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:…relative abundance in
different structures of the brain and spinal cord (Benke et al. 1999; Billinton et al. 1999;
Bischoff et al. 1999; Fritschy et al. 1999; Kaupmann et al. 1998b; Liang et al. 2000;
Malitschek et al. 1998; Poorkhalkali et al. 2000; Princivalle et al. 2000; Towers et al. 2000).
Category: