A dog is conditioned to salivate at a tone, then the association is extinguished, then it reappears. Order the events in the life of that one association.

1. The tone means nothing and produces no salivation.
2. The tone is repeatedly followed by food, and salivation to the tone appears.
3. The tone is now presented alone, and salivation to it fades away.
4. A day passes with no tone and no food at all.
5. The tone is sounded again and salivation returns, weaker than before.

The last two steps are the informative ones. Nothing whatever happened during the delay - no tone, no food, no training of any kind - and yet the response came back. That is spontaneous recovery, and it is the clearest evidence available that extinction did not erase anything.

What extinction laid down was a second, competing association, and the second one fades faster than the first. After a rest the older learning is once again the stronger of the two, and the response returns.

The practical consequence runs right through this lesson. A fear that has been extinguished in a clinic has not been deleted; it has been overlaid, and the overlay is the more fragile of the two. That is why relapse after successful exposure therapy is expected rather than a sign that the treatment failed.