The response may occur in the absence of the typical discriminative stimuli if the level of deprivation is sufficient, as in the case of a man dying from thirst in the desert.
An environmental change or event with the following features:
Precedes the response to which it is functionally related
Increases the effectiveness of a particular stimulus change as reinforcement
The type of environmental event
The cause of the environmental event
The type of stimulus that gains reinforcing effectiveness