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:Evidence in the literature suggests that a non-CB1 receptor exists on excitatory
terminals in the hippocampus (Hajos and Freund, 2002; Rouach and Nicoll, 2003), but this remains controversial (Hoffman et al., 2005; Kawamura et al., 2006; Takahashi and Castillo, 2006).
Category: