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:There are two mechanisms by which rTMS can decrease cortical and corticospinal excitability (Chen et al., 1997; Maeda et al., 2000; Muellbacher et al., 2000; Fierro et al., 2001; Touge et al., 2001; Fitzgerald et al., 2002; Romero et al., 2002; Tsuji and Rothwell, 2002) while increasing rCBF (synaptic activity).
Category: