Where does extinction collide?

In learning theory, extinction is the gradual weakening and disappearance of a conditioned response once the reinforcement, or the unconditioned stimulus, that used to follow it is withheld: a dog that stops salivating to a bell no longer paired with food has undergone extinction. Bouton's 2004 review Context and behavioral processes in extinction argued that this does not erase the original learning but adds new, context-dependent learning that competes with it. The clearest evidence is spontaneous recovery: after a rest the extinguished response returns, showing the first association was never destroyed. The referent here is a change in one individual's behavior across training trials, tracked as a falling response rate, and it has nothing to do with a species dying out, light fading in glass, or a touch that goes unreported.[1]

Unit

A conditioned response that fades once reinforcement is withheld, read as a falling response rate

Recovery means

Spontaneous recovery: after a pause the response reappears, which is why extinction is treated as new learning laid over the old rather than as erasure

Measured by

Recorded across trials as the decline in how often, how fast, or how strongly the response occurs after the reinforcer is removed

In evolutionary biology, extinction is the permanent loss of a species or lineage when its last members die and leave no descendants. A slow background rate runs through all of geological time, punctuated by rare mass extinctions in which a large share of species disappear over a short interval. Barnosky and colleagues' 2011 paper Has the Earth's sixth mass extinction already arrived? compared recent losses against that fossil baseline and asked whether current rates approach the five great mass extinctions. Pimm and colleagues' 2014 study The biodiversity of species and their rates of extinction, distribution, and protection put modern extinction rates at roughly a thousand times the background level. The referent is the irreversible end of a lineage, counted as extinctions per species per year, and it shares only its spelling with an unlearned response, dimming light, or a clinical sign.[2][3]

Unit

A lineage that ends when its last members die, counted as extinctions per species per year

Recovery means

None for the species itself: unlike an extinguished response a lost lineage does not come back, though surviving lineages may later diversify to fill the gap

Measured by

Estimated from the fossil record and from documented losses, and expressed as an extinction rate set against a background baseline

In optics, extinction is the total attenuation of a beam of light as it travels through a medium, combining what the medium absorbs with what it scatters out of the path. The two losses together set how much light gets through, and their sum is carried by the extinction coefficient in the Beer-Lambert law, which holds that transmitted intensity falls off exponentially with path length and concentration. Swinehart's 1962 note The Beer-Lambert Law laid out that relationship and what the coefficient means for chemists reading absorbance. The referent is a physical dimming of radiation in a material, measured as a dimensionless optical depth or as an absorbance, and it is unrelated to a fading response, a dying species, or a neurological sign.[4]

Unit

The fraction of light lost to absorption plus scattering, fixed by the extinction coefficient over a path length

Recovery means

Not applicable

Measured by

Measured with a spectrophotometer as absorbance or optical depth, from the ratio of transmitted to incident intensity

In clinical neurology, extinction is a sign in which a person can detect a single touch, sight, or sound on the affected side, yet fails to report that same stimulus when both sides are stimulated at once. It is linked to hemispatial neglect, most often after damage to the right parietal region, and it points to a limit of attention rather than a loss of basic sensation. Driver and Vuilleumier's 2001 review Perceptual awareness and its loss in unilateral neglect and extinction treated the sign as a window onto how attention gates what reaches awareness. The referent is an observed pattern of missed reports under simultaneous double stimulation, noted during a bedside examination, and it bears no relation to an unlearned response, a vanished species, or attenuated light. This entry describes the sign and is not medical advice.[5]

Unit

A stimulus reported alone but missed when a competing one is present on the other side, noted on examination

Recovery means

It can lessen as the underlying neglect resolves, but that describes a clinical course and is not guidance for any individual

Measured by

Assessed clinically by comparing single-side stimulation against simultaneous double stimulation on each sense