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:To control for spatial attention modulation that could be synchronous with stimulus presentation, we required the subjects to perform a visual task superimposed on the fixation square during the entire duration of the scan (Beauchamp et al., 1997; Kastner et al., 1998; Somers et al., 1999; Tootell et al., 1998).
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