Useful Notes on the Laboratory Illustrations of Positive Reinforcer (Thorndike and Skinner)

A positive reinforcer is a stimulus or event which, when it is contingent on a response, makes, that response, more likely to occur.

There are two laboratory illustrations of positive reinforcer which we will discuss briefly:

(i) Thorndike’s Experiment’s: Cats in a Puzzle Box:

One of the first systematic study of positive reinforcement principle was an experi­ment carried out by Thorndike on a cat in a puzzle box. The experiment is briefly summarised as follows:

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“A hungry cat was locked in a “puzzle box” made of slots through which the cat could see a dish of food on the floor outside. A string from the door latch led over a pulley to a wire loop hanging in the box. If the cat clawed at the loop, the door would open and the cat could escape from the box and eat.

When first put in the box, the hungry cat actively scrambled around, scratching and clawing at the sides of the box. Eventually, in its random movement around the box, the cat happened to pull the loop, thus, opening the door and escaping.

Thorndike then put the cat back in the box for a second trial. Again the cat scrambled around until it accidentally pulled the loop, escaped and was then put back in the box for a third trial. Thorndike and the cat kept this up for many trials.”

Thorndike found that over a period of 24 tri­als, the time taken in seconds decreased consider­ably from 160 seconds in first trial to 30 seconds in second trials and gradually to 1 or 2 seconds by 24th trial. Mazes: Besides, the puzzle box, mazes have also been used by Thorndike in the study of instrumental conditioning with positive reinforce­ment.

(ii) Skinner’s Experiment’s: Rats and Pi­geons in an Operant Chamber:

In 1930s, B. F. Skinner began his pioneering study. He used an instrument that is called as operant chamber or Skinner box. An operant chamber is a simple box with a device at one end that can be worked by the animal in the box. For rats, cats, and monkeys, the device is a lever; for pigeons, the device is a small panel, called a “key”, which can be pecked.

The lever and key arc really switches that activate, when positive reinforcement is being used, a food- delivery or water-delivery mechanism. Thus, posi­tive reinforcement is contingent upon pressing a lever or pecking a key. Since these responses are positively reinforced, they increase in frequency.

Following is the example of how conditioning in operant chamber occurs. “The first step in the operant conditioning of a hungry rat is to get it to eat the food pellets when they are delivered by the experimenter, who operates the pellet-delivery mechanism from a push-button switch outside the operant chamber.

The pellets are delivered one by one; after a time, the rat eats each pellet as soon it drops. This first step is necessary if the food reinforcement is to be effective later, when the rat will deliver the food pellets to itself by pressing a lever.

Next, the experimenter stops releasing the pellets, and the rat is left alone in the box with the lever, which will release the pellets. After an initial period of inactivity, the rat, being hungry, begins to explore the box. Eventually, it presses the lever accidentally. A pellet of food is released; that is, reinforcement in contingent upon pressing the lever.

After eating the food pellet, the rat con­tinues exploring, stopping to groom itself from time to time. After a while, it presses the lever again, and again a pellet is released; then it presses the lever a third time. Usually after the fourth or fifth press, the rat begins to cress the lever more rapidly, and operant behaviour is in full swing.

The experimenter measures the number of times the lever has been pressed or the rate of response (i.e. number of times a lever is pressed in a particular unit of lime). This can be measured with a help of cumulative recorder. It is a graph which tells us about the rate of response.