Page 68 - Foundations of Cognitive Psychology : Core Readings

The Appeal of Parallel Distributed Processing 67

otherhand, tendstocause themiddlefinger on thelefthandtomoveupand to
cause the whole hand to move up also.The r unit also causes the left index
fingertomove upand thelefthandtomoveupwithit.
The extent of the influences of each letter on the hand and finger it directs
depends on the extent of the activation of the letter.Therefore, at first, in typing
the word very,the v exerts the greatest control.Because the e and r are simul-
taneously pulling the hand up, though, the v is typed primarily by moving the
index finger, and there is little movement on the whole hand.
Once a finger is within a certain striking distance of the key to be typed, the
actual pressing movement is triggered, and the keypress occurs.The keypress
itself causes a strong inhibitory signal to be sent to the unit for the letter just
typed, thereby removing this unit from the picture and allowing the unit for the
next letter in the word to become the most strongly activated.
This mechanism provides a simple way for all of the letters to jointly deter-
mine the successive configurations the hand will enter into in the process of
typing a word.This model has shown considerable success predicting the time
between successive keystrokes as a function of the different keys involved.
Given a little noise in the activation process, it can also account for some of the
different kinds of errors that have been observed in transcription typing.
The typing model represents an illustration of the fact that serial behavior—
a succession of key strokes—is not necessarily the result of an inherently serial
processing mechanism.In this model, the sequential structure of typing emerges
from the interaction of the excitatory and inhibitory influences among the pro-
cessing units.
Reaching for an Object without Falling Over Similar mechanisms can be used to
model the process of reaching for an object without losing one’s balance while
standing, as Hinton (1984) has shown.He considered a simple version of this
task using a two-dimensional ‘‘person’’ with a foot, a lower leg, an upper leg, a
trunk, an upper arm, and a lower arm.Each of these limbs is joined to the next
at a joint which has a single degree of rotational freedom.The task posed to this
person is to reach a target placed somewhere in front of it, without taking any
steps and without falling down.This is a simplified version of the situation in
which a real person has to reach out in front for an object placed somewhere in
the plane that vertically bisects the body.The task is not as simple as it looks,
sinceifwejustswing an armout in front ofourselves,itmay shift our center of
gravity so far forward that we will lose our balance.The problem, then, is to
find a set of joint angles that simultaneously solves the two constraints on the
task.First, the tip of the forearm must touch the object. Second, to keep from
falling down, the person must keep its center of gravity over the foot.
To do this, Hinton assigned a single processor to each joint.On each compu-
tational cycle, each processor received information about how far the tip of the
hand was from the target and where the center of gravity was with respect to
the foot.Using these two pieces of information, each joint adjusted its angle so
as to approach the goals of maintaining balance and bringing the tip closer to
the target.After a number of iterations, the stick-person settled on postures that
satisfied the goal of reaching the target and the goal of maintaining the center
of gravity over the ‘‘feet.’’