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:Synaptic heterogeneity is not the only evidence for plasticity of several synaptic parameters: (1) Hebbian pairing of pre and postsynaptic neurons was shown to result in a selective increase in low frequency synaptic transmission (Markram and Tsodyks, 1996) which is due to a specific change in U (Tsodyks and Markram, 1997); (2) while presynaptic regulation of neurotransmitter release is typically considered as a mechanism to block or enhance transmission between neurons, this action rather serves to change the frequency dependence of transmission; (3) paired pulse experiments that demonstrated changes in paired pulse ratios rather demonstrated changes in frequency dependence; (4) we have recently found that an initial stage of LTDa is preceded by a selective loss of facilitation (LTDf) (A.
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