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:108, 11262–11267. doi: 10.1073/pnas.1011284108 Calvert, G. A., and Thesen, T. (2004). Multisensory integration: methodological approaches and emerging principles in the human brain. J. Physiol. Paris 98, 191–205. doi: 10.1016/j.jphysparis.2004.03.018 Calvo, J. M., Datta, S., Quattrochi, J., and Hobson, J. A. (1992). Cholinergic microstimulation of the peribrachial nucleus in the cat. II. Delayed and prolonged increases in REM sleep. Arch. Ital. Biol. 130, 285–301. Cartwright, R. D. (2010). The Twenty-four Hour Mind: The Role of Sleep and Dreaming in Our Emotional Lives. Oxford: Oxford University Press, 37–38. Castro, S., Falconi, A., Chase, M. H., and Torterolo, P. (2013). Coherent neocortical 40-Hz oscillations are not present during REM sleep. Eur. J. Neurosci. 37, 1330– 1339. doi: 10.1111/ejn.12143 Chalmers, D. J. (1996). The Conscious Mind: In Search of a Fundamental Theory. New York: Oxford University Press. Chawla, D., Lumer, E. D., and Friston, K. J. (1999). The relationship between synchronization among neuronal populations and their mean activity levels.
Category: