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:…cell firing rates generated by stimulation of a patch of granule cells were sufficient to smoothly alter the firing rates of the two populations of Purkinje cells to levels somewhat comparable to those observed during behavioral tasks (Frysinger et al. 1984; Lisberger and Fuchs 1978; Thach 1970).
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