Page 67 - Foundations of Cognitive Psychology : Core Readings

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

Figure 4.4
The interaction of activations in typing the word very.The very unit is activated from outside the
model.It in turn activates the units for each of the component letters. Each letter unit specifies the
target finger positions, specified in a keyboard coordinate system.L and R stand for the left and
righthands,and Iand Mfor theindex andmiddlefingers.Theletterunits receiveinformation
about the current finger position from the response system.Each letter unit inhibits the activation of
all letter units that follow it in the word: inhibitory connections are indicated by the lines with solid
dots at their terminations.(From ‘‘Simulating a Skilled Typist: A Study of Skilled Motor Perfor-
mance’’ by D.E. Rumelhart and D.A. Norman, 1982, Cognitive Science, 6, p.12.Copyright 1982 by
Ablex Publishing.Reprinted with permission.)

From the point of view of optimizing the efficiency of the typing motion,
these different patterns seem very sensible.In the first case, the hand as a whole
is maintained in a good compromise position to allow the typist to strike both
letters reasonably efficiently by extending the fingers up or down.In the second
case, the need to extend the fingers is reduced by moving the whole hand up,
putting it in a near-optimal position to strike either key.
Rumelhart and Norman (1982) have simulated these effects using PDP mech-
anisms.Figure 4.4 illustrates aspects of the model as they are illustrated in
typing the word very.In brief, Rumelhart and Norman assumed that the deci-
sion to type a word caused activation of a unit for that word.That unit, in turn,
activated units corresponding to each of the letters in the word.The unit for the
first letter to be typed was made to inhibit the units for the second and follow-
ing letters, the unit for the second to inhibit the third and following letters, and
so on.As a result of the interplay of activation and inhibition among these
units, theunitfor thefirstletterwas at first themoststronglyactive, andthe
units for the other letters were partially activated.
Each letter unit exerts influences on the hand and finger involved in typing
the letter.The v unit, for example, tends to cause the index finger to move
down and to cause the whole hand to move down with it.The e unit, on the