Page 73 - Foundations of Cognitive Psychology : Core Readings

72 Jay L. McClelland, David E.Rumelhart, and Geoffrey E.Hinton

Figure 4.7
The unit for the letter T in the first position of a four-letter array and some of its neighbors.Note
that the feature and letter units stand only for the first position; in a complete picture of the units
needed from processing four-letter displays, there would be four full sets of feature detectors and
four full sets of letter detectors.(From ‘‘An Interactive Activation Model of Context Effects in Letter
Perception: Part 1. An Account of Basic Findings’’ by J.L.McClelland and D.E. Rumelhart, 1981,
Psychological Review, 88, p.380. Copyright 1981 by the American Psychological Association.
Reprinted by permission.)

attention on the fourth position where R and K are both equally consistent with
the active features.Here, the activations of the detectors for R and K start out
growing together, as the feature detectors below them become activated.As
these detectors become active, they and the active letter detectors for W, O,and
R in the other positions start to activate detectors for words which have these
letters in them and to inhibit detectors for words which do not have these let-
ters.A number of words are partially consistent with the active letters, and
receive some net excitation from the letter level, but only the word WORK
matches one of the active letters in all four positions.As a result, WORK be-
comes more active than any other word and inhibits the other words, thereby
successfully dominating the pattern of activation among the word units.As it
grows in strength, it sends feedback to the letter level, reinforcing the activa-
tions of the W, O, R,and K in the corresponding positions.In the fourth posi-
tion, this feedback gives K the upper hand over R, and eventually the stronger
activation of the K detector allows it to dominate the pattern of activation,
suppressing the R detector completely.
This example illustrates how PDP models can allow knowledge about what
letters go together to form words to work together with natural constraints on