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:A suspension of cerebellar granule cells was prepared from 10-15 day old ICR mice (Taconic Biotechnology, Inc., Germantown, NY) using the procedures described in previous investigations from our laboratory (Oyama et al., 1996; Carpenter et al., 1997; Boldyrev et al., 2000).
Category: