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:Six cortical fields each contained a separate representation of sound fre-
quency (2–32 kHz), and the range of BFs mapped in all regions covered the range of pinna-related frequency and ILD cues in the rat (Koka et al., 2008; Tollin and Koka, 2009).
Category: