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:Both peptides are required for the normal display of behavioral rhythms in constant conditions—producing short period rhythms or arrhythmicity—and both affect the rhythmicity of cellular pacemaking (VIP: Harmar et al., 2002; Aton et al., 2005; PDF: Peng et al., 2003; Klarsfeld et al., 2004; Lin et al., 2004).
Category: