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:Previous studies have also shown that lower levels of BDNF were associated with lower levels of NPY in both rat brain and cultured neurons (Barnea and Roberts, 2001; Nawa et al., 1993, 1994).
Category: