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HEARING IS CRUCIAL FOR LOCALIZING and identifying sound; for humans, it is particularly important because of its role in the understanding and production of speech. The auditory system has several noteworthy features. Its subcortical pathway is longer than that of other sensory systems. Unlike the visual system, sounds can enter the auditory system from all directions, day and night, when we are asleep as well as when we are awake. The auditory system processes not only sounds emanating from outside the body (environmental sounds, sounds generated by others) but also self-generated sounds (vocalizations and chewing sounds). The location of sound stimuli in space is not conveyed by the spatial arrangement of sensory afferent neurons but is instead computed by the auditory system from representations of the physical cues.
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Sounds Convey Multiple Types of Information to Hearing Animals
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Hearing helps to alert animals to the presence of unseen dangers or opportunities and, in many species, also serves as a means for communication. Information about where sounds arise and what they mean must be extracted from the representations of the physical characteristics of sound at each of the ears. To understand how animals process sound, it is useful first to consider which cues are available.
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Most vertebrates take advantage of having two ears for localizing sounds in the horizontal plane. Sound sources at different positions in that plane affect the two ears differentially: Sound arrives earlier and is more intense at the ear nearer the source (Figure 28–1A). Interaural time and intensity differences carry information about where sounds arise.
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