Page 72 - Foundations of Cognitive Psychology : Core Readings

The Appeal of Parallel Distributed Processing 71

have been observed in visual perception of words since the work of Pillsbury
(1897).
Two of us have proposed a model describing the role of familiarity in per-
ception based on excitatory and inhibitory interactions among units standing
for various hypotheses about the input at different levels of abstraction (Mc-
Clelland & Rumelhart, 1981; Rumelhart & McClelland, 1982).The model has
been applied in detail to the role of familiarity in the perception of letters in
visually presented words, and has proved to provide a very close account of
the results of a large number of experiments.
The model assumes that there are units that act as detectors for the visual
features which distinguish letters, with one set of units assigned to detect the
features in each of the different letter-positions in the word.For four-letter
words, then, there are four such sets of detectors.There are also four sets of
detectors for the letters themselves and a set of detectors for the words.
In the model, each unit has an activation value, corresponding roughly to the
strength of the hypothesis that what that unit stands for is present in the per-
ceptual input.The model honors the following important relations which hold
between these ‘‘hypotheses’’ or activations: First, to the extent that two hypoth-
eses are mutually consistent, they should support each other.Thus, units that
are mutually consistent, in the way that the letter T in the first position is con-
sistent with the word TAKE,tendtoexciteeachother.Second,tothe extent that
two hypotheses are mutually inconsistent, they should weaken each other.
Actually, we can distinguish two kinds of inconsistency: The first kind might be
called between-level inconsistency.For example, the hypothesis that a word
begins with a T is inconsistent with the hypothesis that the word is MOVE.The
second might be called mutual exclusion.For example, the hypothesis that a
word begins with T excludes the hypothesis that it begins with R since a word
can only begin with one letter.Both kinds of inconsistencies operate in the
word perception model to reduce the activations of units.Thus, the letter units
in each position compete with all other letter units in the same position, and the
word units compete with each other.This type of inhibitory interaction is often
called competitive inhibition.In addition, there are inhibitory interactions be-
tween incompatible units on different levels.This type of inhibitory interaction
is simply called between-level inhibition.
The set of excitatory and inhibitory interactions between units can be dia-
grammed by drawing excitatory and inhibitory links between them.The whole
picture is too complex to draw, so we illustrate only with a fragment: Some of
the interactions between some of the units in this model are illustrated in figure
4.7.
Let us consider what happens in a system like this when a familiar stimulus
is presented under degraded conditions.For example, consider the display
shown in figure 4.8. This display consists of the letters W, O,and R,completely
visible, and enough of a fourth letter to rule out all letters other than R and
K.Before onset of the display, the activations of the units are set at or below
0.When the display is presented, detectors for the features present in each po-
sition become active (i.e., their activations grow above 0). At this point, they
begin to excite and inhibit the corresponding detectors for letters.In the first
three positions, W, O,and R are unambiguously activated, so we will focus our