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:…(Polo-Parada et al., 2004, 2005), also caused transmission failures with high-, but not low-frequency stimulation, further supporting the importance of the MLCK–myosin II pathway in maintaining synaptic transmission with high, but physiological (Gorassini et al., 2000), stimulus frequencies.
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