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:Neuroimaging studies and event-related brain potential research have established that the ACC is central to performance monitoring (Carter et al., 1998; Gehring and Knight, 2000; Luu et al., 2000; Ullsperger and Von Cramon, 2003).
Category: