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:First, there is no further increase in the percentage of time the membrane potential spends close to threshold, as the average membrane potential 〈V〉 becomes almost independent of 〈I 〉 (see Figure 3B; see also Holt & Koch, 1997).
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