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 selectivity of M1 antagonists was confirmed by testing the efficacy of either pirenzepine or trihexyphenidyl to block the
M1-mediated membrane depolarization induced by muscarine (60 mM) or by the selective M1 agonist McN-A343 (3 mM) (Bonsi et al., 2008).
Category: