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:…and central VTA) (Valenti et al., 2012; Lammel et al., 2012), whereas short-term stress or other different chronic stressors (e.g., repeated intermittent stress) (Shaham et al., 2000 & 2003) may increase activity of these cells (Moore et al., 2001, Valenti et al., 2011, Valenti et al., 2012).
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