5.4 Hearing

ANATOMY OF THE AUDITORY SYSTEM

   The ear can be separated into multiple sections. The outer ear includes the pinna, which is the visible part of the ear that protrudes from our heads, the auditory canal, and the tympanic membrane, or eardrum. The middle ear contains three tiny bones known as the ossicles, which are named the malleus (or hammer), incus (or anvil), and the stapes (or stirrup). The inner ear contains the semi-circular canals, which are involved in balance and movement (the vestibular sense), and the cochlea. The cochlea is a fluid-filled, snail-shaped structure that contains the sensory receptor cells (hair cells) of the auditory system (figure below).

 The ear is divided into outer (pinna and tympanic membrane), middle (the three ossicles: malleus, incus, and stapes), and inner (cochlea and basilar membrane) divisions.

   Sound waves travel along the auditory canal and strike the tympanic membrane, causing it to vibrate. This vibration results in movement of the three ossicles. As the ossicles move, the stapes presses into a thin membrane of the cochlea known as the oval window. As the stapes presses into the oval window, the fluid inside the cochlea begins to move, which in turn stimulates hair cells known as cilia, which are auditory receptor cells of the inner ear embedded in the basilar membrane. The basilar membrane is a thin strip of tissue within the cochlea that houses the cilia which allow the component pieces of the sound to be broken down into different frequencies.

CILIA AND THE PERCEPTION OF PITCH

   The activation of cilia is a mechanical process in that stimulation of the hair cell occurs when the hair cell is bent in response to a frequency of signal from structures of the middle ear causing a chemical reaction that triggers electrical action potentials for further processing of the auditory information in the brain.  Auditory information is transmitted via the auditory nerve to the inferior colliculus (upper sections of the brainstem), the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe of the brain for processing. Like the visual system, there is also evidence suggesting that information about auditory recognition and localization is processed in parallel streams (Rauschecker & Tian, 2000; Renier et al., 2009).

 Frequency organization of the cochlea. A shows activation that occurs at each frequency. B,C and D show the cochlea unraveled and illustrate how high, medium and low frequencies propagate respectively across the basilar membrane (adapted from Encyclopedia Britannica, 1997).

   Several theories have been proposed to account for pitch perception. We’ll briefly discuss three of them here: temporal theory, volley theory and place theory. The temporal theory of pitch perception asserts that frequency is coded by the activity level of a sensory neuron and that the firing rate of cilia or groups of cilia encode constant pitch perception. This entails a given hair cell or group of hair cells sends action potentials related to the frequency of the sound wave. At high amplitudes (loud sounds) temporal theory suggests that even when large groups of cilia are firing (sending action potentials) there is a periodicity to the firing, which corresponds to the periodicity of the auditory stimulus (Javel & Mott, 1988). Neurons also have a maximum firing frequency that exists between the frequencies humans are able to perceive, therefore in order to completely explain pitch perception, temporal theory must somehow explain how we are able to perceive pitches above the maximum firing rate of the neurons that encode the signal (Shamma, 2001). In response to this, volley theory describes firing patters of groups of neurons that fire in and out of phase in order to create coding for firing rates above what would be possible for a single neuron. Ernest Wever and Charles Bray, in the 1930s, proposed that neurons could fire in a volley and summate in frequency to recreate the frequency of the original sound stimulus (Wever & Bray, 1937). However because later studies determined phase synchrony is only able to code up to 10,000 Hz, volley theory is also not able to account for all the sounds we are able to hear (Goldstein, 1973). Place theory on the other hand suggests that the basilar membrane of the cochlea has specific sensitive areas where the cilia trigger action potentials for different frequencies of sound.

Contrary to temporal and volley theories, Hermann von Helmholtz proposed (though it was many accepted this theory before Helmholtz time) the place model of auditory transmission that suggests our perception of pitch is created by different places on the basilar membrane being activated depending on the frequency of sound (Barnes, 1897). Place theory allows for a description of extremely high frequencies as well as discriminating sound of many types and frequencies at the same time. Additionally, experiments using cochlear implants in order to control for overall vibration of the cochlea during normal experience of loud mixtures of sound, have demonstrated at low stimulation rates, ratings of pitch were proportional to the log of rate of stimulation and also decreased with distance from the round window (middle ear). At higher rates of stimulation, effects of rates of firing were weak, whereas effects of place along the basilar membrane were strong suggesting that hearing follows rules of temporal and volley theory below 1000 Hz and place theory encodes frequencies above 5000 Hz. Frequencies between 1000 and 5000 may utilize all three modes of encoding to encode for both the basilar membrane location as well as the neuron and group firing rates (Fearn, Carter & Wolfe, 1999). Further research has demonstrated similar findings suggesting both the rate of action potentials and place contribute to our perception of pitch. However, frequencies above 5000 Hz can only be encoded using place cues (Shamma, 2001).

SOUND LOCALIZATION

   The ability to locate sound in our environments is an important part of hearing. Localizing sound could be considered similar to the way that we perceive depth in our visual fields. Like the monocular and binocular cues that provided information about depth, the auditory system uses both monaural (one-eared) and binaural (two-eared) cues to localize sound.

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 is helpful in locating sounds that occur above or below and in front or behind us. The sound waves received by your two ears from sounds that come from directly above, below, in front, or behind you would be identical; therefore, monaural cues are essential (Grothe, Pecka, & McAlpine, 2010).

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 between our two ears. If a sound comes from an off-center location, it creates two types of binaural cues: interaural level differences and interaural timing differences. 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 because of the attenuation of the sound wave as it passes through your head. Interaural timing difference refers to the small difference in the time at which a given sound wave arrives at each ear (figure below). Certain brain areas monitor these differences to construct where along a horizontal axis a sound originates (Grothe et al., 2010).

   Deafness is the partial or complete inability to hear. Some people are born deaf, which is known as congenital deafness. Many others begin to suffer from conductive hearing loss because of age, genetic predisposition, or environmental effects, including exposure to extreme noise (noise-induced hearing loss, as shown in figure below), certain illnesses (such as measles or mumps), or damage due to toxins (such as those found in certain solvents and metals).

Environmental factors that can lead to conductive hearing loss include regular exposure to loud music or construction equipment. (a) Rock musicians and (b) construction workers are at risk for this type of hearing loss. (credit a: modification of work by Kenny Sun; credit b: modification of work by Nick Allen)

   Given the mechanical nature by which the sound wave stimulus is transmitted from the eardrum through the ossicles to the oval window of the cochlea, some degree of hearing loss is inevitable. With conductive hearing loss, hearing problems are associated with a failure in the vibration of the eardrum and/or movement of the ossicles. These problems are often dealt with through devices like hearing aids that amplify incoming sound waves to make vibration of the eardrum and movement of the ossicles more likely to occur.

Hearing problems associated with a failure to transmit neural signals from the cochlea to the brain are referred to as sensorineural hearing loss. Ménière’s disease refers to sensorineural hearing loss which results in a degeneration of inner ear structures that can lead to hearing loss, tinnitus (constant ringing or buzzing), vertigo (a sense of spinning), and an increase in pressure within the inner ear (Semaan & Megerian, 2011). This kind of loss cannot be treated with hearing aids, but some individuals can be treated with cochlear implants. Cochlear implants are electronic devices that consist of a microphone, a speech processor, and an electrode array that stimulate specific parts of the inner ear that transducing . The device receives incoming sound information and directly stimulates the auditory nerve to transmit information to the brain. Cochlear implants have not only aided individuals who had previously been hearing impaired, but these have also lead to new understandings of cochlear function and organizational processing of sound as discussed above in terms of the temporal, volley, and place theories of pitch perception (Moore, 2003).