Extinction (psychology)

The dominant account of extinction involves associative models. However, there is debate over whether extinction involves simply "unlearning" the unconditional stimulus (US) – Conditional stimulus (CS) association (e.g., the Rescorla–Wagner account) or, alternatively, a "new learning" of an inhibitory association that masks the original excitatory association (e.g., Konorski, Pearce and Hall account). A third account concerns non-associative mechanisms such as habituation, modulation and response fatigue. Myers and Davis laboratory work with fear extinction in rodents has suggested that multiple mechanisms may be at work depending on the timing and circumstances in which the extinction occurs.[3]

Given the competing views and difficult observations for the various accounts researchers have turned to investigations at the cellular level (most often in rodents) to tease apart the specific brain mechanisms of extinction, in particular the role of the brain structures (amygdala, hippocampus, the prefrontal cortex), and specific neurotransmitter systems (e.g., GABA, NMDA).[3] A recent study in rodents by Amano, Unal and Paré published in Nature Neuroscience found that extinction of a conditioned fear response is correlated with synaptic inhibition in the fear output neurons of the central amygdala that project to the periaqueductal gray that controls freezing behavior. They infer that inhibition derives from the ventromedial prefrontal cortex and suggest promising targets at the cellular level for new treatments of anxiety.[4]

Learning extinction can also occur in a classical conditioning paradigm. In this model, a neutral cue or context can come to elicit a conditioned response when it is paired with an unconditioned stimulus. An unconditioned stimulus is one that naturally and automatically triggers a certain behavioral response. A certain stimulus or environment can become a conditioned cue or a conditioned context, respectively, when paired with an unconditioned stimulus. An example of this process is a fear conditioning paradigm using a mouse. In this instance, a tone paired with a mild footshock can become a conditioned cue, eliciting a fear response when presented alone in the future. In the same way, the context in which a footshock is received such as a chamber with certain dimensions and a certain odor can elicit the same fear response when the mouse is placed back in that chamber in the absence of the footshock.

In this paradigm, extinction occurs when the animal is re-exposed to the conditioned cue or conditioned context in the absence of the unconditioned stimulus. As the animal learns that the cue or context no longer predicts the coming of the unconditioned stimulus, conditioned responding gradually decreases, or extinguishes.

In the operant conditioning paradigm, extinction refers to the process of no longer providing the reinforcement that has been maintaining a behavior. Operant extinction differs from forgetting in that the latter refers to a decrease in the strength of a behavior over time when it has not been emitted.[5] For example, a child who climbs under his desk, a response which has been reinforced by attention, is subsequently ignored until the attention-seeking behavior no longer occurs. In his autobiography, B.F. Skinner noted how he accidentally discovered the extinction of an operant response due to the malfunction of his laboratory equipment:

My first extinction curve showed up by accident. A rat was pressing the lever in an experiment on satiation when the pellet dispenser jammed. I was not there at the time, and when I returned I found a beautiful curve. The rat had gone on pressing although no pellets were received. ... The change was more orderly than the extinction of a salivary reflex in Pavlov's setting, and I was terribly excited. It was a Friday afternoon and there was no one in the laboratory who I could tell. All that weekend I crossed streets with particular care and avoided all unnecessary risks to protect my discovery from loss through my accidental death.[6]

When the extinction of a response has occurred, the discriminative stimulus is then known as an extinction stimulus (SΔ or S-delta). When an S-delta is present, the reinforcing consequence which characteristically follows a behavior does not occur. This is the opposite of a discriminative stimulus which is a signal that reinforcement will occur. For instance, in an operant chamber, if food pellets are only delivered when a response is emitted in the presence of a green light, the green light is a discriminative stimulus. If when a red light is present food will not be delivered, then the red light is an extinction stimulus (food here is used as an example of a reinforcer). However, some make the distinction between extinction stimuli and "S-Delta" due to the behavior not having a reinforcement history, i.e. in an array of three items (phone, pen, paper) "Which one is the phone" the "pen" and "paper" will not produce a response in the teacher but is not technically extinction on the first trial due to selecting "pen" or "paper" missing a reinforcement history. This still would be considered as S-Delta.

There is a strong body of evidence to suggest that extinction alters across development.[20][21] That is, learning extinction may differ during infancy, childhood, adolescence and adulthood. During infancy and childhood, learning extinction is especially persistent, which some have interpreted as erasure of the original CS-US association,[22][23][24] but this remains contentious. In contrast, during adolescence and adulthood extinction is less persistent, which is interpreted as new learning of a CS-no US association that exists in tandem and opposition to the original CS-US memory.[25][26]

• Operant conditioning
• Posttraumatic stress disorder