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:…et al., 2005) and the proposed impairment in the processing of visual oscillatory (flicker-) stimuli in HE patients (Kircheis et al., 2002; Timmermann et
al., 2008), the aim of the present study was to follow-up on these findings and characterize alterations in the behavioral CFF,…
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