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:The striatum consists of two parallel motor circuits that have antagonistic effects on motor behavior, direct pathway neurons activated through D1 receptors that facilitate motor behaviors and indirect pathway neurons inhibited by D2 receptor activity that prevent motor cortex activity (Albin et al., 1989; Gradinaru et al., 2009).
Category: