Stimulus discrimination is a term used in both classical and operant conditioning. It refers to the ability to distinguish between one stimulus and another, and to respond differently to each one. In psychology, a discriminative stimulus is a stimulus that generates a particular response when it is present, and the response is usually faster, more frequent, and more resistant to extinction. The responding behavior is then subjected to discriminative stimulus control. A discriminative stimulus (Sd or SD) is created when the response is reinforced in its presence, but not when it is absent. Discriminative stimuli have control over a particular behavior because the behavior is reliably reinforced through positive or negative reinforcement and punishment when the stimuli are present and not when they are absent.

Stimulus discrimination is an effective treatment for difficulty in PTSD patients. Clients are guided to deliberately attend to differences between then (danger at the time of the trauma) and now (safety in the present) . Stimulus discrimination can be framed to clients as a ‘brain training’ exercise with the rationale that their mind is attending to stimuli associated with their trauma and incorrectly interpreting them as signs of danger in the here-and-now. Clients should be guided to record where they were when their memories of the trauma were triggered and what was happening.

Here are some examples of discriminative stimulus:

• When a child asks for candy, she always gets one during grandma’s visit, but not in her absence. Grandma’s visiting is an Sd that controls the child’s asking behavior.
• When the traffic light turns green, drivers keep their car going forward, but not when the light turns red. The green light is then an Sd for going while the red light is for stopping.
• You give a dog a treat when it barks at a certain person, but not when it barks at another person. Then that first person becomes an Sd that controls the dog’s barking behavior.