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:First, depression is observed at all synaptic connections between layer 5 pyramidal neurons even though U varies from 0.1 to 0.95 (Markram and Tsodyks, 1996, Tsodyks and Markram, 1997).
Category: