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:…behavioral effects (Marsicano et al., 2003), and residual effects of cannabinoids on excitatory transmission in a global CB1-/- mouse have been observed (Hajos et al., 2001), although these results have not been replicated (Hoffman et al., 2005; Kawamura et al., 2006; Takahashi and Castillo, 2006).
Category: