hearing

(neuroscience)

The general perceptual behavior and the specific responses that are made in relation to sound stimuli. The auditory system consists of the ear and the auditory nervous system. The ear comprises outer, middle, and inner ear. The outer ear, visible on the surface of the body, directs sounds to the middle ear, which converts sounds into vibrations of the fluid that fills the inner ear. The inner ear contains the vestibular and the auditory sensory organs. See also Ear (vertebrate).

The auditory part of the inner ear, known as the cochlea because of its snaillike shape, analyzes sound in a way that resembles spectral analysis. It contains the sensory cells that convert sounds into nerve signals to be conducted through the auditory portion of the eighth cranial nerve to higher brain centers. The neural code in the auditory nerve is transformed as the information travels through a complex system of nuclei connected by fiber tracts, known as the ascending auditory pathways. They carry auditory information to the auditory cortex, which is the part of the sensory cortex where perception and interpretation of sounds are believed to take place. Interaction between the neural pathways of the two ears makes it possible for a person to determine the direction of a sound's source. See also Binaural sound system; Brain.

Role of the ear

The pinna, the projecting part of the outer ear, collects sound, but because it is small in relation to the wavelengths of sound that are important for human hearing, the pinna plays only a minor role in hearing. The ear canal acts as a resonator: it increases the sound pressure at the tympanic membrane in the frequency range between 1500 and 5000 Hz. The difference between the arrival time of a sound at each of the two ears and the difference in the intensity of the sound that reaches each ear are used by the auditory nervous system to determine the location of the sound source.

Sound that reaches the tympanic membrane causes the membrane to vibrate, and these vibrations set in motion the three small bones of the middle ear: the malleus, the incus, and the stapes. The footplate of the stapes is located in an opening of the cochlear bone—the oval window. Moving in a pistonlike fashion, the stapes sets the cochlear fluid into motion and thereby converts sound (pressure fluctuations in the air) into motion of the cochlear fluid. Motion of the fluid in the cochlea begins the neural process known as hearing.

There are two small muscles in the middle ear: the tensor tympani and the stapedius muscles. The former pulls the manubrium of the malleus inward, while the latter is attached to the stapes and pulls the stapes in a direction that is perpendicular to its pistonlike motion. The stapedius muscle is the smallest striated muscle in the body, and it contracts in response to an intense sound. This is known as the acoustic middle-ear reflex. The muscle's contraction reduces sound transmission through the middle ear and thus acts as a regulator of input to the cochlea. Perhaps a more important function of the stapedius muscle is that it contracts immediately before and during a person's own vocalization, reducing the sensitivity of the speaker's ears to his or her own voice and possibly reducing the masking effect of an individual's own voice. The role of the tensor tympani muscle is less well understood, but it is thought that contraction of the tensor tympani muscle facilitates proper ventilation of the middle-ear cavity. These two muscles are innervated by the facial (VIIth) nerve for the stapedius and the trigeminal (Vth) nerve for the tensor tympani. The acoustic stapedius reflex plays an important role in the clinical diagnosis of disorders affecting the middle ear, the cochlea, and the auditory nerve.

Vibrations in the cochlear fluid set up a traveling wave on the basilar membrane of the cochlea. When tones are used to set the cochlear fluid into vibration, one specific point on the basilar membrane will vibrate with a higher amplitude than any other. Therefore, a frequency scale can be laid out along the basilar membrane, with low frequencies near the apex and high frequencies near the base of the cochlea.

The sensory cells that convert the motion of the basilar membrane into a neural code in individual auditory nerve fibers are located along the basilar membrane. They are also known as hair cells, because they have hairlike structures on their surfaces. The hair cells in the mammalian cochlea function as mechanoreceptors: motion of the basilar membrane causes deflection of the hairs, starting a process that eventually results in a change in the discharge rate of the nerve fiber connected to each hair cell. This process includes the release of a chemical transmitter substance at the base of the hair cells that controls the discharge rate of the nerve fiber.

The frequency selectivity of the basilar membrane provides the central nervous system with information about the frequency or spectrum of a sound, because each auditory nerve fiber is “tuned” to a specific frequency. The frequency of a sound is also represented in the time pattern of the neural code, at least for frequencies up to 5 kHz. Thus, the frequency or spectrum of a sound can be coded for place and time in the neural activity in the auditory nervous system. See also Audiometry; Pitch.

