Chapter 8: The Auditory System

The auditory system mediates hearing, a sensory experience that is as broad as the sound spectrum itself. From signals of impending danger, like a car horn, to the pleasing sounds that fill a concert hall, much of our daily behavior is determined by the sounds around us. The auditory system is also our principal communication portal, allowing us to understand speech. This system, like the somatic sensory and visual systems, has a topographic organization determined by the peripheral receptive sheet. And similar to the other systems, the auditory system consists of multiple parallel pathways that engage multiple cortical regions, either directly or via complex corticocortical networks. Each auditory pathway is hierarchically organized and has the connections and properties to mediate different aspects of hearing.

The complexity of the auditory pathways derives from the particular properties of natural sounds, with their diverse frequency characteristics, multiple sources of origin, and large dynamic ranges. However, an added measure of complexity is imposed on the human auditory system by the demands of producing and understanding speech. Although the physical characteristics of a spoken word may be simpler than many sounds that are not part of our lexicon, the linguistic quality of the stimulus engages unique cortical areas. This chapter first considers the general functional organization of the auditory system. Then it examines key levels through the brain stem and thalamus, where auditory information is processed. Finally, the complex connections of the auditory and speech centers of the cerebral cortex are examined.

Parallel Ascending Auditory Pathways Are Involved in Different Aspects of Hearing

The process of hearing begins on the body surface, as sounds are conducted by the auricle and external auditory meatus to the tympanic membrane. Mechanical displacement of the tympanic membrane, produced by changes in sound pressure waves, is transmitted to the inner ear by tiny bones termed the middle ear ossicles (see Figure 8–3). The inner ear transductive machinery is located within the temporal bone in a coiled structure called the cochlea. This is the location of the auditory receptors, termed hair cells because they each have a bundle of hair-like stereocilia on their apical surface. Each auditory receptor is sensitive to a limited frequency range of sounds. Hair cells in the human cochlea are not mitotically replaced, and their numbers decline throughout life. This reduction can be exacerbated by conditions such as ear infections, exposure to loud sounds or drugs with ototoxic properties.

A topographic relationship exists between the location of a hair cell in the cochlea and the sound frequency to which the receptor is most sensitive. As discussed later, from the base of the cochlea to the apex, the frequency to which a hair cell is maximally sensitive changes systematically from high frequencies to low frequencies. This differential frequency sensitivity of hair cells along the length of the cochlea is the basis of the tonotopic organization of the auditory receptive sheet. Many of the components of the auditory system are tonotopically organized. The topographic relationship between the receptor sheet and the central nervous system is similar to that of the somatic sensory and visual systems, where the subcortical nuclei and cortical areas have a somatotopic or retinotopic organization. In each of these cases, the topographic organization of the central representations is determined by the spatial organization of the peripheral receptive sheet. An important difference exists, however. The receptor sheets of the somatic sensory and visual systems are spatial maps representing stimulus location (eg, hand versus foot, central versus peripheral vision). The cochlea represents the frequency of sounds. Localizing where a sound originates is determined by central nervous system auditory neurons, computed on the basis of the timing, loudness, and spectral characteristics of sounds (see below).

Hair cells are innervated by the distal processes of bipolar primary sensory neurons located in the spiral ganglion. The central processes of the bipolar neurons form the cochlear division of the vestibulocochlear nerve (cranial nerve VIII). These axons project to the ipsilateral cochlear nuclei (Figure 8–2), which are located in the rostral medulla. The cochlear nuclei consist of the ventral cochlear nucleus, which has anterior and posterior subdivisions, and the dorsal cochlear nucleus. Neurons in these three components have distinct connections with the rest of the auditory system and give rise to parallel auditory pathways that serve different aspects of hearing. A key function of the ventral cochlear nucleus is horizontal localization of sound. In addition, some neurons in the posteroventral division contribute to a system of connections that regulate hair cell sensitivity. The ventral cochlear nucleus projects to the superior olivary complex, a cluster of nuclei in the caudal pons. Most neurons in the superior olivary complex project via an ascending pathway called the lateral lemniscus to the inferior colliculus, located in the midbrain. The projection from the ventral cochlear nucleus to the inferior colliculus is bilateral, reflecting the importance of binaural mechanisms for horizontal (side-to-side) localization of sounds. The dorsal cochlear nucleus is thought to play a role in identifying sound source elevation as well as identifying complex spectral characteristics of sounds. It projects directly to the contralateral inferior colliculus, also via the lateral lemniscus. The inferior colliculus is the site of convergence of all lower brain stem auditory nuclei. It is tonotopically organized and contains a spatial map of the location of sounds.

In sequence, the next segment of the ascending auditory pathway is the medial geniculate nucleus, the thalamic auditory relay nucleus. The medial geniculate nucleus projects to the primary auditory cortex, located within the lateral sulcus (also called the Sylvian fissure) on the superior surface of the temporal lobe. The primary auditory cortex contains multiple tonotopically organized territories, all located on Heschl's gyri (Figure 8–2B, inset; see Figure 8–8). The primary cortex forms a central core surrounded by multiple secondary auditory areas that form a belt around the primary core. Neurons in the primary core are activated by simple tones, whereas those in the surrounding belt of secondary areas are better activated by complex sounds. Several higher-order auditory areas adjoin the secondary areas on the superior and lateral surfaces of the temporal lobe in the superior temporal gyrus and sulcus (Figure 8–2B). This is where several areas are located that are important for understanding speech (see below).

There is logic to the organization of the projections from the primary cortex, much like that of the visual system's what and where (or how) pathways (see Figure 7–15). There is a ventral stream that originates anteriorly and projects to the ventral portion of the frontal lobe, including Broca's area. This path may be analogous to the "what" path and is thought to be important in identifying the source of speech, such as a bark from a dog or meow from a cat. It is also a path involved in analyzing the linguistic meaning of sounds. There is a dorsal stream that originates caudally and projects to the parietal lobe and, from there, preferentially to the dorsolateral prefrontal and premotor cortical areas of the frontal lobe. This path is thought to be more important for spatial localization of the source of sounds and for using sounds for actions.

The auditory pathways contain decussations and commissures—where axons cross the midline—at multiple levels, so that sounds from one ear are processed by both sides of the brain. The bilateral representation of sounds provides a mechanism for sound localization (see below) and enhancing the detection of sounds through summation of converging inputs. Apart from sound localization, what is the clinical significance of this bilateral organization of central auditory connections? Unilateral brain damage does not cause deafness in one ear unless the injury destroys the cochlear nuclei or the entering fascicles of the cochlear nerve. Unilateral deafness is thus a sign of injury to the peripheral auditory organ or the cochlear nerve. As discussed in later sections of the chapter, unilateral damage to central auditory centers produces impairment in localizing and interpreting sounds or linguistic disorders, not deafness.

The Auditory Sensory Organs Are Located Within the Membranous Labyrinth

The membranous labyrinth is a complex sac within the bony labyrinth, cavities in the petrous portion of the temporal bone (Figure 8–3). The membranous labyrinth consists of the auditory sensory organ, the cochlea, and five vestibular sensory organs, the three semicircular canals, and the saccule and utricle (Figure 8–3A). (Another name for the semicircular canals, utricle, and saccule is the vestibular labyrinth.) The morphological complexity of the auditory and vestibular sensory organs rivals that of the eyeball. Vestibular sensory organs mediate our sense of acceleration, such as during takeoff in a jet, and are important in balance and eye movement control. The vestibular system is considered in Chapter 12. Much of the membranous labyrinth is filled with endolymph, an extracellular fluid resembling intracellular fluid in its ionic constituents. Endolymph has a high potassium concentration and low sodium concentration. Perilymph, a fluid resembling extracellular fluid and cerebrospinal fluid, fills the space between the membranous labyrinth and the temporal bone.

The cochlea is a coiled structure about 30 mm long (Figure 8–3A). The hair cells are located in the organ of Corti, a specialized portion of the cochlear duct that rests on the basilar membrane (Figure 8–3C). Hair cells of the organ of Corti are covered by the tectorial membrane (Figure 8–3C). The basilar membrane, hair cells, and tectorial membrane collectively form the basic auditory transductive apparatus. Two kinds of hair cells are found in the organ of Corti, and their names reflect their position with respect to the axis of the coiled cochlea: inner and outer hair cells. Inner hair cells are arranged in a single row, whereas outer hair cells are arranged in three or four rows. Although there are fewer inner than outer hair cells (approximately 3500 vs 12,000), the inner hair cells are responsible for frequency and other fine discriminations in hearing. This is because most of the axons in the cochlear division of cranial nerve VIII innervate the inner hair cells. Each inner hair cell is innervated by as many as 10 auditory nerve fibers, and each auditory fiber contacts only a single, or at most a few, inner hair cells. This is a high-resolution system, like that of the innervation of the fingertips and the fovea. By contrast, only a small fraction of auditory nerve fibers innervates the outer hair cell population. Each fiber branches to contact multiple outer hair cells. Research has shown that outer hair cells are important as efferent structures, modulating the sensitivity of the organ of Corti (see the section on the olivocochlear system, below).

The organ of Corti transduces sounds into neural signals. This organ is mechanically coupled to the external environment by the tympanic membrane and the middle ear ossicles (malleus, incus, and stapes), the smallest bones of the body (Figure 8–3A). Pressure changes in the external auditory meatus, resulting from sound waves, cause the tympanic membrane to vibrate. The middle ear ossicles—the malleus, incus, and stapes—conduct the external pressure changes from the tympanic membrane to the scala vestibuli of the inner ear (Figure 8–3B). These pressure changes are conducted from the scala vestibuli through the fluid to the other compartments of the cochlea, the scala media to the scala tympani (Figure 8–3B). Pressure changes resulting from sounds set up a traveling wave along the compliant basilar membrane (Figure 8–3C), on which the hair cells and their support structures rest. Because the hair cells have hair bundles that are embedded in the less compliant tectorial membrane, the traveling wave results in shearing forces between the two membranes. These shearing forces cause the hair bundles to bend, resulting in a membrane conductance change in the hair cells.

Hearing thus depends on movement of the basilar membrane produced by sounds. Outer hair cells can enhance this movement, thereby amplifying the signal generated by the organ of Corti in response to sound. They do so by changing their length in response to sounds (see section on the olivocochlear system, below). This results in a small additional displacement of the basilar membrane that increases the mechanical oscillation produced by changes in sound pressure on the tympanic membrane.

The traveling wave on the basilar membrane, established by changes in sound pressure impinging on the ear resulting from sounds, is extraordinarily complex. High-frequency sounds generate a wave on the basilar membrane with a peak amplitude close to the base of the cochlea; consequently, these sounds preferentially activate the basal hair cells. As the frequency of the sound source decreases, the location of the peak amplitude of the wave on the basilar membrane shifts continuously toward the cochlear apex. This results in the preferential low-frequency activation of hair cells that are located closer to the cochlear apex. Although the mechanical properties of the basilar membrane are a key determinant of the auditory tuning of hair cells and the tonotopic organization of the organ of Corti, other factors play important roles. For example, the length of the hair bundle varies with position within the cochlea. The bundles act as miniature tuning forks: The shorter bundles are tuned to high frequencies (and are located on hair cells at the cochlear base), whereas the longer bundles are tuned to low frequencies (and are located on hair cells at the apex). The electrical membrane characteristics of hair cells also contribute to frequency tuning. As is discussed in the next section, the tonotopic organization underlies the topography of connections in the central auditory pathways.

The Cochlear Nuclei Are the First Central Nervous System Relays for Auditory Information

The cochlear nuclei, located in the rostral medulla, comprise the ventral cochlear nucleus, which has anterior and posterior subdivisions, and the dorsal cochlear nucleus (Figure 8–4C). The dorsal and ventral cochlear nuclei are each tonotopically organized and have distinctive functions. The ventral cochlear nucleus is important for horizontal sound localization. In addition, some of the neurons in the posteroventral component engage a system for regulating hair cell sensitivity. The ventral cochlear nucleus projects bilaterally to the superior olivary complex. Whereas we know much about the physiological characteristics of neurons in the dorsal cochlear nucleus—many process the spectral characteristics of sounds—its perceptional functions are not as well understood. The dorsal cochlear nucleus is thought to be important for vertical sound localization, which depends on spectral information (see next section), and for analyzing complex sounds. It projects directly to the contralateral inferior colliculus, bypassing the superior olivary complex.

Most of the axons from each division of the cochlear nucleus decussate and reach the superior olivary complex or the inferior colliculus by one of three paths, all located in the caudal pons. First, the principal auditory decussation is the trapezoid body (Figure 8–4B), which contains crossing axons of the ventral cochlear nucleus as they travel to the superior olivary nucleus. Second, the dorsal acoustic stria carries the axons from the dorsal cochlear nucleus, as they cross to project to the inferior colliculus. Third, some axons of the posterior division of the ventral cochlear nucleus decussate in the intermediate acoustic stria. Of the three auditory decussations, only the trapezoid body is shown in Figure 8–4B because it is the only one that can be discerned without using special tracer techniques; it is also the most ventral. The trapezoid body obscures the medial lemniscus at this level.

The cochlear nucleus is the most central site in which a lesion can produce deafness in the ipsilateral ear. This is because it receives a projection from only the ipsilateral ear. Lesions of the other central auditory nuclei do not produce deafness, because at each of these sites there is convergence of auditory inputs from both ears. The anterior inferior cerebellar artery supplies the cochlear nuclei, and unilateral occlusion can produce deafness in one ear (see Figure 3–2).

The Superior Olivary Complex Processes Stimuli From Both Ears for Horizontal Sound Localization

The superior olivary complex (Figure 8–4B) contains three major components: the medial superior olivary nucleus, the lateral superior olivary nucleus, and the nucleus of the trapezoid body. The superior olivary complex should be distinguished from the inferior olivary nucleus (Figure 8–4C), which contains neurons that are important in movement control (see Chapter 13). The superior olivary complex receives input from the ventral cochlear nucleus, and gives rise to the pathway for horizontal localization of sounds (Figure 8–5). To understand how the anatomical connections between the anteroventral cochlear nucleus and the superior olivary complex contribute to this function, consider how sounds in the horizontal plane are localized. A sound is recognized as coming from one side of the head or the other by two means, depending on its frequency. Low-frequency sounds activate the two ears at slightly different times, producing a characteristic interaural time difference. The farther a sound source is located from the midline, the greater the interaural time difference. For high-frequency sounds, the interaural time difference is very small and is thus an ambiguous cue. However, the head acts as a shield and attenuates these sounds. A high-frequency sound arriving at the distant ear is softer than at the closer ear. This is because sound energy is absorbed by the head, resulting in an interaural intensity difference. This is the duplex theory of sound localization because the mechanisms for low and high frequencies differ.

There are distinct neuroanatomical substrates for the localization of low- and high-frequency sounds (Figure 8–5). Neurons in the medial superior olivary nucleus are sensitive to interaural time differences, and in accord with the duplex theory, they respond selectively to low-frequency tones. Individual neurons in the medial superior olive receive monosynaptic connections from the ventral cochlear nuclei on both sides. Remarkably, these inputs are spatially segregated on the dendrites of medial superior olive neurons (Figure 8–5). This segregation of inputs is thought to underlie the sensitivity to interaural time differences. In contrast, neurons in the lateral superior olivary nucleus are sensitive to interaural intensity differences, and they are tuned to high-frequency stimuli. Sensitivity to interaural intensity differences is thought to be determined by convergence of a monosynaptic excitatory input from the ipsilateral ventral cochlear nucleus and a disynaptic inhibitory connection from the contralateral ventral cochlear nucleus, relayed through the nucleus of the trapezoid body (Figure 8–5).

Sounds can also be localized along the vertical axis. Here the structure of the external ear is important. The ridges in the auricle reflect sound pressure in complex ways, creating sound spectra that depend on the direction of the source. Specialized neurons within the dorsal cochlear nucleus appear to use this information to determine the elevation of the sound source. Not surprisingly, the ascending projection of the dorsal cochlear nuclei bypasses the superior olivary complex to reach the inferior colliculus directly.

The Olivocochlear System Regulates Auditory Sensitivity in the Periphery

Some neurons in the superior olivary complex are not directly involved in processing the horizontal location of the source of sounds. These neurons receive auditory information from the ventral cochlear nucleus (primarily the posteroventral subdivision) and give rise to axons that project back to the cochlea via the vestibulocochlear nerve. This efferent pathway is called the olivocochlear bundle. This olivocochlear projection regulates the sensitivity of the peripheral auditory system. This system is thought to improve auditory signal detection, to help the listener attend to particular stimuli in a noisy background, and to protect the peripheral auditory system from damage caused by overly loud sounds.

There are separate medial and lateral efferent control systems; both use acetylcholine as their neurotransmitter but affect sensitivity differently. The medial system originates from neurons near the medial superior olivary nucleus and synapse directly on outer hair cells. This system influences the mechanical properties of the basilar membrane. In vitro studies have shown that outer hair cells contract when acetylcholine is directly applied to the receptor cell. This mechanical change can modulate cochlea sensitivity and frequency tuning by boosting the basilar membrane traveling wave. The other olivocochlear efferent system originates more laterally in the superior olivary nucleus and synapses on the auditory afferent fibers, just beneath the inner hair cells. This system affects auditory afferent activity directly, not through a mechanical action on the basilar membrane.

Auditory Brain Stem Axons Ascend in the Lateral Lemniscus

The lateral lemniscus is the ascending brain stem auditory pathway (Figure 8–4A, B). (The lateral lemniscus should be distinguished from the medial lemniscus [Figure 8–4B], which relays somatic sensory information to the thalamus.) The lateral lemniscus carries axons primarily from the contralateral dorsal cochlear nucleus and the superior olivary complex (medial and lateral nuclei) to the inferior colliculus (Figure 8–6). Many of the axons in the lateral lemniscus, especially those from part of the ventral cochlear nucleus, also send collateral (ie, side) branches into the nucleus of the lateral lemniscus (Figure 8–4A). The nucleus of the lateral lemniscus contains mostly inhibitory neurons that project to the inferior colliculus. It is another site in the auditory pathway where information crosses the midline.

The Inferior Colliculus Is Located in the Midbrain Tectum

The inferior colliculus is located on the dorsal surface of the midbrain, caudal to the superior colliculus (Figure 8–6A). The inferior colliculus is an auditory relay nucleus where virtually all ascending fibers in the lateral lemniscus synapse. Recall that the superior colliculus is part of the visual system. It is not a relay nucleus but, rather, participates in visuomotor control (see Chapters 7 and 12). Although the two colliculi look similar on myelin-stained sections, they can be distinguished by the configuration of structures within the center of the midbrain at the two levels (Figure 8–6B1, 2). The superior and inferior colliculi are imaged parasagittally in the MRI in Figure 8–6B3.

Three-component nuclei comprise the inferior colliculus: the central and external nuclei and the dorsal cortex. The central nucleus of the inferior colliculus is the principal site of termination of the lateral lemniscus. This nucleus receives convergent input from three major sources: (1) pathways originating from the superior olivary nuclei, (2) the direct pathway from the dorsal cochlear nucleus, and (3) axons from the nucleus of the lateral lemniscus. The central nucleus, receiving convergent information from the ventral and dorsal cochlear nuclei for horizontal and vertical sound source localization, contains a map of auditory space. The central nucleus is tonotopically organized and laminated (although not apparent on myelin-stained sections): Neurons in a single lamina are maximally sensitive to similar tonal frequencies. As in the somatic sensory and visual systems, lamination is used in the auditory system for packaging neurons with similar functional attributes or connections. The central nucleus gives rise to a tonotopically organized ascending auditory pathway to the thalamus, which continues to the primary auditory cortex.

The functions of the external nucleus and dorsal cortex are not well understood. Animal studies suggest that the external nucleus may participate in acousticomotor function, such as orienting the head and body axis to auditory stimuli. This function of the external nucleus may also use somatic sensory information, which is also projected to this nucleus from the spinal cord and medulla, via the spinotectal and trigeminotectal tracts.

The tract through which the inferior colliculus projects to the thalamus is located just beneath the dorsal surface of the midbrain, the brachium of inferior colliculus (Figure 8–6A). As different as the superior and inferior colliculi are, so too are their brachia. The brachium of the superior colliculus brings afferent information to the superior colliculus, whereas that of the inferior colliculus is an efferent pathway carrying axons away from the inferior colliculus to the medial geniculate nucleus (see next section).

The Medial Geniculate Nucleus Is the Thalamic Auditory Relay Nucleus

The medial geniculate nucleus is located on the inferior surface of the thalamus, medial to the visual relay, the lateral geniculate nucleus (Figures 8–6A and 8–7). The medial geniculate nucleus comprises several divisions, but only the ventral division is the principal auditory relay nucleus (see Figure AII–15. This component, referred to simply as the medial geniculate nucleus, is the only portion that is tonotopically organized. It receives the major ascending auditory projection from the central nucleus of the inferior colliculus. Although not observable on the myelin-stained section, the ventral division of the medial geniculate nucleus is laminated. Like the central nucleus of the inferior colliculus, individual laminae in the medial geniculate nucleus contain neurons that are maximally sensitive to similar frequencies. The medial geniculate nucleus, like the lateral geniculate nucleus (Figure 7–11), terminates predominantly in layer IV of the primary auditory cortex.

The other divisions of the medial geniculate nucleus (dorsal and medial) receive inputs from the three components of the inferior colliculus as well as somatic sensory and visual information. Rather than relaying auditory information to the cortex, they seem to serve more integrated functions, such as participating in arousal mechanisms. (The dorsal division is shown in Figure AII–15.)

The Primary Auditory Cortex Comprises Several Tonotopically Organized Representations Within Heschl's Gyri

The auditory cortical areas have a concentric, hierarchical organization. The primary cortex is surrounded by the secondary auditory cortex, which is surrounded by higher-order auditory areas (Figure 8–8). The primary auditory cortex (cytoarchitectonic area 41) is located in the temporal lobe within the lateral sulcus, on Heschl's gyri (Figures 8–8 and 8–9). These gyri, which vary in number from one to several depending on the side of the brain and the individual, run obliquely from the lateral surface of the cortex medially to the insular region (Figure 8–9). The orientation of Heschl's gyri is nearly orthogonal to the gyri on the lateral surface of the temporal lobe, hence the frequently used term transverse gyri of Heschl. The primary cortex, receiving direct thalamic inputs from the medial geniculate nucleus, processes basic auditory stimulus attributes. The primary auditory cortex is tonotopically organized along the axis of Heschl's gyri, from low frequencies lateral to high frequencies medial. Although not yet well characterized in humans, several tonotopically organized subregions are found within this primary sensory area. This organization of multiple representations of the receptor sheet may be similar to the primary somatic sensory cortex, which has multiple somatotopically organized subdivisions (see Figure 4–13). As in other sensory cortical areas, the primary auditory cortex has a columnar (or vertical) organization: Neurons sensitive to similar frequencies are arranged across all six layers, from the pial surface to the white matter. Within the primary cortex, neurons represent other features of auditory stimuli besides frequency, including particular binaural interactions, stimulus timing, and additional tuning characteristics.

Caudal Secondary and Higher-order Auditory Areas Give Rise to Projections for Distinguishing the Location of Sounds

The secondary and higher-order auditory areas form concentric belts surrounding much of the primary core region (Figure 8–8). The secondary areas receive their principal input from the primary areas and, in turn, provide information to higher-order areas. Primary cortex neurons respond to simple stimulus attributes. Not surprisingly, primary cortex neurons respond to simple pure tones as well as the tonal qualities of more complex sounds. By contrast, neurons in the secondary and higher-order areas respond selectively to more complex aspects of sounds (Figure 8–9). In animals, neurons in the higher-order auditory areas respond to species-specific calls, and in humans, to speech.

There are a myriad of cortical auditory areas, up to 15 by some counts, with at least two major streams of auditory information flow that are strikingly similar to the "what" and "where-how" paths of the visual system (see Figure 7–15). Research in animals, using anatomical tracing techniques, and in humans, using noninvasive functional imaging techniques, has revealed a dorsal pathway for localizing sound sources and using sounds to guide movements. This "where–how" pathway originates from the primary cortex and projects to caudal portions of the secondary and then higher-order areas in the superior temporal gyrus (Figure 8–10). Studies in animals using axon tracing techniques and in humans using diffusion tensor imaging (DTI; see Box 2–2) revealed a long-distance connection between the posterior temporal lobe and the parietal lobe (Figure 8–11A). Receiving converging information from the somatic sensory and visual systems, together with this auditory information, the posterior parietal lobe constructs a representation of extrapersonal space that the brain uses to help establish where we are and where stimuli occur in relation to the world around us. Also using DTI, another connection has been demonstrated between the posterior superior temporal gyrus and two areas of the frontal lobe, the premotor cortex and dorsolateral prefrontal cortex (Figure 8–11B). These frontal areas participate in the planning of movements, receiving information about "where" we wish to move from the parietal lobe (Figure 8–10), and transmitting control signals to the motor cortex about "how" to move. Interestingly, this connection to the frontal lobe travels in the arcuate fasciculus, a C-shaped pathway (Figures 8–10B and 8–11) that curves around the lateral sulcus.

Rostral Secondary and Higher-Order Auditory Areas Give Rise to Projections for Processing the Linguistic Characteristics of Sounds

A second cortical auditory pathway is involved in processing nonspatial characteristics of sounds. This path originates from the primary auditory cortex and projects to rostral portions of the secondary and higher-order areas in the superior temporal gyrus and then to the inferior frontal lobe (Figure 8–10). In monkeys, neurons in this area respond to species-specific calls. Using DTI in humans, a long-distance pathway between the rostral superior temporal gyrus and Broca's area, the motor speech area, has been revealed (Figure 8–11C). In addition to serving a linguistic function, it is thought that the connections between the rostral superior temporal gyrus and ventral frontal lobe are important for identifying the source of speech: who is speaking or "what" is emitting sounds. This pathway may travel within the uncinate fasciculus (Figure AII–22). DTI also reveals a link between Broca's area and the inferior parietal lobule (Figure 8–11C), an area long known for its importance in language.

Damage to Frontotemporal Regions in the Left Hemisphere Produces Aphasias

Several higher-order auditory cortical areas on the lateral surface of the left temporal lobe in the human brain (cytoarchitectonic areas 42 and 22) comprise important substrates for understanding speech. Damage to certain areas of the brain can produce a language impairment, or aphasia. Damage to the left temporal lobe produces an impairment in understanding speech. Remarkably, words can be spoken well but their positions in sentences are often meaningless. This is sometimes referred to as a "word salad." This kind of impairment has been attributed to an interruption in the function of Wernicke's area, and is termed Wernicke's aphasia. Wernicke's area is thought to be located in the posterior superior temporal gyrus, in cytoarchitectonic area 22 (see Table 2–2 and Figure 2–19). However, modern neuropsychological studies point to more significant speech disorders with rostral superior temporal gyrus lesions. Indeed, in Wernicke's original description of the effects of temporal lobe lesion, he placed the critical area along the entire extent rostro-caudal length of the superior temporal gyrus, not just caudally.

Whereas the left temporal lobe is important in understanding or the sensory processing of speech, Broca's area, in the left inferior frontal gyrus, is the motor speech area. This region includes the frontal operculum and corresponds approximately to cytoarchitectonic areas 44 and 45 (see Table 2–2 and Figure 2–19). Damage to Broca's area impairs the ability to express language; this is termed Broca's aphasia. Speech is labored; it is slow to start and frequently halted.

Homotopic areas in the right hemisphere are important for the rhythm, intonation, and emphasis of speech, not for choosing the correct words or for structuring proper sentences. These areas are especially important in emotional intonation in speech. For example, damage to the right superior temporal gyrus can impair understanding intonation and emotional content, whereas damage to the right inferior frontal gyrus impairs the ability to convey emotion in speech. Interestingly, damage to the linguistic areas of both hemispheres impairs the understanding and production of sign language.

Peripheral Auditory Apparatus

The auditory transductive apparatus, the organ of Corti, is located in the cochlea, a coiled structure within the temporal bone (Figure 8–3A, B). The hair cells (Figure 8–3C) are the auditory receptors. They are organized into a receptive sheet within the cochlea. This sheet has a precise tonotopic organization: Receptors sensitive to high frequencies are located near the cochlear base, and those sensitive to low frequencies are located near the apex. The hair cells are innervated by the peripheral processes of bipolar cells, whose cell bodies are located in the spiral ganglion. The central processes of the bipolar cells collect into the cochlear division of the vestibulocochlear (VIII) nerve (Figure 8–2).

Medulla and Pons

The cochlear division of the vestibulocochlear nerve synapses in the cochlear nuclei. The cochlear nuclei, which are located in the rostral medulla, have three main divisions (Figures 8–2A and 8–4C): the anteroventral cochlear nucleus, the posteroventral cochlear nucleus, and the dorsal cochlear nucleus. Many neurons in the anteroventral cochlear nucleus project to the superior olivary complex in the pons (Figures 8–4B and 8–5), on either the ipsilateral or the contralateral side. Neurons in the superior olivary complex project to either the ipsilateral or the contralateral inferior colliculus via the lateral lemniscus. Some of these decussating axons form a discrete commissure, the trapezoid body (Figures 8–4B and 8–5). The function of this pathway is in the horizontal localization of sounds. The posteroventral nucleus is involved in regulating hair cell sensitivity, together with the olivary cochlear system. Most of the neurons in the dorsal cochlear nucleus give rise to axons that decussate and then ascend in the lateral lemniscus (Figure 8–4A, B) to terminate in the inferior colliculus (Figures 8–6 and 8–7).

Midbrain and Thalamus

The inferior colliculus contains three main nuclei (Figure 8–6). The central nucleus, the principal auditory relay nucleus in the inferior colliculus, has a precise tonotopic organization. It projects to the medial geniculate nucleus (Figures 8–6A and 8–7), which in turn projects to the primary auditory cortex (cytoarchitectonic area 41) (Figure 8–8). The other two nuclei, the external nucleus and the dorsal cortex of the inferior colliculus, give rise to diffuse thalamocortical projections, primarily to higher-order auditory areas (Figure 8–8).

Cerebral Cortex

The primary auditory cortex is located largely on the superior surface of the temporal lobe, in Heschl's gyri (Figures 8–8 and 8–9). It has a tonotopic organization. The higher-order auditory areas, which encircle the primary area (Figures 8–8 and 8–9), receive their principal input from the primary auditory cortex. At least two projections emerge from higher-order areas. One projection, important in sound localization (ie, where), targets the posterior parietal cortex and the dorsolateral prefrontal cortex (Figures 8–10 and 8–11). A second projection—which is thought to be important in processing of complex sounds, including linguistic functions in humans—terminates in the ventral and medial prefrontal cortex. Wernicke's area is a part of the higher-order auditory cortex on the left side that is important in interpreting speech (Figures 8–2B, inset, and 8–11).