If we did not know the name of the animal, the letter or sound “a” would not be sufficient to evoke the response “aardvark.” Similarly, if we had the response in our repertoire, but there was no aardvark present, we would have almost no tendency to say “aardvark” only as a result of hearing the “a” sound.
A type of controlling variable with the following features:
It sums with another controlling variable to evoke a response in a case of multiple controlling variables
The supplemental variable, by itself, is never likely to be sufficient to evoke the response
The type of controlling variable