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:IP, receptors are not detected in the adjacent olfactory bulb, contrasting with the high density of PKC in the external plexiform layer of the olfactory bulb reported previously (Worley et al., 1986a).
Category: