A person may say that they saw an animal at the zoo today but cannot remember the name of it. You might ask, “Did its name begin with an ‘a’?” That is a probe because the person providing the supplement is in no way being affected by the animal that was seen by the speaker. The provider cannot identify the upcoming response.
A discriminative stimulus with the following features:
It is a supplementary stimulus
The person providing the supplementary stimulus can identify the response that the speaker is likely to emit
It is a supplementary stimulus
The person providing the supplementary stimulus cannot identify the response that the speaker is likely to emit
Whether the controlling relationship is formal or thematic
Who provides the supplementary stimulus (either another person or the speaker him or herself)