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:Previous investigators have changed ionic strength to screen charge to study the interaction of various toxin blockers with Ca(21)-activated K(1) (KCa) channels (MacKinnon and Miller, 1988; Anderson et al., 1988; Lucchesi and Moczydlowski, 1991; Giangiacomo et al., 1992) and the block induced by tetrodotoxin and saxitoxin in Na(1) channels (Moczydlowski et al.
Category: