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:In this study, we restrict perturbations of the kinematic environment to rotations of the visual feedback of the position of the end-effector, since this kind of transformation is well studied in both healthy human subjects and those with neurological disorders (Pine et al. 1996; Contreras-Vidal and Buch 2003).
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