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:Reduced CREB activity has been shown to reduce excitability of nucleus accumbens neurons (Dong et al., 2006), whereas loss of c-Fos leads to increased neuronal excitability (Zhang et al., 2002).
Category: