Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:There have been a number of in vivo mode ls with anesthetized animals that have examine d pyloric motor activity in response to intraarte rial and intrave nous stimuli, as well as stimulation of extrinsic and intrinsic nerves (4, 17, 28, 29) .
Category: