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:A large body of evidence indicates that cholinergic and glutamatergic functions regulate cortical development and may contribute to postnatal critical periods (Bear and Singer, 1986; Hohmann et al., 1991; Robertson et al., 1998; for review, see Hohmann and Berger-Sweeney, 1998; Fox et al., 1999; Yuste and Sur, 1999).
Category: