where and SD is a discriminative stimulus, R is a response, and SR is a reinforcer.
In operant conditioning, an animal must first make a response; that response is usually preceded by a discriminative stimulus, and sometimes followed by a reinforcer. Notice, that is not the case in classical conditioning.
Operant conditioning occurs when the association of response and reinforcer causes the animal to make the response again later in a similar situation.The discriminative stimulus signals the animal that a response at a given time is likely to be reinforced. The response MUST be made, for without it, the reinforcement will not occur. Finally, the reinforcer has the property of making the response that precedes it more likely to recur.
Think of a traffic light. It's a complex discriminative stimulus. Green means go, Red means stop. Here's another example:
Imagine that you are in a future classroom and a green light appears on your console. Were you to raise your hand and ask a question, you would get a $20 bill. If you never raise your hand then no money. But, if you do raise your hand and get the money you'll probably raise your hand again.
But, when you raise your hand when the green light is not on, nothing happens, no $20 bill. Soon, the green light comes on, you raise your hand and the money is dispensed. Now, the green light has become a discriminative stimulus. If it is on AND you raise your hand you'll get the money. If it is off and you raise your hand nothing happens. What do you think you'll do when that green light appears?
The apparatus used to study operant conditioning removes many distractors. An animal in an operant chamber will eventually direct its attention to the manipulandum (e.g., the lever or key) that activates the reinforcer.
So, operant conditioning can be defined as a procedure in which a response followed by a stimulus recurs. It recurs because of the stimulus. That stimulus is called a reinforcer. Reinforcers are stimuli that make the response that preceded them more likely to recur. Typical reinforcers include the giving of food or drink, and the removal of shock or pain. I will discuss the properties of reinforcers more below.
Discriminative stimuli set the occasion for a response and its associated reinforcer to occur. For example, a green traffic light is the discriminative stimulus for the response of pressing on the accelerator of your car. The facial expression of your boss may set the occasion for you NOT to ask your boss for a raise (because of a likely negative answer to your question).