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:One series of sections was Nissl stained with cresyl violet, whereas a second series of sections was processed to visualize the marker SMI-32, as previously described (Wannier et al. 2005; Beaud et al. 2008).
Category: