consider the differences between the auditory nervous system in animals and humans. The most obvious difference between the classical auditory nervous system in humans and that of commonly used experi- mental animals is that the auditory nerve is much longer in humans than in animals (2.5 cm [126] vs 0.8 cm in the cat [59]). The fiber tracts of the ascending auditory pathways are also in general longer in man than in small animals such as the cat [59, 126, 158] (Fig. 5.9) which has the implication that the neural travel time becomes longer in humans than in the small ani- mals commonly used in auditory research [158, 205]. We can only speculate about the functional impor- tance of these differences between the ascending audi- tory pathways in humans and that of animals that are commonly used in studies of the auditory system. The differences in the length of the auditory nerve and the length of the fiber tracts, however, are known to be important for the interpretation of auditory evoked potentials (ABR) (see Chapter 7). 4. NON-CLASSICAL ASCENDING AUDITORY PATHWAYS The term “non-classical pathways” is used in this book for the ascending auditory pathways that are different from the classical pathways. Other investiga- tors have used other names for these pathways such as “the diffuse system” that relates to the fact that neurons in the non-classical system are not as clearly tuned and they are not as clearly organized anatomi- cally as those of the classical ascending pathways. The use of the term “the polysensory system” reflects the finding that the non-classical pathways receive input from other sensory systems. Graybiel [71] described the basic anatomy of the non-classical ascending auditory system in the early 1970s. Later studies of the anatomy [3, 270, 312] have provided a general understanding of the connections in these pathways. Chapter 5 Anatomy of the Auditory Nervous System 85 FIGURE 5.8 Schematic diagram of the ascending auditory pathways from the left cochlea showing the main nuclei and their connections including the connections between the two sides (based on Ehret and Romand, 1997, The Central Auditory Pathway. New York: Oxford University Press, with permission from Oxford University Press). There are two specific differences between the clas- sical and the non-classical auditory pathways. While the ICC is a part of the classical ascending auditory system the ICX and the DC are parts of the non-classical auditory system. The neurons of the DC deliver their output to the diffuse thalamocortical auditory system. The ICX receives input from the somatosensory system (dorsal column nuclei) and provides input to the medial portion of the MGB, and to acoustic reflex pathways (other than the acoustic middle ear reflex) (Fig. 5.10) [2]. The IC has been described and labeled as the auditory reflex center as it connects to the supe- rior colliculus (SC) to control eye movements and other motor responses to auditory stimuli that are important for directional hearing (see p. 148). While the classical sensory pathways are inter- rupted by synaptic contacts with neurons in the ventral parts of the MGB of the thalamus, the non- classical sensory pathways use the dorsal and medial division of the MGB as relay (Fig. 5.10) [122]. These divisions of the MGB receive their input from the ICC and the ICX. The posterior division of the MGB (PO) receives input from the ICC and projects to the AAF cortical area. The neurons in the ventral por- tion of the auditory thalamus project to the primary auditory cortex but the neurons in the dorsal and medial parts of the thalamus project to secondary (AII) auditory cortex and association cortices thus bypassing the AI. Neurons in the dorsal auditory thalamus also proj- ect to other parts of the brain such as the lateral nucleus of the amygdala thereby providing a subcor- tical connection to the amygdala (see p. 89). These projections have functional implications that will be discussed in Chapter 10. Neurons in the non-classical pathways respond both to sound and to other sensory stimulations such as touch [4] and light while neurons in the classical auditory pathways up to and including the AI cortex only respond to sound stimulation. Neurons in the non-classical auditory pathways thus receive input from other sensory systems such as the somatosensory [4] and visual systems [11] (Fig. 5.10). While early studies have shown that the non- classical pathways branch off from the classical path- ways at the inferior colliculus [3] (Fig. 5.11) more recent studies indicate that the non-classical pathways branch off as early as the cochlear nucleus where neurons receive projections from the somatosensory system [270]. It is, however, the ICX of the IC and the 86 Section II The Auditory Nervous System FIGURE 5.9 Length of the main paths of the ascending auditory system in humans (modified from Lang et al., 1991, with permission from Springer-Verlag). BOX 5.2. ANATOMICAL DIFFERENCES BETWEEN HUMANS AND ANIMALS The differences in the nuclei in humans and those in the animals commonly used for auditory research are greatest in the superior olivary complex [158]. There are fewer small neurons in the lateral olivary nucleus, the nucleus of the trapezoidal body in humans, compared with animals such as the cat. Small cells are also fewer in the human cochlear nucleus than in the cat and the dorsal cochlear nucleus is much smaller and less developed in humans compared with the cat or other animals that are used in auditory research. Groups of large neurons are more developed in the human CN, the medial superior olivary (MSO) nuclei, and periolivary nuclei [158]. DC of the IC that usually are associated with the non-classical auditory system [3, 191, 295]. In addition to receiving auditory input, neurons of the ICX also receive input from other sensory sys- tems such as the somatosensory system (the dorsal column nuclei) [3] and from the visual and the vestibu- lar systems [11]. The dorsal division of the MGB proj- ects to the AII and the PAF cortical fields (Fig. 5.10) rather than the primary auditory cortex (AI) that is the target of the classical pathways. Another pathway from the IC to the primary cortex is via the posterior nucleus of the thalamus that sends axons to the AAF. The neurons of the medial division of the MGB project to the AAF, which may send collaterals to the reticular nucleus (RE) of the thalamus. The RE controls the excitability of neurons in the MGB. There neurons receive both inhibitory and excitatory input from the somatosensory system and probably also from the visual system. There are indications that the non-classical ascend- ing pathways are dormant in adults but active in children [220]. There are also indications that the non-classical pathways are abnormally active in con- nection with certain pathologies such as tinnitus [219] and hyperacusis (see p. 258) where it may cause phono- phobia and perhaps depression (see Chapter 10). There are some indications that the non-classical audi- tory pathways may function abnormally in certain developmental disorders (autism) [218]. 5. PARALLEL PROCESSING AND STREAM SEGREGATION The information that travels in the auditory nerve is separated in different ways while being processed in the nervous system. Two fundamentally different principles of such separation have been identified. One is parallel processing, which means that the same information is processed in different populations of neurons. The other is stream segregation, which means that different kinds of information are processed in Chapter 5 Anatomy of the Auditory Nervous System 87 DCN AN PVCN AVCN Dorsal column Nucleus Trigeminal nucleus LL ICC SAG ICX DC A I PAF AAF Auditory cortex Hypothalamus A II Dorsal MGB Ventrobasal amygdala Inferior colliculus Midline Reticular formation FIGURE 5.10 Simplified drawing of the non-classical ascending auditory pathways (Reprinted from Møller, 2006, with permission from Cambridge University Press). FIGURE 5.11 Schematic drawing of the connections from the ICC to the ICX and the DC, and connections from these structures to other nuclei. Also shown is the efferent input from the cerebral cortex to the ICX (From Møller, 2003, with permission from Elsevier). different populations of neurons. Stream segregation was first studied in the visual systems but it has later been shown to occur in other sensory systems. 5.1. Parallel Processing Parallel processing is based on branching of the ascending auditory pathways. It begins peripherally where each auditory nerve fiber bifurcate twice to connect to neurons in each of the three main divisions of the CN (Fig. 5.5B) [159, 305] (p. 80). The fiber tract that connects the ICC with the MGB (BIC) has approximately 10 times as many nerve fibers as the auditory nerve and that is another sign of parallel processing and it means that information that is represented in the neural code in the auditory nerve is divided into many separate channels before it reaches the cerebral cortex. Another example of par- allel processing is the classical and the non-classical ascending pathways. 5.2. Stream Segregation It has been demonstrated in several sensory sys- tems that populations of cells that process different kinds of information are anatomically segregated and that populations of cells with common properties are anatomically grouped together [70, 164, 330]. That different kinds of information are processed by dif- ferent populations of cells in association cortices was first recognized in studies of the visual system where it was found that spatial and object information was processed in two anatomically separate locations (streams) in the association cortices [154, 301]. These two locations were also known as the “where” and “what” streams; “where” (spatial information) was found to be processed in a dorsal part of the cortex and a ventral stream processed the “what” (object) information (Fig. 5.12). Recently, stream segregation was studied in the auditory system [104, 239, 248, 298] and it was shown that directional information (“where”) is processed in anatomically separate locations from where object information was processed. Studies in the rhesus monkey have shown that processing of different kinds of information occurs in the lateral belt of audi- tory cortex where neurons in the anterior portion of this belt prefer complex sounds such as species spe- cific communication sounds (“what”) whereas neu- rons in the caudal portion of the belt region show the greatest spatial specificity (“where”) [237, 297, 298]. Neurons in the superior temporal gyrus of the monkey (macaques) is organized in two areas with different functions. One, the most rostral stream, seems to be involved in processing of object infor- mation such as that carried by complex sounds (for instance vocalization) while neurons in the other pop- ulation of neurons that is located more caudally are involved in processing of spatial information. Auditory spatial information (directional infor- mation) is not related to the location on a receptor surface as is the case for visual and somatosensory information but spatial auditory information is derived from manipulation of information from the two ears, thus computational rather than related to a receptor surface. Speech perception is better when listening with the right ear (right ear advantage) [84] while there is no hemispheric difference with regard to identifica- tion of a speaker [118], an indication that information regarding speech perception and speech recognition is processed in different parts of the brain. More recently, neuroimaging techniques have been used to explore the anatomical site of processing of different kinds of sounds in humans [78] and it has been shown that motion produced stronger activation in the medial part of the planum temporale, and frequency-modulation produced stronger activation in the lateral part of the planum temporale, 1 as well as an additional non-primary area lateral to Heschl’s gyrus. The results of these studies were taken as indications of the existence of segregation of spatial and non-spatial auditory information. The study also 88 Section II The Auditory Nervous System 1 Planum temporale: An important structure for language [78] is the posterior surface of the superior temporal gyrus of the cerebral cortex located in the temporal lobe. It is normally larger on the left side than on the right. FIGURE 5.12 Illustration of the anatomical separation of infor- mation into two principal streams. Connections between the visual (striate) cortex and association cortices in the brain of the monkey (according to Mishkin et al., 1983, with permission from Elsevier). suggested that the superior parietal cortex is involved in the spatial pathways and that it is dependent on the task of motion detection and not simply on the presence of acoustic cues for motion. These findings indicate that engagement of processing streams is dependent on the listening task. The psychoacoustic aspects of stream segregation have been studied extensively [156, 282] and it has been related to hearing impairment (see p. 88). 5.3. Connections to Non-auditory Parts of the Brain Auditory information can reach many parts of the brain. Naturally, auditory information can control motor systems such as extraocular muscles and neck muscles. Sound can also activate reflexes such as the acoustic middle-ear reflex and the startle reflex, and it can affect wakefulness and sounds can influence the autonomic system and the endocrine systems. The IC has often been regarded as the motor center of the auditory system although it is not involved in the acoustic middle-ear reflex (see Chapter 8) but it is involved in righting reflexes through its connection to the superior colliculus. Cells in the IC connect to many other parts of the brain with much less known function. Many of these connections are dormant in adults but the synaptic efficacy of the connections to these sys- tems is dynamic and can be modulated by expression of neural plasticity. Auditory information can reach the emotional brain known as the limbic system through two fundamentally different routes (Fig. 5.13) [132]. Input to the amygdala from the auditory system can evoke fear. Both the classical and the non-classical pathways provide input to the amygdala, but through very different routes. The classical pathways provide input to the amygdala through a long route involving the primary auditory cortex, secondary auditory cortex and association cortices while the non-classical pathways provide a much shorter and subcortical route to the amygdala (Fig. 5.13). Subcortical connections from auditory pathways to limbic structures are important because the infor- mation that is mediated through such connections is probably not under conscious control. This route may be activated in certain forms of tinnitus where it can mediate fear without conscious control [219]. The non- classical pathways also have abundant projections to the reticular formation controlling wakefulness [191]. 6. DESCENDING PATHWAYS The descending pathways are at least as abundant as the ascending pathways [311, 312, 314] but much less is known about the descending pathways than the classical ascending pathways. The descending auditory pathways have often been described as two separate pathways, the corticofugal and the cortico- cochlear systems [76]. The most central part of the corticofugal system originates in the auditory cerebral cortex (Fig. 5.14A) and the cortico-cochlear system projects from the auditory cortex to the cochlear Chapter 5 Anatomy of the Auditory Nervous System 89 BOX 5.3 STREAM SEGREGATION STUDIED IN THE FLYING BAT Other evidence of stream segregation in the auditory system comes from studies of the flying bat. Bats emit sounds and use information about the reflected sound for navigation and location of prey (echolocation). In bats, the cortical representation of distance to an object is the interval between the emission of a high frequency sound and receiving of the echo of that sound. This time difference is coded in the discharge pattern of individual neurons. Sound intervals (duration of silence) that are coded in some neurons in the auditory pathways (see p. 137) [197, 321] may therefore be regarded as spatial information because it refers to a location. Bats use low frequency sounds for communication while flying and that may be regarded as object information. Studies have shown that these two kinds of information are separated at the midbrain level (inferior colliculus [IC]) but the two streams are joined again in the cerebral cortex where the same neurons process both kinds of information [241]. Sound duration may also be coded specifically in the auditory system [24, 240]. While the coding of these kinds of sounds has been studied in animals, features like duration of sounds and duration of silent intervals are important features for discrimination of speech sounds. nucleus and cochlea (Fig. 5.14B). Both systems include crossed and uncrossed pathways. The descending pathways from the auditory cortices to the thalamic sensory nuclei are especially abundant [312] and extensive descending pathways reach auditory nuclei in the brainstem [314]. Instead of classifying the descending pathways separately, it seems more appropriate to regard the descending pathways as reciprocal pathways to the ascending pathways. One large descending fiber tract originates in layers V and VI of the primary auditory cortex (Fig. 5.7B). Uninterrupted fiber tracts that originate in neurons of layer VI make synaptic connections with neurons in the MGB and neurons of layer V project to both the MGB and IC [38, 315]. The descending projections to the IC reach mainly neurons in the ICX and DC [311, 314]. The descending connections from layers V and VI may be regarded as reciprocal innervation to the ascending connections but they are often referred to as a separate descending auditory system. Descending pathways from the SOC reach the cochlear nucleus [76, 279], and even cochlea hair cells receive abundant efferent innervation (Fig. 5.15) [303]. The descending system that projects from SOC to the cochlea has two parts, one that projects mainly to the ipsilateral cochlea and the fibers of which travel close to the surface of the floor of the fourth ventricle (Fig. 5.15B) [72]. The other part of the olivocochlear system projects mainly to the contralateral cochlea and the fibers of that system travel deeper in the brainstem. The ipsilateral fibers originate in the lateral part (LSO) of the SOC. The system that mainly projects to the contralateral cochlea originates from medial part of the SOC (MSO). Both systems project to hair 90 Section II The Auditory Nervous System Dorsal medial MGB AII Ventral MGB Thalamus AAF Endocrine Behavioral Autonomic AI ICX DC ICC Amygdala Association cortices AL ABL ACE Nucleus basalis Arousal and plasticity Cerebral cortex “High Route” “Low Route” Polymodal association cortex Other cortical areas FIGURE 5.13 Schematic drawing of the connections between the classical and the non-classical routes and the lateral nucleus of the amygdala (AL), showing the “high route” and the “low route”. Connections between the basolateral (ABL) and the central nuclei (ACE) of the amygdala and other CNS structures are also shown (reprinted from Møller, 2006, with permission from Cambridge University Press; based on LeDoux, 1992). cells in the cochlea but the pathways that originate in the LSO mainly terminate on afferent fibers of inner hair cells, whereas axons of the medial system termi- nate mainly on the cell bodies of the outer hair cells. This description refers to the cat, and the olivocochlear system may be different in different animal species including humans. The fact that the response of single auditory nerve fibers are affected by contralateral sound stimulation has been attributed to the efferent innervations of cochlear hair cells [304]. The finding that cochlear microphonics is affected by electrical stimulation of the efferent bundle is taken as an indication of efferent innervations of outer hair cells [163]. Chapter 5 Anatomy of the Auditory Nervous System 91 FIGURE 5.14 Schematic drawings of the two descending systems in the cat. (A) Cortico-thalamic system. (B) Cortico-cochlear and olivocochlear systems: P =principle area of the auditory cortex; LGB = lateral genic- ulate body; D = dorsal division of the medial geniculate body; V = ventral division of the medial geniculate body; m = medial (magnocellular) division of the medial geniculate body; PC = pericentral nucleus of the inferior colliculus; EN = external nucleus of the inferior colliculus; LL = lateral lemniscus; CN (dm) = dorsal medial part of the central nucleus of the inferior colliculus; DCN = dorsal cochlear nucleus; VCN = ventral cochlear nucleus; DLPO = dorsolateral periolivary nucleus; DMPO = dorsomedial periolivary nucleus; and RF = reticular formation (reprinted from Harrison and Howe, 1974, with permission from Springer-Verlag). 92 Section II The Auditory Nervous System FIGURE 5.15 (A) Origin of efferent supply to the cochlea (reprinted from Schucknecht HF, 1974 Pathology of the Ear. Cambridge, MA: Harvard University Press, with permission from Harvard University Press). (B) Olivocochlear system in the cat. The uncrossed olivocochlear bundle (UCOCB) and the crossed olivocochlear bundle (COCB) are shown (redrawn from Pickles, 1988, with permission from Elsevier). 93 HEARING: ANATOMY, PHYSIOLOGY, Copyright © 2006 by Academic Press, Inc. AND DISORDERS OF THE AUDITORY SYSTEM Second Edition All rights of reproduction in any form reserved. 1. ABSTRACT 1. Frequency selectivity is a prominent property of the auditory nervous system that can be demonstrated at all anatomical levels. The frequency selectivity of the basilar membrane is assumed to be the originator of the frequency tuning of auditory nerve fibers and cells in the classical ascending auditory pathways. 2. The threshold of the responses of an auditory nerve fiber is lowest at one frequency known as that fiber’s characteristic frequency (CF) and a fiber is said to be tuned to that frequency. Different auditory nerve fibers are tuned to different frequencies. 3. A plot of the threshold of an auditory nerve fiber as a function of the frequency of a tone is known as a frequency threshold curve, or tuning curve. 4. Tuning curves of cells of the nuclei of the classical ascending auditory pathways have different shapes. 5. Nerve fibers of the auditory nerve, cells of auditory nuclei and those of the auditory cerebral cortex are arranged anatomically according to their characteristic frequency. This is known as tonotopical organization. 6. An auditory nerve fiber’s response to one tone can be inhibited by presentation of a second tone when that tone is within a certain range of frequencies and intensities (inhibitory tuning curves). 7. Analysis of the discharge pattern of single auditory nerve fibers in response to continuous broad band noise reveals great similarity with the tuning of the basilar membrane over a large range of stimulus intensities. 8. The waveform of a tone or of complex sounds is coded in the time pattern of discharges of single auditory nerve fibers, known as “phase-locking.” Phase-locking can be demonstrated experimentally in the auditory nerve for sounds with frequencies at least up to 5 kHz but may also exist at higher frequencies. The upper frequency limit for phase locking in auditory nuclei is lower than it is in the auditory nerve. 9. Convergence of input from many nerve fibers on one nerve cell improve the temporal precision of phase locking by a process similar to that of signal averaging. 10. The cochlea delivers a code to the auditory nervous system that yields information about both the (power) spectrum and the waveform (periodicity) of a sound. One of these two representations or both is the basis for discrimination of frequency. 11. The frequency selectivity of the basilar membrane is the basis for the place principle of frequency discrimination. Coding of the temporal pattern of sounds in the discharge pattern of auditory nerve fibers is the basis for the temporal principle of frequency discrimination. 12. Because place coding is affected by the sound intensity it may not be sufficiently robust to explain auditory frequency discrimination. The neural coding of vowels in the cat’s auditory nerve shows a higher degree of robustness of the temporal code compared with the place code. 13. The exact mechanisms of decoding the temporal code of frequency are unknown but similar CHAPTER 6 Physiology of the Auditory Nervous System neural circuits as those decoding directional information may decode temporal information about frequency. 14. The most important function of cochlea may be that it prepares sounds for temporal coding by dividing the spectra of complex sounds into (narrow) bands before conversion into a temporal code occurs. 15. Auditory nerve fibers and cells in the nuclei of the classical ascending auditory pathways respond poorly to steady state sounds. The discharge rate of most neurons reaches a plateau far below the physiologic range of sound intensities. 16. Changes in intensity or frequency of sounds are coded in the discharge pattern over a larger range of stimulus intensities than constant sounds or sounds with slowly varying frequency or intensity. 17. The response to complex sounds (the frequency or intensity of which changes) cannot be predicted from knowledge about the response to steady sounds or tone bursts. 18. Hearing with two ears improves discrimination of sounds in noise and helps select listening to one speaker in an environment where several people are talking at the same time. 19. Hearing with two ears (binaural hearing) is the basis for directional hearing, which has been of great importance in phylogenic development but it is of less apparent importance for humans than it is in many other species. 20. The physical basis of directional hearing in the horizontal plane is the difference in the arrival time and the difference in the intensity of sounds at the two ears, both factors being a function of the azimuth. 21. The time between the arrival of sounds at the two ears can be detected by neurons that receive input from both ears. The neural processing of interaural intensity differences is more complex and less studied than that of interaural time differences. 22. The physical basis for directional hearing in the vertical plane is the dependence of the elevation on the spectrum of the sounds that reaches the ear canal. This is a result of the outer ears and the shape of the head. 2. INTRODUCTION All information that is available to the auditory nervous system is contained in the neural discharge pattern of auditory nerve fibers. This information undergoes an extensive transformation in the nuclei of the classical ascending auditory pathways, which per- forms hierarchical and parallel processing of informa- tion. I have shown in the previous chapter that the auditory nervous system is more complex anatomically than that of other sensory system. It is therefore not surprising that also the processing of auditory informa- tion that occurs in the ascending auditory pathways is complex and extensive. Recognition of the existence of two parallel ascending pathways, the classical and the non-classical pathways, adds to the complexity of infor- mation processing in the auditory system. The inter- play between these two systems and the role of the vast descending pathways is not understood. The non- classical auditory system may be analogous to the pain pathways of the somatosensory system [187] and that may explain the similarities between hyperactive disor- ders of the hearing and central neuropathic pain [192]. It seems reasonable to assume that a better understand- ing of these aspects of the function of the auditory nerv- ous system is important for understanding many disorders of the auditory system and it is a necessity for developing better treatments of disorders of the audi- tory system. The introduction of cochlear implants and cochlear nucleus implants (auditory brainstem implants [ABIs]) (see Chapter 11) have made understanding of the anatomy and physiology of the auditory nervous system of clinical importance. Most studies of the function of the auditory system have aimed at the coding of different kinds of sounds in the auditory nerve and how this code changes as the information travels up the neural axis towards the cerebral cortex in the classical auditory pathways. Peripheral parts of the ascending auditory pathways have been studied more extensively than central por- tions. The physiology of the auditory nervous system has been studied mostly in experiments in animals such as the rat, guinea pig and cat. Little is known about the difference between the function of the audi- tory system in small animals and humans. The information processing that occurs in the non- classical (adjunct or extralemniscal) ascending audi- tory pathways has not been studied to any great extent and therefore little is known about the coding and transformation of information in these systems. In fact little is known about the activation of the non-classical auditory pathways in humans [220]. The function of the vast descending pathways is practically unknown with the exception of its most peripheral parts. We will therefore in this chapter focus on the processing of auditory information that occurs in the classical ascending auditory nervous system including the auditory cortex. 94 Section II The Auditory Nervous System [...]... broader band This means that the product of time and bandwidth is a constant Chapter 6 Physiology of the Auditory Nervous System 3.2 Frequency Selectivity in the Auditory Nervous System Frequency tuning of single neurons is prominent at all levels of the classical ascending auditory nervous system Auditory nerve cells of the nuclei of the ascending auditory nervous system and those of the auditory. .. hair cells make the conversion of the mechanical stimulation of hair cells into the discharge rate of single auditory nerve fibers to become non-linear Insufficient understanding of how the firing rate of single auditory nerve fibers are related to the displacement of the basilar membrane complicates interpretation of the results of studies of the frequency selectivity of the auditory system that use... selectivity of the basilar membrane while the coding of the temporal pattern of a sound is a result of the ability of hair cells to modulate the discharge pattern of single auditory nerve fibers with the waveform of the vibration of the basilar membrane (Fig 6.1) Each point on the basilar membrane can be regarded as a band-pass filter and the vibration amplitude at different points along the basilar membrane... that there is a redundancy of the representation of the spectrum of sounds in the auditory nerve Each auditory nerve fiber (type I, see Chapter 5) innervates only one inner hair cell, and the discharges of a single auditory nerve fiber are thus controlled by the vibration of a small segment of the basilar membrane This is the basis for the frequency selectivity of single auditory nerve fibers Auditory. .. The shape of the frequency tuning curves obtained by recordings from cells in the different nuclei are therefore different from those obtained from fibers of the auditory nerve This is one of the several signs of the transformation of the frequency tuning that occurs in the classical ascending auditory pathways The frequency tuning of auditory nerve fibers is a result of the frequency selectivity of. .. 42 , 179, 180] These studies showed that the frequency selectivity decreased when the intensity of the test sounds was increased above threshold The reason that the frequency selectivity of single auditory nerve fibers is intensity dependent is the non-linearity of the vibration of the basilar membrane These studies made use of the fact that the temporal pattern of discharges of single auditory nerve... Formants are the results of the acoustic properties of the vocal tract and the frequencies of the formants uniquely characterize a vowel In the time domain, each formant contributes a damped oscillation to the total waveform of a vowel The frequencies of these damped oscillations are the formant frequencies, and these damped oscillations are repeated with the frequency of the vocal cords, i.e., the fundamental... single point on the basilar membrane It has been claimed that the location of the skirts of the frequency tuning curves of the basilar membrane might vary less than the location of the peak of the basilar membrane motion when the sound intensity is changed The high frequency skirts of frequency Since the pitch of sounds changes little with sound intensity [285], the findings that the tuning of the basilar... (solid lines) and the width of the tuning of a single auditory nerve fiber (dashed line) in the auditory nerve of a rat as a function of the stimulus intensity The width is given a “Q10 dB” which is the center frequency divided by the width at 10 dB above the peak (reprinted from Møller, 1977, with permission from the American Institute of Physics) 18 Chapter 6 Physiology of the Auditory Nervous System 101... the auditory cortex is anatomically organized according to the frequency to which neurons are tuned (tonotopic organization) These tonotopic maps depend on the separation of sound on the basis of their frequencies that occurs in the cochlea, but they are altered through the processing that occurs in the nuclei of the ascending auditory pathways and the cerebral cortex The functional importance of the . between the clas- sical and the non-classical auditory pathways. While the ICC is a part of the classical ascending auditory system the ICX and the DC are parts of the non-classical auditory system. . function of the auditory nerv- ous system is important for understanding many disorders of the auditory system and it is a necessity for developing better treatments of disorders of the audi- tory system. . projections from the somatosensory system [270]. It is, however, the ICX of the IC and the 86 Section II The Auditory Nervous System FIGURE 5.9 Length of the main paths of the ascending auditory system