Tyler Wynn
The ability to locate sound in our environment is an important part of hearing. Our brains can localize sound through monaural (one-eared) and binaural (two-eared) cues. The human outer ear, or pinna, forms direction-selective filters. Depending on the sound input direction, different filter resonances become active, which can be evaluated by the auditory system for sound localization. The superior olivary nucleus also plays a role in sound localization, as whichever superior olivary nucleus is more strongly activated by sound from the ear determines the direction of the sound.
| Characteristics | Values |
|---|---|
| Structure | Superior olivary nucleus |
| Cochlea | |
| Auditory cortex | |
| Pinna | |
| Outer ear | |
| Monaural cues | |
| Binaural cues |
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What You'll Learn
The superior olivary nucleus
The medial superior olive (MSO) is a specialised nucleus that measures the time difference of arrival of sounds between the ears (the interaural time difference or ITD). This time difference is crucial for determining the azimuth of sounds, i.e., localising them on the azimuthal plane by determining their degree to the left or the right. The MSO can distinguish time differences as small as 10 microseconds, which is significantly less than the roughly 700 microseconds it takes for sound to travel around the human head.
The lateral superior olive (LSO), on the other hand, is involved in measuring the difference in sound intensity between the ears (the interaural level difference or ILD). This difference in intensity is another crucial cue in determining the azimuth of high-frequency sounds.
The superior olivary complex is divided into three primary nuclei: the MSO, LSO, and the Medial nucleus of the trapezoid body, along with several smaller periolivary nuclei. These nuclei are considered part of the ascending azimuthal localisation pathway. While the superior olivary complex processes horizontal data from two different ears to localise sound on the azimuth axis, it does not process sound elevation cues.
In summary, the superior olivary nucleus, particularly through the functions of the MSO and LSO, plays a vital role in sound localisation by measuring interaural time and intensity differences, respectively. This allows the brain to determine the direction of a sound source, specifically its azimuth or degree to the left or right.
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The cochlea
Damage to the hair cells within the cochlea can lead to sensorineural deafness, resulting in a failure of the transmission of auditory nerve impulses to the brain. This type of hearing loss can occur due to genetic predisposition, age, or exposure to extreme noise, certain illnesses, or toxins. Thus, the integrity of the cochlea and its hair cells is vital for maintaining our ability to localize and interpret sounds accurately.
In summary, the cochlea is a vital structure in our auditory system, enabling us to localize sound through its amplification properties and the presence of hair cells. Its unique shape and functionality enhance our hearing sensitivity, allowing us to interpret and respond to sounds in our environment effectively.
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The auditory cortex
In summary, the auditory cortex is a complex and highly individualized structure that is essential for hearing. It enables us to localize and interpret sounds, process temporal sequences, and understand complex auditory stimuli such as speech and music. While its specific functions are still being elucidated, it is clear that the auditory cortex plays a pivotal role in our auditory perception and cognitive processing of sound.
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Monaural and binaural cues
Monaural cues are essential for locating sounds that occur above, below, in front of, or behind us. Each pinna interacts with incoming sound waves differently, depending on the sound's source relative to our bodies. This interaction provides a monaural cue that helps us locate the sound.
The sound waves received by our two ears from sounds that come from directly above, below, in front, or behind us would be identical. Therefore, monaural cues are crucial for sound localization. For instance, if a sound comes from an off-center location, it creates two types of binaural cues: interaural level differences and interaural timing differences.
The interaural level difference refers to the fact that a sound coming from the right side of your body is more intense at your right ear than at your left ear due to the attenuation of the sound wave as it passes through your head. On the other hand, the interaural timing difference refers to the small time difference in which a given sound wave arrives at each ear. Certain brain areas monitor these differences to construct where along a horizontal axis a sound originates.
Binaural cues, on the other hand, provide information on the location of a sound along a horizontal axis. They rely on differences in the patterns of vibration of the eardrum between our two ears. Binaural localization was possible with lower frequencies, likely because the pinna is small enough to interact with sound waves of high frequency.
Both sighted and blind humans can use echoes, or binaural and monaural cues, to identify the lateral position of a virtual object or discern characteristics of otherwise silent objects.
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The pinna and the external ear canal
The human outer ear, consisting of the pinna and the external ear canal, plays a crucial role in sound localization. The pinna, the visible portion of the ear, interacts with incoming sound waves, creating monaural cues that help us locate sounds above, below, in front, or behind us. This is achieved through direction-selective filters, which activate different filter resonances based on the sound input direction. These resonances, in turn, imprint direction-specific patterns into the frequency responses of our ears, forming the outer ear transfer functions.
The shape and size of the pinna contribute to these direction-dependent resonances, which act as additional localization cues. This is evident in mammals with pronounced structures in the pinna, near the entry of the ear canal. These structures enable the detection of sound elevation through the use of two detectors positioned at different heights. The ability to discern sound elevation is also observed in animals that tilt their heads to localize sound precisely.
The pinna's role in sound localization is further complemented by the external ear canal. Together, they form the outer ear, which captures sound waves and directs them toward the eardrum. The external ear canal's length and shape influence how sound reaches the eardrum, contributing to our ability to interpret sound sources accurately.
The monaural cues provided by the pinna and the external ear canal are essential for sound localization, particularly when sounds come from above, below, in front, or behind. Binaural cues, on the other hand, come into play when sounds originate from the sides, as they rely on the differences in sound intensity and vibration patterns between the two ears.
In summary, the pinna and the external ear canal, as part of the human outer ear, are integral structures for sound localization. They create direction-specific patterns in our ears' frequency responses, allowing us to determine the direction and elevation of sound sources. This intricate system enhances our ability to interpret and respond to sounds in our environment accurately.
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Frequently asked questions
The superior olivary nucleus.
As sound reaches each superior olivary nucleus from the ear, whichever nucleus is more strongly activated determines the direction of the sound.
The human outer ear, i.e. the structures of the pinna and the external ear canal, form direction-selective filters. The auditory system uses both monaural (one-eared) and binaural (two-eared) cues to localize sound.
Monaural cues are helpful in locating sounds that occur above or below and in front or behind us. Binaural cues, on the other hand, provide information on the location of a sound along a horizontal axis by relying on differences in patterns of vibration of the eardrum.
Sound localization is the ability to locate sound in our environments, an important part of hearing.
