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:Inhibition of PDE4 has been found to increase the survival of cultured rat dopaminergic neurons in vitro (Yamashita et al. 1997) and dopamine synthesis in the striatum in vivo (Nishi et al. 2008).
Category: