We can often demonstrate this by presenting an additional controlling variable, itself not generally sufficient to evoke the response, which when combined with the effects of the other stimulus conditions is sufficient to evoke the response. The additional controlling variable may be considered supplementary. Features irrelevant to the classification of supplementary stimulation include the type of controlling variable.
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