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:Thus, a role for glutamate and excitotoxicity in the direct mechanisms of ALS is appearing tenuous (Martin, 2010a), and the concept has so far failed to bear effective therapeutic fruit after more than 20 years (Rothstein et al., 1992).
Category: