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:Accordingly, loss of striatal muscarinic M2/M4 / autoreceptors, a condition that causes an increase in endogenous acetylcholine (Zhang et al., 2002), impairs LTD but
not LTP (Bonsi et al., 2008).
Category: