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 is at peak levels between WD30 and WD90 and is
accompanied by increased BDNF levels, incubation of sucrose seeking in rats is much less persistent, returning to basal levels by WD90, and is not associated with increased BDNF levels in the NAc (Grimm et al., 2003; Lu et al., 2004c).
Category: