Although the function of the auditory system is to analyze and recognize natural sounds, it is often probed with artificial sounds which differ greatly from the sounds to which animals are exposed in their natural habitats (Merzenich et al., 1975; Middlebrooks et al., 1980; Phillips and Irvine, 1981; Phillips, 1988; Schreiner and Mendelson, 1990; Phillips and Kelly, 1992; Schreiner and Sutter, 1992). It has been argued in the past that in order to understand the function of the auditory system, it is necessary to probe the system with meaningful sounds (Suga, 1992). This approach led for example to the detailed understanding of the bat echolocation system (O’Neill and Suga 1979; Suga et al., 1979; Schuller and Radtke, 1990; Dear et al., 1993). However, when using this approach to study the auditory system of nonspecialized mammals such as ferrets, cats and monkeys, it is not clear what characterizes the class of meaningful sounds. There has been a heavy emphasis on the use of single animal vocalizations, both species-specific and not (Wollberg and Newman, 1972; Newman and Wollberg, 1973; Sovijarvi, 1975; Glass and Wollberg, 1983; Yeshurun et al., 1985; Steinschneider et al., 1990; Pelleg-Toiba and Wollberg, 1991). These are obviously important, but they are not the only sounds which are analyzed by the auditory system. Clean single vocalizations may in fact be relatively rare in the natural environment. They almost invariably appear against a background composed of a mixture of other sounds, which may be vocalizations of the same species, of different species, or sounds produced by non-animal sources. Unpublished observations from our lab suggest that the response to a vocalization is often modified by the acoustic background.
Specializations of the auditory system for the analysis of natural sounds
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