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:Previous publications from this laboratory have shown that when a brief DC electric field is applied during a trough in the NAD(P)H oscillation, the NAD(P)H oscillations, as well as Ca2+ oscillations and other parameters, grow in amplitude (Kindzelskii and Petty 2000; Petty 2000).
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