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 insertion of this time delay is critical because in previous work, it has been shown to allow the development of an inhibitory relationship between the outcome and the stimulus that follows; that is, it allows S1 to predict the absence of subsequent O1 delivery and S2 to predict the absence of subsequent O2 delivery; that is, stimuli trained in such manner exhibit properties (such as retardation) identical to those display by other conditioned inhibitors (Rescorla, 1969; Maier et al., 1976; Delamater et al., 2003).
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