**1. Introduction**

### **1.1 The central auditory nervous system**

The central nervous system is bilaterally symmetrical, and it is composed of seven main regions: the spinal cord, the bulb, the bridge, the cerebellum, the midbrain, the diencephalon, and the cerebral hemispheres. Each of these neural regions performs a number of specific functions. Additionally, each function, whether it is a sensory, a motor, or another integrative task, is performed by more than one neural pathway [1, 2].

The linguistical cerebral functions are mainly located in the auditory cortex, which is divided into four anatomically distinct lobes: the frontal, the parietal, the occipital, and the temporal lobe. The latter is responsible for the function of hearing as well as for various aspects of learning, of memory, and of emotions (**Figure 1**).

The cerebral hemispheres are characterized by two important organizational features:


Broca, Wernicke, and Penfield were pioneers in unraveling the functions of the temporal lobe. Penfield found that a stimulation of primary auditory areas produced gross auditory sensations, whereas stimulation in the superior temporal gyrus produced altered perception of auditory sounds, illusions, and hallucinations.

**Figure 1.** *The four cerebral lobes of the auditory cortex.*

Studies on epileptic patients, whose hemispheres were separated by a section of the corpus callosum, have allowed us to understand numerous details related to the concept of hemispheric specialization [3].

An important contribution for the understanding of hemispheric function was achieved through the development of the Wada test [4]. The latter was developed in order to determine the dominant hemisphere for speech, so that inadvertent lesions of the speech centers, during neurosurgical procedures, could be avoided. During the test, the patient is instructed to count aloud, while sodium nitrite, a fast-acting barbiturate, is injected into the left or right carotid artery. The drug preferentially accesses the hemisphere on the same side as the injection, causing a brief speech dysfunction. If the dominant hemisphere's speech center is affected, the patient usually stops counting. With this test, the relationship between hemispheric specialization and laterality can be assessed, especially in left-handed subjects. Data in the literature [4] suggest that 96% of the right-handed people have dominant speech centers in the left hemisphere, while 15% of left-handed people have dominant speech centers in the right hemisphere. In some left-handed people, speech is controlled by both hemispheres. In such cases the administration of sodium nitrite does not suppress speech. Similar results were observed in trials involving hearing. When sounds were presented at the same time in both ears, it was found that in right-handed people the left ear performed better with nonverbal sounds.

The auditory system can be considered a high-performance signal-processing region, presenting a complex built-in hierarchy. Within the system each structure has a specific function, and progressing upwards along the pathways, these functions become more specific and dependent on the functional and physiological integrity of the previous structures. The auditory system plays an essential role for the communication among the members of the same species. Additionally, humans use the sensory inputs of the auditory system to identify different sounds leading to immediate actions, such as the status of alertness caused by the perception of siren sounds from police cars and ambulances.

From a simplified point of view, the auditory system consists of two areas, namely, the auditory periphery and the central auditory system. In each there are several structures which are stimulated when a sound stimulus is presented. Some researchers [5], however, have disputed this simple division of the auditory system suggesting instead a three-stage depiction:

i.The periphery, which captures and converts the acoustic sound stimuli into electrical neural pulses.

**53**

*Neuroplasticity and the Auditory System DOI: http://dx.doi.org/10.5772/intechopen.90085*

acoustical stimuli

familiar, though memory connections [7].

the auditory cortex to the hair cells [8].

symbolic noise in patients with a left lesion [11].

children never achieved good performance in language tasks.

ii.The brainstem, which performs the initial processing of the information

iii.The thalamocortical region, which is responsible for more advanced functions and produces emotional, cognitive, and linguistic responses from the

The efficiency of this set of interconnected structures depends primarily on the auditory experience. The simplest auditory task is influenced by high-level func-

From a functional point of view, the following functions (among others) are assigned to CANS: the ability to detect and discriminate sound sources, the separation of acoustic stimuli from the background noise, the process of understanding the incoming stimuli, and the process of recognizing the stimuli as something

There are two pathways in the CANS: the ascending (afferent) and the descending (efferent) pathway. The afferent pathway is the path that an impulse, generated in the hair cells, travels along to the auditory cortex), whereas the efferent auditory pathway is a similar path, but in the opposite direction, conducting impulses from

It should be emphasized that information from sound stimuli is sent to the brain through both ipsilateral and contralateral pathways, the result of which provides data on signal timing and stimulus intensity. These are passed on to associative areas which process the data in a differentiated way, with the left hemispheric dominance for the processing of language and the right hemisphere for the process of melody. The data on the hemispheric dominance are derived from studies in patients with cortical lesions revealing a loss of recognition of family songs and prosody in patients with right-sided injury and poor recognition of verbal language and

Rees [12] identified auditory perception deficits as one of the causes of language disorders and concluded that auditory abilities seem to play a major role in language and learning. Lubert [13] reported that a deficiency in the ability to detect acoustic characteristics of an auditory signal was overwhelmingly important that affected

There is a strong interest in the literature on the impact of auditory processing deficits on language skills and reading [14]. However, the nature of the relationship between auditory and speech processing continues to be debated [15]. The starting point for differentiating auditory processing from that of language consists of a knowledge of acoustic, phonemic, and linguistic characteristics in a behavioral and neurological way. Individuals who have primary deficits in their auditory perceptual

The afferent and efferent pathways act in an integrated way. The afferent auditory pathway has a bilateral and predominantly contralateral auditory representation. The propagation of auditory information occurs via the cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate body up to the auditory area of the temporal lobe in the cerebral cortex. The efferent auditory pathway is composed by the medial and lateral olivocochlear bundles which have anatomical and physiological differences, coordinating the independent function of the two ears [9]. The function of the efferent auditory feedback includes the electrical modulation of the outer hair cells in the cochlea, the reduction of the cochlear nerve action potentials, the protection against noise, the localization of a sound source, the improvement in the detection of sound sources in noisy environments, and the focusing of attention to the incoming acoustic stimuli, which is less effective in patients with tinnitus [9, 10] (**Figure 2**).

through modulation and interaction of the signals.

tions that include motivation, memory, and decision-making [6].

*The Human Auditory System - Basic Features and Updates on Audiological Diagnosis and Therapy*

Studies on epileptic patients, whose hemispheres were separated by a section of the corpus callosum, have allowed us to understand numerous details related to the

An important contribution for the understanding of hemispheric function was achieved through the development of the Wada test [4]. The latter was developed in order to determine the dominant hemisphere for speech, so that inadvertent lesions of the speech centers, during neurosurgical procedures, could be avoided. During the test, the patient is instructed to count aloud, while sodium nitrite, a fast-acting barbiturate, is injected into the left or right carotid artery. The drug preferentially accesses the hemisphere on the same side as the injection, causing a brief speech dysfunction. If the dominant hemisphere's speech center is affected, the patient usually stops counting. With this test, the relationship between hemispheric

specialization and laterality can be assessed, especially in left-handed subjects. Data in the literature [4] suggest that 96% of the right-handed people have dominant speech centers in the left hemisphere, while 15% of left-handed people have dominant speech centers in the right hemisphere. In some left-handed people, speech is controlled by both hemispheres. In such cases the administration of sodium nitrite does not suppress speech. Similar results were observed in trials involving hearing. When sounds were presented at the same time in both ears, it was found that in right-handed people the left ear performed better with nonverbal sounds.

The auditory system can be considered a high-performance signal-processing region, presenting a complex built-in hierarchy. Within the system each structure has a specific function, and progressing upwards along the pathways, these functions become more specific and dependent on the functional and physiological integrity of the previous structures. The auditory system plays an essential role for the communication among the members of the same species. Additionally, humans use the sensory inputs of the auditory system to identify different sounds leading to immediate actions, such as the status of alertness caused by the perception of siren

From a simplified point of view, the auditory system consists of two areas, namely, the auditory periphery and the central auditory system. In each there are several structures which are stimulated when a sound stimulus is presented. Some researchers [5], however, have disputed this simple division of the auditory system

i.The periphery, which captures and converts the acoustic sound stimuli into

concept of hemispheric specialization [3].

*The four cerebral lobes of the auditory cortex.*

**Figure 1.**

sounds from police cars and ambulances.

suggesting instead a three-stage depiction:

electrical neural pulses.

**52**


The efficiency of this set of interconnected structures depends primarily on the auditory experience. The simplest auditory task is influenced by high-level functions that include motivation, memory, and decision-making [6].

From a functional point of view, the following functions (among others) are assigned to CANS: the ability to detect and discriminate sound sources, the separation of acoustic stimuli from the background noise, the process of understanding the incoming stimuli, and the process of recognizing the stimuli as something familiar, though memory connections [7].

There are two pathways in the CANS: the ascending (afferent) and the descending (efferent) pathway. The afferent pathway is the path that an impulse, generated in the hair cells, travels along to the auditory cortex), whereas the efferent auditory pathway is a similar path, but in the opposite direction, conducting impulses from the auditory cortex to the hair cells [8].

The afferent and efferent pathways act in an integrated way. The afferent auditory pathway has a bilateral and predominantly contralateral auditory representation. The propagation of auditory information occurs via the cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate body up to the auditory area of the temporal lobe in the cerebral cortex. The efferent auditory pathway is composed by the medial and lateral olivocochlear bundles which have anatomical and physiological differences, coordinating the independent function of the two ears [9]. The function of the efferent auditory feedback includes the electrical modulation of the outer hair cells in the cochlea, the reduction of the cochlear nerve action potentials, the protection against noise, the localization of a sound source, the improvement in the detection of sound sources in noisy environments, and the focusing of attention to the incoming acoustic stimuli, which is less effective in patients with tinnitus [9, 10] (**Figure 2**).

It should be emphasized that information from sound stimuli is sent to the brain through both ipsilateral and contralateral pathways, the result of which provides data on signal timing and stimulus intensity. These are passed on to associative areas which process the data in a differentiated way, with the left hemispheric dominance for the processing of language and the right hemisphere for the process of melody. The data on the hemispheric dominance are derived from studies in patients with cortical lesions revealing a loss of recognition of family songs and prosody in patients with right-sided injury and poor recognition of verbal language and symbolic noise in patients with a left lesion [11].

Rees [12] identified auditory perception deficits as one of the causes of language disorders and concluded that auditory abilities seem to play a major role in language and learning. Lubert [13] reported that a deficiency in the ability to detect acoustic characteristics of an auditory signal was overwhelmingly important that affected children never achieved good performance in language tasks.

There is a strong interest in the literature on the impact of auditory processing deficits on language skills and reading [14]. However, the nature of the relationship between auditory and speech processing continues to be debated [15]. The starting point for differentiating auditory processing from that of language consists of a knowledge of acoustic, phonemic, and linguistic characteristics in a behavioral and neurological way. Individuals who have primary deficits in their auditory perceptual

#### **Figure 2.**

*Schematic representation of the auditory pathways.*

abilities therefore have similar symptoms to those who have other pathologies such as dyslexia and attention deficit hyperactivity disorder, and as a consequence they may have attention and executive deficits as well [16].