Auditory nervous system

The ascending auditory nervous system consists of a complex chain of clusters of nerve cells (nuclei), connected by nerve fibers (nerve tracts). The chain of nuclei relays and transforms auditory information from the periphery of the auditory system, the ear, to the central structures, or auditory cortex, which is believed to be associated with the ability to interpret different sounds. Neurons in the entire auditory nervous system are, in general, organized anatomically according to the frequency of a tone to which they respond best, which suggests a tonotopical organization in the auditory nervous system and underscores the importance of representations of frequency in that system. However, when more complex sounds were used to study the auditory system, qualities of sounds other than frequency or spectrum were found to be represented differently in different neurons in the ascending auditory pathway, with more complex representation in the more centrally located nuclei. Thus, the response patterns of the cells in each division of the cochlear nucleus are different, which indicates that extensive signal processing is taking place. Although the details of that processing remain to be determined, the cells appear to sort the information and then relay different aspects of it through different channels to more centrally located parts of the ascending auditory pathway. As a result, some neurons seem to respond only if more than one sound is presented at the same time, others respond best if the frequency or intensity of a sound changes rapidly, and so on.

Another important feature of the ascending auditory pathway is the ability of particular neurons to signal the direction of sound origination, which is based on the physical differences in the sound reaching the two ears. Certain centers in the ascending auditory pathway seem to have the ability to compute the direction to the sound source on the basis of such differences in the sounds that reach the ears.

Knowledge of the descending auditory pathway is limited to the fact that the most peripheral portion can control the sensitivity of the hair cells. See also Hearing impairment; Loudness; Masking of sound; Phonoreception; Signal detection theory; Sound.

Hearing (vertebrate)

The ability to perceive sound arriving from distant vibrating sources through the environmental medium (such as air, water, or ground). The primary function of hearing is to detect the presence, identity, location, and activity of distant sound sources. Sound detection is accomplished using structures that collect sound from the environment (outer ears), transmit sound efficiently to the inner ears (via middle ears), transform mechanical motion to electrical and chemical processes in the inner ears (hair cells), and then transmit the coded information to various specialized areas within the brain. These processes lead to perception and other behaviors appropriate to sound sources, and probably arose early in vertebrate evolution.

Sound is gathered from the environment by structures that are variable among species. In many fishes, sound pressure reaching the swim bladder or another gas-filled chamber in the abdomen or head causes fluctuations in volume that reach the inner ears as movements. In addition, the vibration of water particles that normally accompany underwater sound reaches the inner ears to cause direct, inertial stimulation. In land animals, sound causes motion of the tympanic membrane (eardrum). In amphibians, reptiles, and birds, a single bone (the columella) transmits tympanic membrane motion to the inner ears. In mammals, there are three interlinked bones (malleus, incus, and stapes). Mammals that live underground may detect ground-borne sound via bone conduction. In whales and other sea mammals, sound reaches the inner ears via tissue and bone conduction.

The inner ears of all vertebrates contain hair-cell mechanoreceptors that transform motion of their cilia to electrochemical events resulting in action potentials in cells of the eighth cranial nerve. Patterns of action potentials reaching the brain represent sound wave features in all vertebrates. All vertebrates have an analogous set of auditory brain centers. See also Ear (vertebrate); Physiological acoustics.

Experiments show that vertebrates have more commonalities than differences in their sense of hearing. The major difference between species is in the frequency range of hearing, from below 1 Hz to over 100,000 Hz. In other fundamental hearing functions (such as best sensitivity, sound intensity and frequency discrimination acuity, time and frequency analysis, and source localization), vertebrates have much in common. All detect sound within a restricted frequency range. All species are able to detect sounds in the presence of interfering sounds (noise), discriminate between different sound features, and locate the sources of sound with varying degrees of accuracy.

The sensitivity range is similar among all groups, with some species in all groups having a best sensitivity in the region of −20 to 0 dB. Fishes, amphibians, reptiles, and birds hear best between 100 and 5000 Hz. Only mammals hear at frequencies above 10,000 Hz. Humans and elephants have the poorest high-frequency hearing.

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