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Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7), copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.

Hearing Loss: Essential Information

1 Why Hearing Is Important in Children

Carol Flexer, Jane R. Madell, Jace Wolfe, and Erin C. Schafer

1 Why Hearing Is Important in Children

Summary

This chapter focuses on the changing world for pediatric audiologists, brought about in large part by brain research and innovations in technology. Because hearing occurs in the brain, not in the ear, auditory neural development will be a focus of this chapter, including a discussion of neuroplasticity, auditory deprivation, and critical periods of auditory development. The hows and whys of spoken language enrichment and literacy development will also be addressed in this chapter. The bottom line is that pediatric audiologists have a key role in determining the future opportunities of a child with a hearing loss, and this chapter sets that precise tone for the rest of the book.

Keywords

hearing, hearing loss, deaf, hard of hearing, children, pediatrics, neuroplasticity, auditory brain, brain, listening, auditory deprivation, critical periods, audiology, language

Key Points

Because o f technology and br ain neuroplastic ity, everyt hing we

•

knew and believed to be true about hearing loss has changed. The problem with hearing loss is that it keeps sound from

•

reaching the brain; the purpose of hearing aids, bone conduction hearing devices, and cochlear implants is to access, activate, stimulate, and grow auditory neural connections throughout the brain as the foundation for spoken language, reading, and academics. There is a distinction between hearing and listening.

•

Today’s child who would previously be referred to as “deaf”

•

without technology, or with technology available 10 or 20 years ago, may function like a child with a mild to moderate hearing loss when provided with hearing aids or a cochlear implant because critical neural connectomes have been developed through meaningful auditory stimulation. Because about 95% of children with hearing loss are born

•

to hearing and speaking families, listening and talking likely will be desired outcomes for the vast majority of families we serve; those outcomes require vigilant, consistent, and caring audiologic management.

1.1 Introduction to Pediatric Audiology

Approximately 12,000 new babies with hearing loss are identi-

ﬁed every year, according to the National Institute on Deafness

and Other Communication Disorders. In addition, estimates are

that another 4,000 to 6,000 infants and young children between

birth and 3 years of age who had passed the newborn screening

test acquire late-onset hearing loss. Therefore, ~ 16,000 to 18,000 new babies and toddlers are identiﬁed with hearing loss per year,

making hearing loss the most common birth defect.

Numerous studies over the decades demonstrate that when

hearing loss of any degree is not adequately diagnosed and treated,

it can negatively aect the speech, language, academic, emotional,

and psychosocial development of young children. secondary eects of hearing loss adversely aect a child’s development as much as or more so than the hearing loss itself does.

Recently there has been a surge of technology and information about testing and managing hearing loss in infants and children. The impetus for this surge has been newborn hearing screening. As a result of identifying and treating hearing loss in neonates,

we now are dealing with a vastly dierent population of children

with hearing loss, a population that never existed before. With

this new population, whose hearing loss is identiﬁed at birth, the secondary developmental and communicative deﬁcits of

hearing loss that were so common can now be prevented. What

has happened in the ﬁeld of hearing loss is revolutionary, and the

pediatric audiologist is in the linchpin position.

How does the pediatric audiologist of today diagnose and treat this new population of babies and children with hearing loss and their families? How does audiology, as a (health) diagnosing and treating profession, collaborate with other health care providers, early interventionists, speech-language pathologists, teachers, and, of course,

families in providing quality services? The ﬁrst step is to recognize

that, because of technology and brain neuroplasticity, everything that we used to know and believe about hearing loss has changed.

This chapter will begin with a discussion of the changing world for pediatric audiologists. Next, auditory neural development will be detailed, along with a discussion of neuroplasticity, auditory deprivation, and critical periods of auditory development. A new

context for the word “deaf” will be posited, and the chapter will

conclude with the distinction between hearing and listening.

1,2

Therefore, the

1.2 Pediatric Audiology Is

Changing

The popular book about change, Who Moved My Cheese? by

Spencer Johnson, M.D. (1998), is particularly meaningful in

the world of hearing loss.3 Changes brought about through technology and early hearing detection and intervention (EHDI) programs have permitted outcomes of listening and talking only dreamed of a few years ago. It is important to realize that the new outcomes available today do not invalidate the treatment decisions made by pediatric audiologists in the past. Audiologists did what was necessary with what was available at the time. For

example, until the 1970s, children with bilateral hearing loss were routinely ﬁtted with only one hearing aid.

With increased knowledge, we can now oer better services.

Audiologists today do the best that can be done in today’s world. Tomorrow’s world will bring new possibilities, and we will need

to “move with the cheese.” Our job as pediatric audiologists is to

I Hearing Loss: Essential Information

prepare today’s babies to be take-charge adults in the world of

2030, 2040, and 2050—not in the world of 1970, 1990, or even

2020. Because information and knowledge are the currencies of today’s cultures, listening, speaking, reading, writing, and electronic technologies must be made available to our babies and children to the fullest degree possible.

1.3 Hearing Occurs in the Brain

As a result of neurobiologic research, conversations about sensory input now focus on the brain. For example, we see with the brain; the eyes are the entryway to the brain for visual information. We smell with the brain; the nose is the pathway to the brain for olfactory stimuli. We hear with the brain; the ears are the doorway to the brain for sound/auditory information. Consequently, hearing loss is primarily a brain issue, not an ear issue; the problem with hearing loss is that it keeps sound from reaching the brain. Continuing with this analogy, hearing loss can be described as a doorway problem, not an ear problem. Hearing loss obstructs that doorway in various ways and to varying degrees, preventing auditory input from reaching the brain.

Hearing technologies break the doorway open to allow access, activation, stimulation, and development of auditory neural pathways with auditory information, including spoken language.

Pearl

The purpose of hearing technology is to get auditory information through the doorway (the ear) to the brain. There is no other purpose!

The child’s listening and spoken language outcomes are not

determined by 16,000 (or likely much fewer) hair cells or by 30,000 auditory nerve ﬁbers but by 100 billion neurons in the

brain, processing 100 trillion pieces of information per second.

There is substantial evidence that “hearing” is indeed the most eective modality for the teaching of spoken language (speech),

reading, and cognitive skills.

ampliﬁcation technologies, including cochlear implants, and early identiﬁcation and intervention, auditory brain access is available

to babies with even the most profound deafness. This brain access allows the use of a developmental model of intervention that prevents the negative developmental outcomes of hearing loss that were so common a few years ago.

1,5,6,7,8,9,10

Furthermore, with today’s

1.4 Neuroplasticity, Auditory

Deprivation, Critical Periods, and Spoken Language Enrichment

Studies of brain development show that sensory stimulation of

the auditory centers of the brain is critical and, indeed, inuences

the actual organization of auditory brain pathways. The fact is, the brain can organize itself only around the stimuli

4,11,12,1 3,14,15,16,17,18

that it receives. If complete acoustic events are received, this is

how the brain will be organized. Conversely, if hearing loss ﬁlters

some or all speech sounds from reaching auditory centers of the

brain, the brain will be organized dierently. “When we want to

remember (or learn) something we have heard, we must hear it clearly because memory can be only as clear as its original signal

. . . muddy in, muddy out.” vided by ampliﬁcation technology, is really about brain stimula-

tion, with subsequent development of auditory-neural pathways.

Signal enhancement, such as that pro-

1.4.1 Review of Auditory Anatomy and Physiology

To fully appreciate auditory brain development, both in the pres-

ence of sucient access to intelligible speech and in the context

of auditory deprivation associated with untreated congenital hearing loss, it is useful to consider a brief review of auditory anatomy and physiology as well as recent studies exploring auditory brain development.

The auditory nervous system is highly complex from the brainstem to the auditory cortex. Although the brainstem is highly involved with several aspects of auditory processing (e.g., sound localization, the processing of speech in noise [e.g., the release from masking, which, in regard to the understanding of speech

in noise, is often referred to as the cocktail party eect], and in the tuning and reﬁnement of the auditory signal to facilitate

processing at higher levels), the focus of this chapter resides at the level of the auditory cortex. The auditory cortex is the level of the auditory system at which acoustic stimuli become especially meaningful to the listener.

Primary Auditory Cortex

Fig. 1.1a and Fig . 1.1b provide illustrations of the ascending auditory pathway. As one may see from Fig . 1.1a, all auditory input from the auditory brainstem and thalamic regions arrives at the primary auditory cortex. The anatomic characteristics of the primary auditory cortex can be elusive and obscure to many new audiology students, because the primary auditory cortex is not readily visible from a surface view of the cerebrum. However, if parts of the frontal and parietal lobes are removed, the primary auditory cortex, which is also known as the Heschl gyrus, may be viewed as a ridge of neural tissue that courses medially and posteriorly in the sylvian ﬁssure (Fig. 1.1b). The tonotopic organization established in the cochlea and preserved throughout the ascending auditory pathway is maintained in the primary auditory cortex, with low-frequency sounds processed at the lateral edge of the Heschl gyrus and high-frequency sounds processed at the medial edge.

Secondary Auditory Cortex

The secondary auditory cortex is less clearly deﬁned than the

primary auditory cortex and includes multiple areas including but not limited to the planum temporale, the angular gyrus, the supramarginal gyrus, the inferior parietal lobe, the posterior frontal lobe, the insula, and the superior temporal gyrus (Fig. 1. 2a and Fig. 1.2b). Health care professionals have long recognized the prominent role that the secondary auditory cortex plays in a listener’s ability to understand speech and to derive meaning

1 Why Hearing Is Important in Children

Fi g . 1.1 Primary auditory cortex. (Adapted from Schuenke M, Schulte E, Schumacher U. Thieme Atlas of Anatomy. Vol. 3, Head, Neck and

Neuroanatomy, 2nd ed. New York, NY: Thieme; 2017 and Gilroy AM, MacPherson BR, Schuenke M. Thieme Atlas of Anatomy, 3rd ed. New York, NY: Thieme; 2016.)

from environmental sounds and music. For example, the German physician Carl Wernicke noted in 1874 that an injury to Brodmann area 42, which resides in the secondary auditory cortex (Fig. 1.2a

and Fig. 1.2b), results in an inability to understand speech.

For persons who have sucient access to intelligible speech

during the ﬁrst several years of life, the auditory areas of the brain

respond robustly when stimulated by speech or other meaningful

sounds. Kevin Green and colleagues21 used positron emission

tomography (PET) to image areas of the brains of participants who had had normal hearing during childhood and had received a cochlear implant after developing hearing loss as adults. When these participants listened to intelligible speech, auditory

I Hearing Loss: Essential Information

Planum temporale

Irsula

a b

Fig. 1. 2 Secondary auditory cortex. (Adapted from Gilroy AM,

MacPherson BR, Schuenke M. Thieme Atlas of Anatomy, 3rd ed. New York, NY: Thieme; 2016.)

Superior temporal gyrus

Left inferior prefrontal cortex

Angular gyrus

Brodman area 42

stimulation from the cochlear implant at one ear in response to the running speech resulted in bilateral activation of both the primary and secondary auditory cortices.

Fig. 1.3 provides an example from the Green study in which

a participant was listening to running speech with a cochlear implant for the left ear.21 As shown, an auditory response is seen at both the primary and secondary auditory cortices not only at the right side of the listener’s brain (i.e., contralateral to the stimulated ear) but at the left side as well. In short, the ipsilateral ascending auditory tracts and the corpus callosum serve to facilitate a bilateral response. However, most importantly, the auditory stimulation produces a broad and robust response that spans throughout the primary and secondary auditory cortices.

This responsiveness of the secondary auditory cortex to speech and other sounds is relevant, because the secondary auditory cortex serves to connect the auditory nervous system functionally to the rest of the brain. Research has shown that the secondary auditory cortex possesses an abundance of connections to other areas of the brain (such as cortical neurons residing in the frontal, parietal, and occipital lobes and also in deeper brain areas such as the hippocampus and amygdala). The connections between the secondary auditory cortex and other areas of the brain are known as intrahemispheric tracts.

Most pediatric hearing health care professionals are aware of interhemispheric tracts, such as the corpus callosum, which exchanges information between the right and left sides of the cerebrum. Intrahemispheric tracts exchange information between two areas located within the same hemisphere (i.e., right or left) of the cerebrum. The arcuate fasciculus, which is shown in Fig.

1.4, is an example of an intrahemispheric tract that delivers information from the auditory areas of the brain to the frontal lobe for higher-order processing. Numerous other intrahemispheric tracts exist to form connections between the auditory areas of the brain and other cortical areas as well as with structures inferior to the cerebrum, such as the amygdala.

The secondary auditory cortex also relays information back to

the primary auditory cortex in the form of e erent tracts. Although the exact function of these e erent tracts is not clearly established, research has suggested that these e erent connections serve to

modulate the response of the primary auditory cortex, allowing it to focus on primary signals of interest. Throughout life, listeners are simultaneously inundated with numerous sounds that overlap in both spectral and temporal composition. To process the most meaningful or interesting sounds in his or her environment, a

listener must be able to focus on the acoustic elements that are unique to the sound he or she desires to comprehend, an ability known as feature extraction or feature representation.

12,13,22

Feature Extraction

Feature extraction involves the listener’s ability to attend

selectively to acoustic elements that are su cient to decode a sound of interest and that su ciently represent a sound to enable successful recognition and identiﬁ cation. To expand,

prominent auditory physiologist, Andrej Kral and colleagues have proposed that the secondary auditory cortex’s modulation of the primary cortex enables detection of important acoustic features necessary for the listener to recognize a sound. Additionally, Kral has suggested that the secondary auditory cortex and other higher-order areas of the brain combine these basic acoustic features into meaningful representations (i.e., auditory objects), enabling the listener to identify and comprehend sound.

13,23

Furthermore, many neurons in the secondary auditory cortex are capable of responding to multiple modes of stimulation, a property referred to as pluripotency. For example, some neurons in the secondary auditory cortex have been shown to be responsive to both auditory and visual stimulation, while others have been shown to respond to both auditory and tactile stimulation.24 It is possible that these pluripotent neurons in the secondary auditory cortex enable multimodal integration (e.g., the integration of auditory and visual information to enable a listener to

Fig. 1. 3 Positron emission tomography (PET) image of areas of the brain that were responsive (arrows) when postlingually deafened adults listened to intelligible speech while using a cochlear implant.

4,23

Fig. 1.4 Intrahemispheric tracts. (arrows). (Adapted from Schuenke M,

Schulte E, Schumacher U. Thieme Atlas of Anatomy. Vol. 3, Head, Neck and Neuroanatomy, 2nd ed. New York, NY: Thieme; 2017.)

associate a certain sound, such as glass shattering, with a visual image, such a baseball shattering a window). Additional research is needed to elucidate the role that the pluripotent neurons in the

secondary auditory cortex play in the recognition, identiﬁ cation,

and comprehension of sound.

Integration between the secondary auditory cortex and the rest of the brain is imperative for sound to elicit a meaningful experience rather than just mere detection. It is important to acknowledge the fact that auditory physiologists do not fully understand the process by which auditory objects (e.g., speech, environmental sounds, or music) are represented and decoded in the brain. However, neuroscience literature suggests that every signal that enters our minds from our sensory systems is reduced to patterns of neural activity.4 Stated di erently, every cognitive experience we have is represented by its own unique network of

neurons that ﬁ re in response to input from our sensory systems to

produce the reality we perceive.

For instance, consider the word “soft.” When we hear “soft,” a certain set of neurons ﬁ res across the brain to generate the cognitive “images” we experience. The acoustical elements of the word “soft” are represented in the primary auditory cortex in the form of the number of neurons that ﬁ re, the timing at which they ﬁ re, and the tonotopic places at which they ﬁ re. As with the example mentioned from Green’s work and depicted in Fig. 1.3, neurons in

the secondary auditory cortex also respond. These neurons may serve to modulate the response of the primary auditory cortex, but more importantly, the neurons in the secondary auditory cortex distribute the auditory stimulation across other areas of the brain, creating a network to process the sound. Fig. 1.5a provides an

overly simpliﬁ ed cartoon representation of the network of neurons that may ﬁ re across the brain in response to the word “soft.” Most of the neurons that ﬁ re reside in the primary and secondary auditory cortices, but neurons also ﬁ re in the inferior parietal

and posterior frontal lobes as well as in occipital regions and in multimodal regions of the secondary auditory cortex.

Involvement of the inferior frontal lobe enables us to extract

higher-order meaning from the word “soft.” For example, we may

associate the word with a soft mattress for our bed; we may associate it with comfort, or in contrast, we may think it aggravates our

1 Why Hearing Is Important in Children

bad back. Further, engagement of neurons in the visual centers of the brain or of pluripotent neurons in the secondary auditory

cortex enables us to form an image of “softness” in our “mind’s eye.” For instance, we may associate the word “soft” with a pillow, a bath towel, a piece of  u y cotton, or a feather gently blowing in

the light breeze. All of these visual images are evoked by the word

“soft” because of the integration that occurs between auditory

and visual areas of the brain. Likewise, we can imagine the tactile

sensation of a cotton ball between our ﬁ ngers because of the

integration that takes place between the auditory, parietal, and frontal areas of the brain. Finally, we can also speak or produce

the word “soft,” because a network formed between the auditory

system and the left inferior prefrontal cortex (LIPC; i.e., the Broca region) enables phonemic awareness and the vocal reproduction of sounds that are modeled in the auditory system.

Fig. 1.5b provides a cartoon depiction of a contrasting yet similarly unique neural network or pattern of neurons that respond to the sound of our alma mater’s marching band playing the school’s

ﬁ ght song. In this example, a larger and more extensive network of neurons is engaged, presumably because the ﬁ ght song produces a deeper and more complex response than the word “soft.”

Once again, we see robust and broad activation of primary and secondary auditory cortices. Also, because the secondary auditory cortex serves as the launching pad from which sound is distributed to the rest of the brain, we see activation throughout other areas

of the brain as well. In our mind’s eye, we may be able to “see”

the marching band belting out each note with pride as the football team scores and takes the lead. Again, this occurs because of

Fig. 1. 5 (a) Representation of the network of neurons that may  re

across the brain in response to the word “soft.” (b) Depiction of a contrasting yet similarly unique neural network or pattern of neurons that respond to the sound of our alma mater’s marching band playing

the school’s  ght song. (Adapted from Gilroy AM, MacPherson BR,

Schuenke M. Thieme Atlas of Anatomy, 3rd ed. New York, NY: Thieme;

2016.)

I Hearing Loss: Essential Information

integration and interaction of auditory-responsive neurons on the secondary auditory cortex with visually responsive neurons. We may also feel our own sense of pride and happiness when we hear the song, because of the engagement of secondary auditory cortex with the frontal lobe and other emotion-mediated areas of the brain, such as the amygdala. We may think of fun times attending

games with friends and family. We may even be able to “smell”

the scent of concession stand hotdogs because of the integration between the secondary auditory cortex and the olfactory system.

Connectomes or Neural Networks

In short, every sound that comes into our minds from our ears is reduced to its own unique pattern of neural activity across the brain. In order for our sound-elicited experience not just to result in simple detection but also to possess higher-order

meaning and “come to life,” auditory stimulation has to extend

beyond the primary auditory cortex to the secondary auditory cortex, where it is delivered to other areas of the brain to form interconnections known as a neural network or connectome. A connectome is a map of functional neural connections in the brain and may be thought of as the brain’s wiring diagram.

Andrej Kral et al have created a theoretical illustration depicting the auditory component of the human brain’s connectome (Fig.

1.6).4 Once again, the secondary auditory cortex serves as the launching pad for a connectome to be formed by the requisite interaction and integration between the auditory system and other areas throughout the brain.

A normal neural connectome is established during the ﬁ rst

several years of life through a number of stimulus-driven processes. First, through a process referred to as synaptogenesis (formation of synapses, the structures that carry signals between two communicating neurons), neuron-to-neuron contacts are formed, establishing neural pathways throughout the brain. New synapses are created through a process known as arborization (from arbor, the Latin word for tree), in which the transmitting axons and receiving dendrites of neurons grow and branch out to come into contact with neighboring neurons, with which they can form synapses. Synaptic counts reach a maximum between the

ﬁ rst and fourth years of life and decline thereafter.

Pearl

Synapses across the brain are sustained when they receive consistent and meaningful input from the peripheral sensory systems or from other areas within the brain and pruned when they do not, a phenomenon that may be summarized as “use it or lose it.”

The creation and maintenance of functional synapses is dependent upon experience. Synapses across the brain are sustained when they receive consistent and meaningful input from sensory systems or from other areas within the brain. In contrast, synapses that are not used (e.g., stimulated by input from the sensory system) are subjected to synaptic pruning, the elimination of synapses in neural pathways that do not receive meaningful stimulation. Synaptic pruning is necessary because otherwise the brain

Language

Attention

Object identity

Auditory cortex

Fig. 1.6 An illustration of the auditory portion of the human brain’s connectome.

Motor planning

Declarative memory

Attention

Working memory

Executive funtion

would be inundated by a cacophony of signals from an abundance of neural connections, many of which do not contribute useful information about one’s environment. Synaptic pruning eliminates synapses that do not provide meaningful signals vis à vis one’s environment, while neural connections that provide a person with the most useful information necessary to survive and succeed in his

or her environment are maintained. Speciﬁ cally, synaptic pruning

enables us to focus the brain’s resources upon the environmental stimuli and information that are most relevant for our well-being.

Although arborization, synaptogenesis, and synaptic pruning

occur throughout the lifespan, these phenomena are far more prev-

alent during the ﬁ rst few years of life (i.e., during the critical period

of development and maturation) and decline substantially throughout childhood. As a result, brain plasticity, or the brain’s ability to change and establish new functional connections, is greatest during the early childhood years. According to Kral and colleagues,

“hearing deprivation during early development prevents functional

maturation, delays cortical synaptogenesis, and increases synaptic

elimination, ultimately a ecting central functions such as intensity

coding, cortical column functioning, cochleotopic representation of auditory space, and corticocortical interactions including top-

down control and auditory object formation.”

1.4.2 Auditory Deprivation

With the preceding review of an auditory neural network or connectome in mind, it is now prudent to review recent studies examining auditory brain development in persons who have access to intelligible speech and those who were deprived of

1 Why Hearing Is Important in Children

access to intelligible speech as children. A large number of imaging studies have explored auditory brain activity in persons with and without hearing loss. One of the pioneering studies in this area of research was conducted by Nishimura and colleagues.25 They used PET scan imaging to evaluate the brain areas that were

responsive to a variety of dierent stimuli including running

speech, sign language, and meaningless hand movements. The study participants were prelingually deafened adults who had had no auditory access to intelligible speech and used sign language prior to receiving a cochlear implant in adulthood.

Fig. 1.7 shows a typical response obtained from these participants while listening to running speech while using their cochlear implants. As shown, speech elicited a neural response in the primary auditory cortex ipsilateral to the ear that was stimulated,

a ﬁnding that indicates that the primary auditory cortex is “hardwired” to respond to auditory stimulation from the peripheral auditory system. Stated dierently, even in the presence of a life-

time of deafness and auditory deprivation, the primary auditory cortex still responds to auditory stimuli. The fact that primary auditory cortex is dedicated to process auditory signals explains why congenitally deafened teenagers and adults who have no

spoken language and who have never had sucient access to auditory stimulation during their ﬁrst decade or longer of life are still able to respond to warble tones presented in the soundﬁeld at 20 to 25 dB hearing level (HL) after receiving a cochlear implant

that is adequately programmed. Primary auditory cortex is committed to audition, and its dedication allows for detection of auditory input from the peripheral auditory system, regardless of an individual’s duration of deafness.

Fig. 1.7 Typical response obtained from prelingually deafened adult participants who had no auditory access to intelligible speech and used sign language prior to receiving a cochlear implant (CI) in adulthood, as they listened to running speech while using their cochlear implant.

Blue: areas activated by visual stimuli (meaningless hand movement); yellow: areas activated by sign language; green: areas activated by

spoken language (CI: left ear).

Of note, it should be mentioned that a long duration of deafness most likely results in some reorganization of the primary auditory cortex and some change or reduction in its responsiveness to sound.14 For instance, research has shown that a larger proportion of primary auditory cortex is employed for the processing of low-frequency stimuli for persons with a long duration of high-frequency hearing loss.25 However, as the Nishimura et al study shows, the primary auditory cortex will continue to be responsive to auditory stimuli after many years of auditory deprivation, a fact that does support simple detection of sound.

The most compelling ﬁnding of the Nishimura et al study came

in the researchers’ observations of the secondary auditory cortex.25 As shown in Fig. 1.7, the scans typically showed no activity present in the secondary auditory cortex in response to auditory stimulation from the cochlear implant. In contrast, robust neural activity was observed in the secondary auditory cortex when the participants observed a story being told via sign language. The activity present in secondary auditory cortex in response to sign language is evidence of cross-modal reorganization.26 In other words, in the absence of sucient access to intelligible speech from the peripheral auditory system, the secondary auditory cortex is functionally

“decoupled” from the primary auditory cortex and transforms to

assist in the processing of visual information.

Colonization of the secondary auditory cortex in the presence of

deafness has been demonstrated in a number of additional studies

over the past 15 years, with research also showing activity in the

secondary auditory cortex in response to tactile stimulation.

12,27,28

Acquisition of the secondary auditory cortex by other sensory modalities likely explains the phenomenon by which many persons who are born deaf and do not have access to auditory stimulation

during the ﬁrst few years of life develop exceptional ability with

other sensory modalities. For example, research has shown that

persons who are born deaf without sucient auditory access

develop better peripheral vision.

29,30

Colonization of the secondary auditory cortex also explains the unsatisfactory outcomes often experienced in persons who are prelingually deafened and receive cochlear implants after the critical period of development. Once the primary and secondary auditory cortices are decoupled from one another, peripheral auditory input cannot be adequately relayed from the primary to the secondary auditory cortex. If auditory input is not delivered to the secondary auditory cortex, then it cannot be distributed to the rest of the brain for higher-order processing that is necessary to convey a meaningful auditory experience. The auditory neural network or connectome is fractured between the primary and secondary auditory cortex. Persons who have a congenital hearing loss and receive a cochlear implant after the critical period of language development can detect sound at low presenta-

tion levels but typically have a dicult time understanding speech.

Interestingly enough, Nishimura and colleagues found no activity in the secondary auditory cortex when the participants watched meaningless hand movements, see Fig. 1.7.25 Instead, activity was recorded only in the occipital lobe (i.e., visual area of the brain) when meaningless hand movements were presented. The presence of activity in the secondary auditory cortex only when a language-based visual stimulus is present, implies that the secondary auditory cortex is at least somewhat predisposed to process language. Thus, one may initially (and erroneously) con-

clude that the provision of sign language during the ﬁrst few years of the life of a child born with deafness is sucient to develop

the necessary synaptic connections and neural networks within and beyond the secondary auditory cortex that are necessary to eventually foster language development and, even more speciﬁcally, auditory and spoken language development. However, upon further consideration, the sole use of visual forms of communica-

tion during the ﬁrst few years of life with an absence of access to

intelligible speech results in colonization of the secondary auditory cortex by the visual system. As a result, the functional neural synapses and connections between the primary and secondary

I Hearing Loss: Essential Information

auditory cortices necessary for the exchange of auditory information to the secondary auditory cortex and then to the rest of the brain do not form.

Pearl

As Nishimura’s work shows, when auditory deprivation occurs during the early years of life, the secondary auditory cortex is forever altered as a mechanism for functionally processing auditory information.

1.4.3 Critical Period of Auditory Brain Development

Of note, Kral’s research with white deaf kittens has provided an explanation for the exact location of the decoupling that occurs between the primary and secondary auditory cortices.

measured auditory-evoked neural responses at dierent layers of the auditory cortex by inserting microelectrodes to dierent

depths into the auditory cortex. As shown, in Fig. 1.8, the audi-

tory cortex, which is 2 to 4 mm in thickness, comprises six layers of neurons. The aerent auditory nerve ﬁbers from the medial

13,30

Kral

geniculate body of the auditory thalamus arrive at layer IV, and a good deal of the processing that takes place within the primary auditory cortex occurs in layers I to III, which are referred to as the supragranular layers. Layers V to VI are known as the infragranular layers of the auditory cortex. The infragranular layers accomplish several functions, but most importantly, they serve as the output circuits of the primary auditory cortex. More

speciﬁcally, auditory information from the primary auditory

cortex is delivered to other areas of the brain, including to the secondary auditory cortex, from layers V and VI of the primary auditory cortex.

Kral’s congenitally deafened cats had received cochlear implants

at dierent ages. For normally hearing cats and kittens who were implanted during the ﬁrst 3 months of life, normal activity was

observed at all layers of the primary auditory cortex (Fig. 1.9a). In

contrast, white deaf cats who received a cochlear implant after 5

months of age showed normal activity in layers I to IV but reduced

or no activity in the infragranular layers (V–VI) (Fig. 1.9b). In short, Kral’s ﬁndings suggest that the decoupling of primary and

secondary auditory cortices occurs at the microscopic level of the infragranular layers of the primary auditory cortex. The absence of auditory evoked responses at the output circuits of the primary auditory cortex prevents the delivery of auditory information to the secondary auditory cortex. As a result, the secondary auditory cortex cannot deliver the auditory signal to the rest of the brain, and the auditory neural network/connectome is eliminated.

Fig. 1. 8 Cortical layers. (Adapted from Schuenke M, Schulte E, Schumacher U. Thieme Atlas of Anatomy. Vol. 3, Head, Neck and Neuroanatomy, 2nd ed. New York, NY: Thieme; 2017.)

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