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:…preferred TFs ranged from 0.25 to 6.5 Hz (mean = 1.1 Hz), which were consistent with previous electrophysiological reports (Niell and Stryker, 2008; Gao et al., 2010; LeDue et al., 2012, 2013), and within the ranges indicated by recent multi-photon imaging studies (Andermann et al., 2011; Marshel…
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