Page 76 - Foundations of Cognitive Psychology : Core Readings

The Appeal of Parallel Distributed Processing 75

no close partial matches which include the letter F in the second position, so
this letter receives no feedback support.As a result, E comes to dominate, and
eventually suppress, the F in the second position.
The fact that the word perception model exhibits perceptual facilitation to
pronounceable nonwords as well as words illustrates once again how behavior
in accordance with general principles or rules can emerge from the interactions
of simple processing elements.Of course, the behavior of the word perception
model does not implement exactly any of the systems of orthographic rules that
have been proposed by linguists (Chomsky & Halle, 1968; Venesky, 1970) or
psychologists (Spoehr & Smith, 1975).In this regard, it only approximates such
rule-based descriptions of perceptual processing.However, rule systems such
as Chomsky and Halle’s or Venesky’s appear to be only approximately hon-
ored in human performance as well (Smith & Baker, 1976).Indeed, some of the
discrepancies between human performance data and rule systems occur in ex-
actly the ways that we would predict from the word perception model (Rumel-
hart & McClelland, 1982).This illustrates the possibility that PDP models may
provide more accurate accounts of the details of human performance than
models based on a set of rules representing human competence—at least in
some domains.

Retrieving Information from Memory
Content Addressability One very prominent feature of human memory is that it
is content addressable.It seems fairly clear that we can access information in
memory based on nearly any attribute of the representation we are trying to
retrieve.
Of course, some cues are much better than others.An attribute which is
shared by a very large number of things we know about is not a very effective
retrieval cue, since it does not accurately pick out a particular memory repre-
sentation.But, several such cues, in conjunction, can do the job.Thus, if we ask
a friend who goes out with several women, ‘‘Who was that woman I saw you
with?’’hemay notknowwhich onewemean—butifwe specify somethingelse
about her—say the color of her hair, what she was wearing (in so far as he
remembers this at all), where we saw him with her—he will likely be able to hit
upon the right one.
It is, of course, possible to implement some kind of content addressability of
memory on a standard computer in a variety of different ways.One way is to
search sequentially, examining each memory in the system to find the memory
or the set of memories which has the particular content specified in the cue.An
alternative, somewhat more efficient, scheme involves some form of indexing—
keeping a list, for every content a memory might have, of which memories have
that content.
Such an indexing scheme can be made to work with error-free probes, but
it will break down if there is an error in the specification of the retrieval cue.
There are possible ways of recovering from such errors, but they lead to the kind
of combinatorial explosions which plague this kind of computer implementation.
But suppose that we imagine that each memory is represented by a unit
which has mutually excitatory interactions with units standing for each of its