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:), it is a direct measure of sympathetic activity that is relatively free of any influence of somatic symptoms (respiratory cycles or muscle tone) [Geer, 1966; Corah and Pantera, 1968; West et al., 1983; Jiménez, 1995; Lundgren et al., 2001].
Category: