Ivan Pavlov, answered from the texts and cited to the page.
When a conditioned reflex has been extinguished — that is, when the conditioned stimulus has been applied repeatedly without the unconditioned stimulus until the response falls to nothing — it does not stay at nothing. Leave the animal in peace for a sufficient interval, and the reflex returns of its own accord, without any fresh pairing with the unconditioned stimulus.1
This is what I call spontaneous recovery. The fact of spontaneous recovery carries a precise theoretical weight: it proves that extinction is not fatigue, not the wearing out of some nervous mechanism, but inhibition — a positive, active suppression of the conditioned response.2 Were the reflex simply exhausted, rest alone could not restore it; yet rest does restore it.
The inhibitory process, therefore, is temporary, and when it lifts, the underlying conditioned connection re-emerges. The recovery is slow in developing but stable when it comes about this way.3 It can be hastened — an extraneous reflex introduced at the moment of zero response will restore the conditioned effect quickly, a phenomenon I called dis-inhibition — but that accelerated recovery is only temporary, persisting no longer than the extraneous stimulus and its after-effect, after which inhibition reasserts itself and the reflex must wait for its spontaneous return.4
The two modes of recovery, spontaneous and dis-inhibitory, were for a time described as fundamentally different in kind; later experiments by Dr. Podkopaev showed that earlier description to be inaccurate, and we revised it.5 Science, even in a well-run laboratory, is a matter of correction as much as discovery.
If a well elaborated and stable conditioned stimulus be repeatedly applied at short intervals, alone, without being followed by the unconditioned stimulus, then it begins to decrease and gradually becomes inactive. This is not a complete destruction of the conditioned reflex, but only a temporary suspension. That this is so is proved by the spontaneous restoration of the reflex after an interval.Lectures on Conditioned Reflexes
That this extinction of the conditioned reflex is not a matter of fatigue is proved by its restoration without the assistance of the unconditioned stimulus.Lectures on Conditioned Reflexes
In the first, spontaneous recovery occurred after a longer or shorter interval of rest; such recovery was slow in developing, but stable.Conditioned Reflexes
The whole group of cases of which the above is an illustration have one common feature. In all of them the removal of the inhibition is only temporary, persisting no longer than the extra stimulus responsible for the removal of inhibition and its after-effect.Conditioned Reflexes
Recently during experiments upon some new aspects of the relation between the conditioned and the unconditioned stimulus these old observations were repeated, and found to be inaccurate [experiments of Dr. Podkopaev].Conditioned Reflexes