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:…of BDNF in the medial prefrontal cortex (mPFC) suppresses cocaine seeking (Berglind et al., 2007, 2009;Sadri-Vakili et al., 2010; Whitfield et al., 2011), whereas elevation of BDNF in the VTA (Lu et al., 2004a) and NAc shell (Graham et al., 2007; Graham et al., 2009) facilitates cocaine seeking.
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