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:…the AN model, each of which has been tested against physiological responses from cats to simple and complex stimuli, including tones, two-tone complexes, broadband noise, and vowels (Carney 1993; Heinz et al. 2001; Zhang et al. 2001; Bruce et al. 2003; Tan and Carney 2003; Zilany and Bruce 2006).
Category: