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:Envelope frequencies of AM signals have been found to be represented topographically (orthogonal to the frequency map) in ®eld L of the mynah bird, a member of the starling family (Hose et al., 1987; Scheich, 1990).
Category: