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:The sensation evoked by CO2 was stinging or burning pain, as has been reported in other studies.(17,18,19,20) The putative neuronal substrate for this is polymodal nociceptors or chemonociceptors.
Category: