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:…spectral-notch attributable to the pinna and the pinna-dependent ILDs was most prominent for sound frequencies between 12 and 24 kHz in the rat (Koka et al., 2008); hence, tone frequency response sensitivities were probed between 2 and 32 kHz as described in detail previously (Polley et al.,…
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