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:…cocaine seeking progressively increases over the first months of withdrawal from extended-access cocaine self-administration and remains elevated even 6 months after the last exposure (Neisewander et al., 2000; Grimm et al., 2001; Lu et al., 2004b; c; Sorge and Stewart, 2005; Pickens et al., 2011).
Category: