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| A Skinner box typically contains one or more levers which an animal
can press, one or more stimulus lights and one or more places in which
reinforcers like food can be delivered. The animal's presses on the levers
can be detected and recorded and a contingency between these presses, the
state of the stimulus lights and the delivery of reinforcement can be set
up, all automatically. It is also possible to deliver other reinforcers
such as water or to deliver punishers like electric shock through the
floor of the chamber. Other types of response can be measured -
nose-poking at a moving panel, or hopping on a treadle - both often used
when testing birds rather than rats. And of course all kinds of
discriminative stimuli may be used.
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| In principle, and sometimes in practice, it is
possible for a rat to learn to press a bar in a Skinner-box by trial and
error. If the box is programmed so that a single lever-press causes a
pellet to be dispensed, followed by a period for the rat to eat the pellet
when the discriminative-stimulus light is out and the lever inoperative,
then the rat may learn to press the lever if left to his own devices for
long enough. This can, however, often take a very long time. The methods
used in practice illustrate how much the rat has to learn to tackle this
simple instrumental learning situation. The first step is to expose the
rat to the food pellets he will later be rewarded with in the Skinner box
in his home cage when he is hungry. He has to learn that these pellets are
food and hence are reinforcing when he is hungry. Now he can be introduced
to the Skinner-box.
Initially there may be a few pellets in the hopper where reinforcers
are delivered, plus a few scattered nearby, to allow the rat to discover
that the hopper is a likely source of food. Once the rat is happy eating
from the hopper he can be left in Skinner box and the pellet dispenser
operated every now and then so the rat becomes accustomed to eating a
pellet from the hopper each time the dispenser operates (the rat is
probably learning to associate the sound of the dispenser operating with
food - a piece of classical
conditioning which is really incidental to the instrumental learning
task at hand). Once the animal has learned the food pellets are
reinforcing and where they are to be found, it would, however, still
probably take some time for the rat to learn that bar-pressing when the SD
light was on produced food. The problem is that the rat is extremely
unlikely to press the lever often by chance. In order to learn an operant
contingency by trial and error the operant must be some behavior which the
animal performs often anyway. Instead of allowing the rat to learn by
trial and error one can use a 'shaping' or 'successive-approximations'
procedure. Initially, instead of rewarding the rat for producing the exact
behavior we require - lever pressing - he is rewarded whenever he performs
a behavior which approximates to lever pressing. The closeness of the
approximation to the desired behavior required in order for the rat to get
a pellet is gradually increased so that eventually he is only reinforced
for pressing the lever. Starting by reinforcing the animal whenever he is
in the front half of the Skinner-box, he is later only reinforced if he is
also on the side of the box where the lever is. After this the
reinforcement occurs if his head is pointing towards the lever and then
later only when he approaches the lever, when he touches the lever with
the front half of his body, when he puts touches the lever with his paw
and so on until the rat is pressing the lever in order to obtain the
reinforcer. The rat may still not have completely learned the operant
contingency - specifically he may not yet have learned that the
contingency between the operant response and reinforcement is signalled by
the SD light. If we now leave him to work in the Skinner-box on his own he
will soon learn this and will only press the lever when the SD light is
on. |
