Most animals show some degree of non-associative learning. This means they change their response to a stimuli without association with a positive or negative reinforcement. Animals frequently subjected to a stimulus will often become habituated to that stimulus--they will show a reduction or total elimination of response to a stimulus without positive or negative reinforcement. If you poke them, sea slugs (Aplysia) will curl inwards. However, if you poke them repeatedly, the response will become less and less extreme until they do not withdraw at all. When presented with a novel stimulus, such as an electric shock, the sea slugs will recover their withdrawal response to poking. This phenomenon in which the habituation disappears is, conveniently, known as dishabituation. Furthermore, the sea slugs can be sensitized, whereby they will show an increased response to poking after first being presented with a strong or novel stimulus. The difference between dishabituation and sensitization is that dishabituation involves the recovery of the original response while sensitization produces a response stronger than the original one.
In 1902, the Russian physiologist, Ivan Pavlov, began his famous experiments on conditioning. Pavlov repeatedly presented a dog with food following the ringing of a bell. When the bell sounded without the presentation of food, the dog would still respond to the bell as if it were food. Pavlov collected the dogs' saliva and found that the amount of saliva produced by bell ringing increased as the dogs were more frequently exposed to the coupling of food presentation and bell ringing. The dog had learned to associate the sound of the bell with food. Pavlov called the food an unconditional stimulus, or UCS, because the dog's normal reaction would be to salivate at the presentation of food. The bell he termed the conditional stimulus, or CS, because response to the bell was conditional upon the association between the bell and food. For the same reasons, salivation in response to food was labeled the unconditional response, or UCR, while salivation in response to the bell was called the conditional response, or CR. Conditioning the dog to salivate at the sound of the bell occurred as a result of a contingency between the UCS and the CS. Pavlov's experiment was an example of positive conditioning. It is also possible to negatively condition an animal by using an unpleasant UCS.
In classical conditioning, the animal receives no benefit from associating the CS with the UCS. However, in operant conditioning, an unassociated behavior becomes associated with a reward. B.F. Skinner designed an apparatus called a "Skinner box" to test the interaction between UCS and CS. A rat was placed inside the Skinner box; if the rat pressed down a lever inside the box then the box would release a food pellet. Soon, the rat pressed the lever far more often than he would just by chance. Most likely, the first time the rat pressed the lever it was by chance. But with each instance of lever pressing, the operant is reinforced by reward with food. The rat learns that pressing the lever is associated with food, and so he will increasingly press it. Almost any operant and reward system can be used effectively.