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:However, surprisingly few human studies have examined the effects of neurotoxic stimulants on the adolescent brain, particularly on the ACC (Lyoo et al. 2015; Sung et al. 2013a), although the use of psychostimulants including MA is often initiated during adolescence (King et al. 2010).
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