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 classical finding (Nachmias and Sansbury, 1974; Legge and Foley, 1980; Wilson, 1980) means that our ability to discriminate contrast is best around a non-zero contrast value, which is typically close to the detection threshold.
Category: