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:In mouse models of amyloid-b accumulation, reduced cerebral blood flow occurs well before any pathological or neuro-
logical abnormalities (Niwa et al., 2002a, b; Iadecola,
2004).
Category: