**1.2 Neuroplasticity**

The human brain has certain time periods—called critical periods—during which it is conducive to neuroplasticity. In these critical periods, capacities are shaped, perfected, or altered as a result of experience.

In humans, cortical neuroplasticity is most pronounced in the first years of life. During this developmental period, cortical neurons are extensively stimulated, and in this way, synapses mature and developed. In addition, various sensory and cognitive systems interact and adjust their functional properties based on prior experience and learning. Younger brains seem to be more able to change as a result of persisting stimuli [17]. These are usually related to changes (i) in behavior, (ii) in the environment, and (iii) in the neural processes.

Over the course of a lifetime, a lack of experience during critical developmental periods can hinder learning [19]. A critical period can be described by the simultaneous presence of these three conditions:


It should be emphasized that simple skills require the use of less specialized neural circuits, while more complex abilities depend on the use of more specialized ones. The simplest neural circuits need first to be activated and to be efficient, before new neural circuits can be made reliable [20, 21].

**55**

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

individual [22, 23].

and create new synapses.

plasticity [34].

multimodal neuronal stimulation.

eliminated from the system [37].

**1.3 Neuroplasticity of the auditory system**

Stimulation of any skill during the critical period of development is an extremely important factor for the success of any intervention process. However, it is important to note that adult brains also have a proven ability to change. Thus, different neuronal systems can be activated regardless of the age of the

in a multimodal cerebral activation, and thus they can enhance neuroplasticity. A good example of such multisensory stimulation is the process of learning to play music [24–26]. Paraskevopoulos et al. [25] demonstrated that musicians who started their training as young adults had a greater activation of the prefrontal cortex than musicians with only short-term training. Data in the literature suggest that a wide range of beneficial effects can be manifested by elderly musical students, including improvements in attention, memory, motor function, executive function,

Intrinsic and extrinsic factors can cause changes in brain cells. Data from the literature suggest that new neurons are present after 6–8 weeks from the time an adult undertakes a new skill [31, 32]. It is therefore suggested that learning and maintaining a new activity should be encouraged in order to activate neural circuits

Neuroplasticity has been associated with a delayed onset of dementia. Broolmeyer et al. [33] state that brain plasticity should be made a priority in dealing with individuals who have dementia. Concomitantly, age-related cognitive decline can be delayed, interrupted, or even reversed by introducing tasks that involve

By recognizing the importance of neuroplasticity, professionals involved in rehabilitation are encouraged to turn their efforts toward stimulating, motivating,

Like other systems, the development of the central auditory nervous system depends on a critical period during the first years of life when responses to different stimuli and sound environments are gradually established. In the auditory system the capacity for anatomical and functional modification is called auditory neuro-

The cortical areas that encompass the auditory system develop rapidly in the first years of life, due to an abundance of neuronal connections [35, 36]. At approximately 4 years of age, the neurons responsible for hearing go through a process which is called pruning, where neurons and synapses which are not activated are

Although the plasticity due to experience is far greater in the first years of life, it is known that the auditory system has some malleability throughout life [37]. Sharma et al. [38] established that there was a difference between what is known as a critical period and a sensitive period. According to Sharma, the critical period ends suddenly, and the neural system is unable to adapt to stimuli; in contrast, the sensitive period is an ideal neuroplastic period during which sound can be introduced into the auditory cortex and promote normal age-appropriate development. Preterm infants who remain long periods in a neonatal intensive care unit (NICU) are often exposed to high ambient noise levels, generated by the hospital equipment. The high-frequency sounds can cause acoustic trauma and hamper the proper development of the central auditory nervous system [39]. According to Zhang et al., excessive noise at critical periods of development can lead to impaired

cortical tonotopic maps, resulting in a reduction in neural synchrony and a

creating, and developing new strategies for the treatment of their patients.

creativity, anxiety reduction, and visual scanning [27–30].

Activities which depend on the integration of different neuronal systems engage

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

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

abilities therefore have similar symptoms to those who have other pathologies such as dyslexia and attention deficit hyperactivity disorder, and as a consequence they

The human brain has certain time periods—called critical periods—during which it is conducive to neuroplasticity. In these critical periods, capacities are

In humans, cortical neuroplasticity is most pronounced in the first years of life. During this developmental period, cortical neurons are extensively stimulated, and in this way, synapses mature and developed. In addition, various sensory and cognitive systems interact and adjust their functional properties based on prior experience and learning. Younger brains seem to be more able to change as a result of persisting stimuli [17]. These are usually related to changes (i) in behavior, (ii) in

Over the course of a lifetime, a lack of experience during critical developmental periods can hinder learning [19]. A critical period can be described by the simulta-

ii.The neural connections need to be intact to be able to process information,

modifications in the morphologies of axons and dendrites and modification

i.The information must be reliable and extremely precise.

either through inhibitory and/or excitatory connections [18].

iii.Mechanisms must be in place to sustain the plasticity process, such as

It should be emphasized that simple skills require the use of less specialized neural circuits, while more complex abilities depend on the use of more specialized ones. The simplest neural circuits need first to be activated and to be efficient,

may have attention and executive deficits as well [16].

*Schematic representation of the auditory pathways.*

shaped, perfected, or altered as a result of experience.

the environment, and (iii) in the neural processes.

neous presence of these three conditions:

to synaptic connections.

before new neural circuits can be made reliable [20, 21].

**1.2 Neuroplasticity**

**Figure 2.**

**54**

Stimulation of any skill during the critical period of development is an extremely important factor for the success of any intervention process. However, it is important to note that adult brains also have a proven ability to change. Thus, different neuronal systems can be activated regardless of the age of the individual [22, 23].

Activities which depend on the integration of different neuronal systems engage in a multimodal cerebral activation, and thus they can enhance neuroplasticity. A good example of such multisensory stimulation is the process of learning to play music [24–26]. Paraskevopoulos et al. [25] demonstrated that musicians who started their training as young adults had a greater activation of the prefrontal cortex than musicians with only short-term training. Data in the literature suggest that a wide range of beneficial effects can be manifested by elderly musical students, including improvements in attention, memory, motor function, executive function, creativity, anxiety reduction, and visual scanning [27–30].

Intrinsic and extrinsic factors can cause changes in brain cells. Data from the literature suggest that new neurons are present after 6–8 weeks from the time an adult undertakes a new skill [31, 32]. It is therefore suggested that learning and maintaining a new activity should be encouraged in order to activate neural circuits and create new synapses.

Neuroplasticity has been associated with a delayed onset of dementia. Broolmeyer et al. [33] state that brain plasticity should be made a priority in dealing with individuals who have dementia. Concomitantly, age-related cognitive decline can be delayed, interrupted, or even reversed by introducing tasks that involve multimodal neuronal stimulation.

By recognizing the importance of neuroplasticity, professionals involved in rehabilitation are encouraged to turn their efforts toward stimulating, motivating, creating, and developing new strategies for the treatment of their patients.

### **1.3 Neuroplasticity of the auditory system**

Like other systems, the development of the central auditory nervous system depends on a critical period during the first years of life when responses to different stimuli and sound environments are gradually established. In the auditory system the capacity for anatomical and functional modification is called auditory neuroplasticity [34].

The cortical areas that encompass the auditory system develop rapidly in the first years of life, due to an abundance of neuronal connections [35, 36]. At approximately 4 years of age, the neurons responsible for hearing go through a process which is called pruning, where neurons and synapses which are not activated are eliminated from the system [37].

Although the plasticity due to experience is far greater in the first years of life, it is known that the auditory system has some malleability throughout life [37]. Sharma et al. [38] established that there was a difference between what is known as a critical period and a sensitive period. According to Sharma, the critical period ends suddenly, and the neural system is unable to adapt to stimuli; in contrast, the sensitive period is an ideal neuroplastic period during which sound can be introduced into the auditory cortex and promote normal age-appropriate development.

Preterm infants who remain long periods in a neonatal intensive care unit (NICU) are often exposed to high ambient noise levels, generated by the hospital equipment. The high-frequency sounds can cause acoustic trauma and hamper the proper development of the central auditory nervous system [39]. According to Zhang et al., excessive noise at critical periods of development can lead to impaired cortical tonotopic maps, resulting in a reduction in neural synchrony and a

decreased sensitivity to particular frequencies [40]. In addition, the extra noise can mask speech sounds, thereby impoverishing the auditory experience. As a result, infants can become more sensitive to noise and focus their attention on this type of sound stimulus instead of ignoring it and focusing on speech [41]. Among preterm infants there is a high rate of impairment of hearing, language, and attention; on the other hand, a home environment rich in post-NICU auditory and linguistic stimuli favors auditory neuroplasticity, meaning that premature infants then have a good chance of developing normal speech, language, and learning [42].

One way to observe plasticity in the auditory system is by monitoring patients undergoing cochlear implantation. Even after a period of auditory deprivation due to hearing loss, it is possible for the brain's auditory system to reorganize and develop better hearing abilities. Research on children implanted at the age of 3, 5, and 7 years has demonstrated that cortical auditory development can be mixed, with some children presenting cortical auditory evoked potential responses (notably P1) within normal limits, while others do not seem to achieve normal central auditory maturity. These findings are consistent with positron emission tomography (PET) imaging tests performed before and after cochlear implantation. It appears that 3.5 years of age is the end of the sensitive period for cochlear implantation in children with congenital deafness; this age is approximately when the observed exponential increase in synaptic density ends and begins to decrease [35]. Beyond 7 years of age, neuroplasticity in the central auditory system is significantly reduced; if new sounds are introduced after this time, the auditory cortex is unable to process auditory information normally [38, 43]. Research on the development of speech and language skills in children has indicated significantly better outcomes in those who received cochlear implants at younger ages [44, 45].

Neuroplasticity can be observed in individuals with central auditory processing disorder (CAPD) who have undergone auditory training. Training is a therapeutic procedure involving auditory stimulation that leads to reorganization (remapping) of the cortex and brainstem, improving synaptic efficiency and increasing neural density. These neurophysiological changes, reflected on behavioral changes, have encouraged the use of this rehabilitation strategy [46–48].
