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:For the STN, we used the published data of Nakanishi et al. (1987), which have been previously used by Gillies and Willshaw (2004) to constrain a similar sigmoid function as per (6), which reproduces the firing properties of the STN neurons.
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