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:Hajos et al. (2001) found that WIN55,212-2 depressed excitatory transmission in the hippocampus of both wildtype and CB1-/- mice, while two other groups have found that excitatory transmission was depressed only in wildtype mice (Kawamura et al., 2006; Takahashi and Castillo, 2006).
Category: