Localization Of Sound

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Location of the source of sound plays a large role in central processing in the auditory system. Localization of sound along the vertical axis can be achieved using input from only one ear. The folds and bulges in the structure of the outer ear apparently produce reflections of sound waves as they enter the auditory canal. The lack of symmetry of the pinna allows for different echoing effects of sounds based on their angle of entry along a vertical plane. Sound entering the ear from below will have a relatively long time delay between the response to the direct sound and the reflected sound; sound from above will have a relatively short delay time. In this way, echo patterns can provide a code for the location of the sound in space. Similar mechanisms are used by bats (and

FIGURE 6 Central auditory pathways. (A) Lateral view of cerebral cortex shows the location of the coronal section below. (B) Coronal section shows the auditory projection from the medial geniculate nucleus to the cerebral cortex. (C) Dorsal view of brain stem and midbrain shows auditory pathways. The auditory-vestibular nerve is cranial nerve VIII, which enters the brain stem at the base of the pons. Auditory information is first processed in the ventral and dorsal cochlear nuclei, and from there the information ascends both ipsilaterally and contralaterally to the superior olive, inferior colliculus, and medial geniculate nucleus. Finally, auditory radiations carry the information to the superior lip of the temporal lobe, designated as Brodmann's areas 41, 42, and 22.

sonar devices on ships) when they emit sounds and analyze echoes in order to locate objects.

Localization of sound in the horizontal plane is more complex and relies on comparisons of subtle differences in input from the two ears. Auditory fibers, as they ascend the brain stem, split in a partial decussation, sending tonotopically matched information from both ears to both left and right superior olivary nuclei. As the first binaural neurons in the auditory pathway, cells in the olivary nuclei represent the first stage at which differences in input from the two ears can be compared. The ears are spaced roughly 20 cm apart. Given the speed of sound at 342 m/s, this creates a time delay (interaural onset delay) of 0.06 ms between the time a sound wave reaches one ear and the time it reaches the ear on the opposite side. This calculation pertains to sounds coming from the left or right side; sounds coming from other angles have correspondingly shorter time delays. Sounds coming from straight ahead have no interaural delay.

This system works at sound onset, but it does not work for a continuous tone. Another method, interaural phase delay, is required when the source of a continuous tone moves from one location to another. In this case, inter-aural delay can still be used by comparing differences in the time a given phase of the sound wave reaches each ear. For example, the peak in the sound wave coming from the right will reach the left ear 0.6 ms after it reaches the right ear; however, there is a problem at higher frequencies, particularly those with wave cycles that are shorter than the 20-cm distance between the ears. For example, one cycle of a 2000-Hz sound wave covers only 1.7 cm; thus, the peaks of these cycles are not separated enough in time to allow separate detection by the two ears.

An additional method is used for sound localization of high-frequency tones based on interaural intensity differences. The head tends to block the propagation of high-frequency sound waves, creating a sound shadow opposite the sound source. For example, sound coming from one side will be loudest in the ear on the same side but will be partially muffled in the ear on the opposite side because of the head's sound shadow. As with other interaural comparisons described earlier, the interaural intensity difference is greatest for sounds originating from the side, less for sounds at other angles, and absent for sounds from straight ahead or behind. Binaural neurons sensitive to differences in intensity use this information to locate the sound. Because low-frequency sound waves tend to diffract around the head, there is no distinct sound shadow for low frequencies. Localization of the full range of sound frequencies in a horizontal plane is achieved by combined calculations of interaural onset and phase delay for tones of 20 to 2000 Hz and of inter-aural intensity difference for tones of 2000 to 20,000 Hz.

Suggested Readings

Altschuler R, Hoffman D, Bobbin D, Clopton B. Neurobiology of hearing, Vol. 3, The central auditory system. New York: Raven Press, 1991.

Corey DP, Hudspeth AJ. Ionic basis of the receptor potential in a vertebrate hair cell. Nature 1979; 281:675-677.

Edelman GM, Gall WE, Cowan WM, Eds. Auditory function: neurobiological bases of hearing. New York: John Wiley & Sons, 1988.

Hudspeth A. Transduction and tuning by vertebrae hair cells. Trends Neurosci 1983; 6:366-389.

Khanna SM, Leonard DGB. Basilar membrane tuning in the cat cochlea. Science 1982; 215:305-306.

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