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:Auditory stimuli were generated on a PC outside the MR environment using the BrainStim software package (Brain Products, Munich) and conducted via a pair of plastic tubes into a set of headphones placed over the subjects’ ears [55, 56].
Category: