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:This idea is supported by reports of a1D expression in cone terminals ~Wilkinson & Barnes, 1996; Taylor & Morgans, 1998; Morgans, 1999, Morgans et al., 2001, 2005! and is consistent with the greater ERG b-wave deficit in mice lacking the VDCC b2 subunit ~Ball et al., 2002!, the only VDCC b subunit…
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