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:This differed from the previous results from pharmacological studies (Visentin et al., 1999), which claimed that there were no homomeric P2X1 or P2X3 receptors on the primary cultured
rat microglial cells.
Category: