Motivating operation: I see the brake lights of the car ahead of me come on.
Discriminative stimulus: Brake pedal in my car.
Behaviour: I lift my foot off of the gas pedal and step onto the brake pedal.
Consequence: I slow down and stop in time enough to avoid hitting the car ahead of me (negative reinforcement)I have not done a four-term contingency in a while but this one came to me while sitting in traffic yesterday. It can be difficult to separate the motivating operation (MO) from the discriminative stimulus (SD), but I think this example highlights quite nicely the 'availability of reinforcement' factor that defines the SD versus the 'need for reinforcement' factor which defines MO. While driving, we have available to us the break pedal for slowing down or stopping our vehicle. But is is not used all the time. We have learned that pressing the brake pedal is a reliable means of avoiding something or someone, keeping you and others safe. There are any number of situations that suddenly require us to behave in a way that will avoid hitting objects or people. These additional environmental changes such as the car's brake lights or a child running out into the road are what establishes the need for reinforcement - in this case avoidance of hitting the car ahead or another person. In summary (or in other words), we always have available to us the potential to brake; however, only do so when there is a need for it.