Page 80 - Foundations of Cognitive Psychology : Core Readings

The Appeal of Parallel Distributed Processing 79

Since they all share the same ‘‘occupation,’’ they work together to fill in that
property for Lance.Of course, there is no reason why this should necessarily be
the right answer, but generally speaking, the more similar two things are in
respects that we know about, the more likely they are to be similar in respects
that we do not, and the model implements this heuristic.
Spontaneous Generalization The model we have been describing has another
valuable property as well—it tends to retrieve what is common to those mem-
ories which match a retrieval cue which is too general to capture any one
memory.Thus, for example, we could probe the system by activating the unit
corresponding to membership in the Jets.This unit will partially activate all the
instances of the Jets, thereby causing each to send activations to its properties.
In this way the model can retrieve the typical values that the members of the
Jets have on each dimension—even though there is no one Jet that has these
typical values.In the example, 9 of 15 Jets are single, 9 of 15 are in their 20s,
and 9 of 15 have only a Junior High School education; when we probe by acti-
vating the Jet unit, all three of these properties dominate.The Jets are evenly
divided between the three occupations, so each of these units becomes partially
activated.Each has a different name, so that each name unit is very weakly
activated, nearly cancelling each other out.
In the example just given of spontaneous generalization, it would not be un-
reasonable to suppose that someone might have explicitly stored a general-
ization about the members of a gang.The account just given would be an
alternative to ‘‘explicit storage’’ of the generalization.It has two advantages,
though, over such an account.First, it does not require any special generalization
formation mechanism.Second, it can provide us with generalizations on unan-
ticipatedlines,on demand.Thus,ifwewanttoknow, for example,whatpeople
in their 20s with a junior high school education are like, we can probe the model
by activating these two units.Since all such people are Jets and Burglars, these
two units are strongly activated by the model in this case; two of them are
divorced and two are married, so both of these units are partially activated. 1
The sort of model we are considering, then, is considerably more than a con-
tent addressable memory.In addition, it performs default assignment, and it
can spontaneously retrieve a general concept of the individuals that match any
specifiable probe.These properties must be explicitly implemented as compli-
cated computational extensions of other models of knowledge retrieval, but in
PDP models they are natural by-products of the retrieval process itself.

Representation and Learning in PDP Models
In the Jets and Sharks model, we can speak of the model’s active representation
at a particular time, and associate this with the pattern of activation over the
units in the system.We can also ask: What is the stored knowledge that gives
rise to that pattern of activation? In considering this question, we see immedi-
ately an important difference between PDP models and other models of cog-
nitive processes.In most models, knowledge is stored as a static copy of a
pattern.Retrieval amounts to finding the pattern in long-term memory and
copying it into a buffer or working memory.There is no real difference between
the stored representation in long-term memory and the active representation in