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 study the role of top-down modulation on speech in noise perception, de Boer and Thornton (2008) examined MOCB activity in young participants who underwent a training program to discriminate speech presented in noisy environments (de Boer & Thornton, 2008).
Category: