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 occurrence of burst suppression in this study contrasts with EEG studies in humans, rats and horses in which no burst suppression was reported over the range 0.6–1.75 MAC halothane (Johnson et al. 1994; Thomsen & Prior 1996; Johnson & Taylor 1998; Antunes et al. 2003; Murrell et al. 2008).
Category: