Auditory Function II: Processing and Perception Flashcards
Properties of Sound
Frequency • Measures in Cycles per second (Hetz: Hz) • High-frequency (15,000Hz) • Low Frequency (100Hz) Wavelength • Speed of sound transmission. • Indication of medium sound travels through. Amplitude • Size of peak or Trough. • Measured in decibels (dB) • Indication of sound volume
Review: Ear Anatomy
Sound waves are collected by the external, cartilaginous part of the ear called thepinna, then travel through the auditory canal and cause vibration of the thin diaphragm called thetympanumor ear drum, the innermost part of theouter ear.
Interior to the tympanum is themiddle ear. The middle ear holds three small bones called theossicles, which transfer energy from the moving tympanum to the inner ear. The three ossicles are themalleus(also known as the hammer), theincus(the anvil), andstapes(the stirrup). The aptly named stapes looks very much like a stirrup. The three ossicles are unique to mammals, and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes.
These bones also function to collect force and amplify sounds. The ear ossicles are homologous to bones in a fish mouth: the bones that support gills in fish are thought to be adapted for use in the vertebrate ear over evolutionary time. Many animals (frogs, reptiles, and birds, for example) use the stapes of the middle ear to transmit vibrations to the middle ear.
Sound Transduction
Sound waves are essentially pressure waves in the air. When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound.
The pressure waves strike the tympanum, causing it to vibrate.
The mechanical energy from the moving tympanum transmits the vibrations to the three bones of the middle ear. The stapes transmits the vibrations to a thin diaphragm called the oval window, which is the outermost structure of the inner ear. The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the most interior portion of the ear.
Here, the energy from the sound wave is transferred from the stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the oval window create pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal. Inside the cochlea, the basilar membrane is a mechanical analyser that runs the length of the cochlea, curling toward the cochlea’s centre.
The mechanical properties of the basilar membrane change along its length, such that it is thicker, tauter, and narrower at the outside of the whorl (where the cochlea is largest), and thinner, floppier, and broader toward the apex, or centre, of the whorl (where the cochlea is smallest). Different regions of the basilar membrane vibrate according to the frequency of the sound wave conducted through the fluid in the cochlea. For these reasons, the fluid-filled cochlea detects different wave
Review: Sound Detection
Sound is detected in the organ of Corti (spiral organ). It is composed of hair cells held in place above the basilar membrane, with their exposed short, hair-like stereocilia contacting or embedded in the tectorial membrane above them.
The inner hair cells are the primary auditory receptors and exist in a single row. The stereocilia from inner hair cells extend into small dimples on the tectorial membrane’s lower surface. The outer hair cells are arranged in three or four rows and function to fine tune incoming sound waves. The longer stereocilia that project from the outer hair cells attach to the tectorial membrane. All the stereocilia are mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel. As a result, the hair cell membrane is depolarized, and a signal is transmitted to the cochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated.
Vestibular Function
The stimuli associated with the vestibular system are linear acceleration (gravity) and angular acceleration and deceleration. Gravity, acceleration, and deceleration are detected by evaluating the inertia on receptive cells in the vestibular system. Gravity is detected through head position. Angular acceleration and deceleration are expressed through turning or tilting of the head.
The vestibular system has some similarities with the auditory system. It utilizes hair cells just like the auditory system, but it excites them in different ways. There are five vestibular receptor organs in the inner ear: the utricle, the saccule, and three semicircular canals. Together, they make up what’s known as the vestibular labyrinth.
The utricle and saccule respond to acceleration in a straight line, such as gravity. The roughly 30,000 hair cells in the utricle and 16,000 hair cells in the saccule lie below a gelatinous layer, with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystals—like tiny rocks. When the head is tilted, the crystals continue to be pulled straight down by gravity, but the new angle of the head causes the gelatin to shift, thereby bending the stereocilia. The bending of the stereocilia stimulates the neurons, and they signal to the brain that the head is tilted, allowing the maintenance of balance. It is the vestibular branch of the vestibulocochlear cranial nerve that deals with balance.
The fluid-filledsemicircular canalsare tubular loops set at oblique angles. They are arranged in three spatial planes. The base of each canal has a swelling that contains a cluster of hair cells. The hairs project into a gelatinous cap called the cupula and monitor angular acceleration and deceleration from rotation. They would be stimulated by driving your car around a corner, turning your head, or falling forward. One canal lies horizontally, while the other two lies at about 45-degree angles to the horizontal axis. When the brain processes input from all three canals together, it can detect angular acceleration or deceleration in three dimensions. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals to the brain. Upon cessation accelerating or decelerating—or just moving—the movement of the fluid within the canals slows or stops. For example, imagine holding a glass of water. When moving forward, water may splash backwards onto the hand, and when motion has stopped, water may splash forward onto the fingers. While in motion, the water settles in the glass and does not splash. Note that the canals are not sensitive to velocity itself, but to changes in velocity, so moving forward at 60mph with your eyes closed would not give the sensation of movement, but suddenly accelerating or braking would stimulate the receptors.
Auditory Detection: Hair Cell Transduction
The tectorial membrane runs parallel to the basilar membrane, so when the basilar membrane vibrates up and down in response to motion at the stapes, so does the tectorial membrane. However, the displacement of the membranes causes them to pivot about different hinging points and this creates a shearing force between the hair cell stereocilia embedded in the tectorial membrane and the hair cells themselves which rest on the basilar membrane. Shearing is a particular form of bending in which, in this case, the top moves more than the bottom. It is this shearing force that transduces mechanical energy into electrical energy which is transmitted to the auditory nerve fibres.
In order for the hair cell to transduce stereocilia shearing (mechanical) forces into an electrical (neural) response, the permeability of the hair cell membrane must change. This happens when the shearing motion, which is a mechanical stimulus, opension channelsin the cell’s plasma membrane and the current flowing through these channels alters the cell’s membrane potential (this is the electrical response). So, in response to a mechanical stimulus, there is an influx of ions into the cell which disturbs the resting potential of the cell membrane, driving the membrane potential to a new level called thereceptor potential. The channels are relatively non-selective about which ions they allow to pass through them.
In fact, when a hair bundle is displaced by a mechanical stimulus, its response depends on the direction and magnitude of the stimulus. In an unstimulated cell about 10 per cent of the ion channels are open. As a result, the cell’s resting potential (about −50 mV) is determined, in part, by the inward flow of current. A positive stimulus that displaces the stereocilia towards the tall edge opens additional channels and the resultant influx of positive ions depolarises the cell by as much as tens of mV. A negative stimulus that displaces the stereocilia towards the short edge shuts the channels that are open at rest and hyperpolarises the cell.
This directional sensitivity of the cells, their arrangement on the organ of Corti and the hypothesised motion of the organ of Corti in response to a stimulus, means that an upward movement of the basilar membrane leads to depolarisation of the cells, whereas a downward deflection elicits hyperpolarisation.
The receptor potential of a hair cell is graded; as the stimulus amplitude increases, the receptor potential grows increasingly larger, up to a maximal point of saturation. The relationship between a bundle’s deflection and the resulting electrical response is S-shaped. This results in a high degree of sensitivity. A small displacement of only 100 nm (100 × 10−9m) represents 90% of the response range of the hair cell (shaded part). Deflection of a hair cell by the width of a hydrogen atom is enough to make the cell respond.
Hair Cell Transduction Tip links
Tip links aid in causing ‘channels’ to open and close near the top of the hair cell. Tip links are filamentous connections between two stereocilia. Each tip link is a fine fibre obliquely joining the distal end of one stereocilium to the side of the longest adjacent process. It is thought that each link is attached at one end or both to the molecular gates of one or a few channels. Under this arrangement, pushing a bundle in one direction increases the tension on the tip link and promotes channel opening while pushing the bundle in the opposite direction slackens the link and the associated channel closes.
Many other sensory receptors, such as photoreceptors and olfactory neurons, employ second messengers in the transduction process. This is not true for hair cells. The rapidity with which they respond makes this impossible. In order to deal with the frequencies of biologically relevant stimuli, transduction must be rapid. The highest frequency humans can hear is about 20000 Hz. This in effect means that hair cells must be able to turn current on and off 20000 times per second (200000 times per second for a bat). Also, localisation of sound sources requires that animals are able to resolve very small time differences, in the order of 10 μs.
In addition to being sensory receptors, hair cells are also presynaptic terminals. The membrane at the base of each hair cell contains several presynaptic active zones, where chemical neurotransmitter is released. When the hair cells are depolarised, chemical transmitter is released from the hair cells to the cells of the auditory nerve fibres. Excited by this chemical transmitter, the afferent nerve fibres contacting the hair cells fire a pattern of action potentials that encode features of the stimulus. As in other synapses, the depolarisation that leads to transmitter release acts through an intermediary, namely calcium ions. Depolarisation opens channels at the base of the hair cell (voltage-gated calcium channels), which allow calcium ions to enter from the surrounding perilymph resulting in the release of transmitter. Calcium also has another function: it opens potassium channels, called calcium-activated potassium channels, which allow potassium ions to leave the cells because the perilymph on the other side is low in potassium. The potassium ions leaving the hair cell via the calcium-activated channels results in repolarisation of the cell.
Auditory Detection: Hair Cell Tuning
We have determined that:
the location of the peak of the travelling wave on the basilar membrane is determined by the frequency of the originating sound.
hair cells run the length of the basilar membrane.
When a certain frequency sound stimulates a point on the membrane, it responds by moving, and the hair cells at that site are stimulated by the shearing force that this movement creates.
Groups of hair cells therefore only respond if certain frequencies are present in the originating sound. The frequency sensitivity of a hair cell can be displayed as a tuning curve. To construct a tuning curve, a single hair cell is stimulated repeatedly with pure tone stimuli of various frequencies. For each frequency, the intensity of the stimulus is adjusted until the response of the hair cell reaches some predefined level. The tuning curve is then the graph of sound intensity against stimulus frequency. Tuning curves for hair cells are characteristically V-shaped. The tip represents the frequency to which the cell is most sensitive.
A sound of this frequency will elicit a response from the cell even when it is of very low intensity. Sounds of greater or lesser frequency require higher intensity to excite the cell to the predetermined level.
The great majority of neurons that carry information from the cochlea to higher levels of the auditory system connect to the inner hair cells. Thus most, if not all, information about sounds is conveyed to the brain via the inner hair cells. Given that the outer hair cells greatly outnumber the inner hair cells, it seems paradoxical that most cochlear output is derived from the inner cells. However, ongoing research suggests that outer hair cells do play an important role in the transduction process. Membranes of the outer cells contain amotor proteinthat changes the length of the outer hair cells in response to stimulation. This change in length effects a change in the mechanical coupling between the basilar and tectorial membranes. Outer hair cells are sometimes said to constitute acochlear amplifierby amplifying the response of the basilar membrane. This causes the sterocilia on the inner cells to bend more, creating a bigger response in the auditory nerve.
Auditory Detection: Neural
The vestibulocochlear nerve enters the brainstem just under the cerebellum and conveys information from the hair cells in the inner ear as well as from the vestibular organs of the inner ear. The cochlear portion of the nerve (auditory nerve ) contains two basic types of auditory nerve fibres: afferent fibres that carry information from the peripheral sense organ (organ of Corti) to the brain; and efferent fibres that bring information from the cerebral cortex to the periphery. Afferent fibres arise from nerve cell bodies in the spiral (or cochlear) ganglion and contact the hair cells. The hair cells themselves do not have axons and therefore do not generate action potentials. Action potentials are first produced by the axons of afferent fibres, about 10 per cent of the ion channels are open when the hair cell is unstimulated This means that in the auditory nerve, there is a continuous low level of discharge of action potentials even when hair cells are unstimulated. Depolarisation of hair cells in response to stereocilia shearing causes an increase in the discharge rate of action potentials above this spontaneous rate (excitation) while hyperpolarisation of hair cells leads to a decrease in the discharge rate of action potentials below the spontaneous discharge rate (inhibition).
Auditory Processing in the Brain
Up till now we have dealt with the anatomy of the auditory periphery and how the basic attributes of sound are coded within the auditory periphery. A great deal of additional processing takes place in the neural centres that lie in the auditory brainstem and cerebral cortex. Because localisation and other binaural perceptions depend on the interaction of information arriving at the two ears, we need to study the central auditory centres, since auditory nerves from the two cochleae interact only at the brainstem and cerebral cortex. This section deals with the structure and function of the central auditory nervous system (CANS).
There are two main components of the auditory pathway:
Primary (lemniscal) pathway – this is the main pathway through which auditory information reaches the primary auditory cortex (A1).
Non-lemniscal pathway – mediating unconscious perception such as attention, emotional response, and auditory reflexes.
Primary Auditory Pathway
Cochlear Nuclei
Fibres from the cochlear nerve bifurcate and information is sent to the cochlear nuclei on each side of the brainstem:
Ventral (anterior) cochlear nucleus – located in the area where the nerve enters the brainstem.
Dorsal (posterior) cochlear nucleus – located posterior to the inferior cerebellar peduncle.
It forms a small bulge on the surface of the brainstem – known as the auditory tubercle.
From the dorsal cochlear nucleus, most fibres cross the midline and ascend in the contralateral lateral lemniscus. Other fibres ascend in the ipsilateral lateral lemniscus.
From the ventral cochlear nucleus, some fibres also ascend in the lateral lemniscus bilaterally. However, most fibres from the ventral cochlear nucleus decussate to the contralateral superior olivary nuclei in a region known as the trapezoid body. Although the ventral cochlear nuclei neurons decussate at the trapezoid body, some fibres synapse at the ipsilateral superior olivary nucleus. The superior olivary nucleus is located just next to the trapezoid body. It also projects upwards through the lateral lemniscus.
In summary, in both the dorsal and ventral nuclei, some fibres decussate while others do not. For that reason, information from both ears travels bilaterally in each lateral lemniscus. This is important because supranuclear lesions (i.e. above the level of the cochlear nucleus) will not lead to serious hearing impairment. Therefore, hearing problems can be conductive or sensorineural but are rarely central.
Inferior Colliculus and Medial Geniculate Body
fibres ascending through the lateral lemniscus from both cochlear nuclei and from the superior olivary nuclei arrive at the inferior colliculus, where all these fibres carrying auditory information converge.
These fibres project to the ipsilateral medial geniculate body (MGB) in the thalamus (recall that vision is relayed on the lateral geniculate body).
The MGB does not act as a simple relay centre: it has reciprocal connections with the auditory cortex and mediates refinement of the incoming information. Projections from the medial geniculate body proceed then to the primary auditory cortex.
Note: A good way to remember what information passes through each geniculate body is that music goes to medial and light goes to lateral.
Primary Auditory Cortex
The primary auditory cortex (A1) is located in the superior temporal gyrus, right under the lateral fissure. The primary auditory cortex is organized tonotopically.
There are, in fact, two distinct pathways that occur in the CANS:
The ‘what’ pathway which is monaural and receives information from only one ear. This pathway is concerned with the spectral (frequency) and temporal (time) features of a sound and is hardly concerned with the spatial aspects. It focuses mainly on identifying and classifying different types of sound.
The ‘where’ pathway which is binaural and receives information from both ears. It is involved in the localisation of a sound stimulus.
Despite the apparent dichotomy of these two processing pathways, the same types of acoustic cues may be important for the analysis that occurs in each. For example, spectral information is used in the ‘where’ pathway for determining a sound’s elevation; and temporal information, used for our perception of frequency in the ‘what’ pathway, is also used in the ‘where’ pathway for determining a sound’s horizontal location.
The ‘What’ Pathway
The main nucleus involved in the ‘what’ pathway is the cochlear nucleus which has three main components, each of which is tonotopically organised; cells with progressively higher characteristic frequencies are arrayed in an orderly progression along one axis. The cochlear nuclei contain neurons of several types, each of which encodes a specific parameter of a stimulus (frequency, intensity, time): stellate cells encode stimulus frequency and intensity, bushy cells provide information about the timing of acoustical stimuli, and are involved in locating sound sources along the horizontal axis, and fusiform cells are thought to participate in the localization of sound sources along a vertical axis.
The ‘Where’ Pathway
The ‘where’ pathway involves the ventral cochlear nuclei, the superior olivary complex and the inferior colliculus. The superior olivary complex is composed of the lateral superior olive (LSO) and the medial superior olive (MSO).
The neurons in the superior olivary complex are the first brainstem neurons to receive strong inputs from both cochleae and are involved in sound localisation.
The MSO receives excitatory inputs from the cochlear nuclei on both sides and is tonotopically organised. It is involved in the localisation of sound in the horizontal plane by processing information about auditory delays. Units in the MSO increase their firing rate in response to sounds from both ears as opposed to one ear, and these excitatory–excitatory (EE) units will increase their discharge rate further in response to sounds that reach both ears with a certain delay. In other words, a unit will discharge at the greatest rate when there is a particular interaural delay. This aids in localising sound in the horizontal plane.
The LSO is also involved with sound localisation but instead of using interaural time delays, it employs intensity differences to calculate where a sound originated. Information from the ipsilateral (same side) inputs to the LSO is usually excitatory and results in an increase in discharge rate of the neuron. Contralateral stimulation of the LSO is usually inhibitory. Thus, stimulation from both ears may decrease the firing rate of the neuron relative to the firing rate when only the ipsilateral ear receives sound. These excitatory–inhibitory (EI) units discharge with a few spikes when there is approximately equal stimulation of both ears and discharge rate increases as a function of changing the interaural level difference. The LSO therefore appears to form a network for processing interaural level differences, which are used to determine the location of sound sources.
The inferior colliculus is part of the tectum and is the most prominent nucleus in the brainstem. It receives inputs from the olivary complex and the cochlear nucleus. Units in the inferior colliculus appear to be mainly EI units although there are EE units as well. They are tonotopically organised in sheets of cells (as in the cochlear nucleus). Cells in different parts of the inferior colliculus are either monaural, in that they respond to input from one ear only, or binaural, responding to bilateral stimulation. Both the spectral processing that takes place in the cochlear nucleus and the binaural processing that occurs in the olivary complex are seen in the inferior colliculus. In fact, the inferior colliculus is the termination of nearly all projections from brainstem auditory nuclei. It is therefore a ‘watershed’ for information processing where the ‘what’ and ‘where’ pathways converge on a single tonotopic map. Outputs of the inferior colliculus project mainly to the medial geniculate nucleus.
The medial geniculate nucleus is also tonotopically organised. Neurons with the same characteristic frequency are arrayed in one layer, so that the nucleus consists of a stack of neural laminae that represent successive stimulus frequencies. Sensitivity to interaural time or intensity differences is maintained. Axons leaving the MGN project to the auditory cortex. The neural responses of cortical cells in response to sound have been studied extensively in primates. In general, neurons are relatively sharply tuned for sound frequency and possess characteristic frequencies covering the audible spectrum of frequencies. In electrode penetrations made perpendicular to the cortical surface, the cells encountered tend to have similar characteristic frequencies, suggesting columnar organisation on the basis of frequency, the so-called ice-cube model of the auditory cortex (Figure 30). Although most of the neurons in the primary auditory cortex are sensitive to stimulation through either ear, their sensitivities are not identical. Instead the cortex is divided into alternating strips of two types. Half of these strips contain EE neurons and respond more to stimulation from both ears than to either ear separately, and the other half consist of EI neurons which are stimulated by unilateral input but inhibited by stimulation from the opposite ear. The strips of EE and EI cells run at right angles to the axis of tonotopic mapping so that the primary auditory cortex is partitioned into columns responsive to every audible frequency and to each type of interaural interaction
