
Carnegie Mellon University
Pittsburgh, PA  15213-3890