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 sequence of intermediates in the hR photocycle and their interconversions have been the subject of numerous studies (Va´ró et al., 1995a, b; Hazemoto et al., 1984; Zimányi and Lanyi, 1989a, b; Zima´nyi et al., 1989; Spencer and Dewey, 1990; Ames et al., 1992; Dioumaev and Braiman, 1997a).
Category: