Auditory Agnosia and its effects on language, hearing and speech

CONTENT

List of Figures

I Introduction

II The auditory system and auditory disorder
2.1. Types of agnosia
2.2. Affected brain area

III Disorders of auditory processing and word recognition
3.1. Cortical deafness
3.1.1. Symptoms
3.1.2. Causes and damage
3.1.3. Treatments
3.2. Auditory agnosia
3.2.1. Symptoms
3.2.2. Causes and damage
3.2.3. Treatments
3.3. Auditory verbal agnosia
3.3.1. Symptoms
3.3.2. Causes and damage
3.3.3. Treatments
3.4. Phonological retrieval disorder

IV Summary

V List of references

List of Figures

Figure 1: Ingram, J. (2007). Neurolinguistics: An Introduction to Spoken Language Processing and its Disorders, 160

Figure 2: http://www.imagecloud.us/primary-motor-cortex-definition/

Figure 3: http://aurinegra.pt/uma-area-com-muitas-areas-wernicke/

I Introduction

Imagine not being able to recognize simple everyday objects. For example your favourite coffee mug. You see it, but you can’t comprehend what you see. Or imagine not being able to recognize the faces of your family and friends. Now imagine not being able to understand what you hear. Nothing anyone says makes sense to you, nor do you recognize the sounds from your environment. You hear everything, but can’t recognize it. Such symptoms can occur in some people, making these individuals suffer from what is known as agnosia.

Agnosia is a neurological disorder that results in an inability to recognize certain objects, persons, smells, or sounds despite normally functioning visual and/or auditory senses; for example they are not able to identify a sound as a sneeze or an object as an apple. It also can be classified as having trouble processing sensory information, like touch, sound, and even light. Agnosia is normally restricted to one sensory modality like vision or hearing and these deficits are not due to memory loss. It can result from strokes, traumatic brain injury, dementia, a tumor, developmental disorders, overexposure to environmental toxins (e.g., carbon monoxide poisoning), or other neurological conditions. Symptoms may vary, according to the area of the brain that is affected. With agnosia the brain develops these lesions that are caused by neurological ailments or even a traumatic brain injury either in the temporal, occipital, or parietal lobe of the brain (Maskill, L., & Grieve, J., 2017).

This paper will mainly focus on auditory agnosia, which affects language, hearing and speech. For better understanding this paper will briefly summarize the different types of agnosia, before moving on to auditory agnosia and explain the different types.

II The auditory system and auditory disorder

2.1. Types of agnosia

Before going into detail about auditory agnosia, this paper will first explain briefly the difference to other agnosias briefly. There are several forms but they depend on the person’s perception of touch, vision, or auditory senses, along with many types and many sub-categories with some experts are not even in agreement with. Therefore this paper will divide and only describe visual agnosia, tactile agnosia, autotopagnosia and auditory agnosia, as they are the most important and later focus on the auditory processing disorders, that are being described by John C. L. Ingram (2007).

Figure 1

Abbildung in dieser Leseprobe nicht enthalten

Visual agnosia is developed when brain damage along the pathways of the brain which connects the occipital lobe to the parietal and temporal lobes occurs (Maskill, L., & Grieve, J., 2017). The occipital lobe is responsible for gathering the incoming visual information, while the temporal and parietal lobes let the individual comprehend the meaning of the incoming information (Maskill, L., & Grieve, J., 2017). There is a chance that visual agnosia could also occur in association with other underlying disorders. Treatment of primary agnosia is symptomatic and supportive; when it is caused by an underlying disorder, treatment of the disorder may reduce symptoms and help prevent further brain damage.

Tactile agnosia is the incapability to identify objects with touch (Maskill, L., & Grieve, J., 2017). A person might be able to feel how heavy the object is, but they might not be able to comprehend the importance or the use of the object. Tactile agnosia is caused by lesion in the parietal lobe.

Autotopagnosia is when an individual loses their capability to position parts of their own body (Kerkar, 2018) and is caused by damage on the left parietal lobe. There is a number of other forms of agnosia, but recently a proposal for a new form of agnosia has been published. This new form is called affective agnosia and it is described as not being able to know one’s own emotions (Taylor, G. J., Bagby, R. M., & Parker, J. A.,2016),which ties into the theory of alexithymia, the incapability to recognize and describe emotions in the self (Taylor et al., 2016).

Auditory agnosias range from the inability to comprehend spoken words to the inability to recognize nonlinguistic sounds and noises and even music (amusia). Basically, it is the incapability to distinguish or process sounds even with undamaged hearing. The individual is still capable of reading, writing, and speaking. In young children, acquired verbal auditory agnosia, which is a symptom of Landau-Kleffner syndrome – a sudden or gradual development of aphasia - , may lead to mutism, or loss of the ability or will to speak.

Phonagnosia is the incapability to distinguish and identify voices that we are used to (Maskill, L., & Grieve, J., 2017). It occurs when the brain undergoes damage to a specific part of the sound association area. This area is situated at the right side of the brain. The individual will still be able to comprehend words spoken by others.

2.2. Affected brain area

Figure 2 [for copyright reasons not part of this publication]

The most common group of areas damaged are the temporal lobes, especially the transverse temporal gyri also known as Heschl's gyri (Wirkowski et al., 2006). Heschl's gyri are unique in their position within the lobes as the only parts that run toward the center of the brain. Difficulty in identifying lesioned areas of the brain not only involves where the lesion is but also when in the process of hearing the lesion affects auditory functions. It is unclear if understanding language can be localized to Heschl's gyri or if auditory, because damage to the temporal lobes bilaterally interrupts language comprehension, due to information being cut off before reaching the part of the brain that interprets language.

A second group of damaged areas involve unilateral lesions in the left temporal lobe (Slevc et al., 2011). Researchers have identified the left hemisphere as the more dominant hemisphere for language functions and aspects of sounds. Specifically, the superior temporal cortex, which processes frequency of sound, and the planuum temporal, which is a triangular region around Wernicke's area, are damaged in auditory agnosia patients and language comprehension is consequently affected due to the damage.

Figure 3 will showcase the importance of Wernicke’s area to auditory agnosia.

Figure 3 [for copyright reasons not part of this publication]

The auditory cortices analyse the sensory properties of verbal material, such as speech and text. The sensory cortices of both hemispheres project this input to Wernicke’s area in the left hemisphere. Wernicke’s area then extracts the linguistic properties of this input, and projects it downstream to Broca’s area. Broca’s area then processes grammatical structure and speech output. In the context of this model, auditory agnosia occurs because of cortical damage to both auditory cortices (bilateral damage), or damage to the fibers that connect the auditory cortices with Wernicke’s area (unilateral left damage).

Although these important parts of the brain have been discovered as damaged in some patients, specific neurobiological causes of the disorder have yet to be identified.

III Disorders of auditory processing and word recognition

3.1. Cortical deafness

3.1.1. Symptoms

Cortical deafness is a disorder characterized by sensorineural hearing loss that is the result of damage to the central nervous system, meaning there is no physical damage but to the primary auditory areas in the superior temporal lobes of the cerebral cortex. This disorder is classified as rare because of the high improbability that both temporal lobes will be lesioned. Diagnosing the disorder is still proving to be difficult, as cortical deafness and auditory agnosia have many similarities.

The main distinction is the capability to detect pure tones, as measured with pure tone audiometry. Auditory agnosia patients have been regularly reported capable of detecting pure tones nearly as good as healthy individuals, while cortical deafness patients found this assignment nearly impossible (Vignolo, 1982).

Patients with cortical deafness - or also called central hearing loss - typically have inconsistent auditory behaviour that may cause them to be misdiagnosed as having "functional" or psychogenic hearing disturbances. Cortically deaf patients may have reactions to environmental sounds, despite the absence of reaction to loud noises, due to the fact that the sub-cortical auditory system is intact. As in patients with cortical disturbances, patients may consider themselves "deaf" despite having reactions to sounds in the room.

3.1.2. Causes and damage

Bilateral damage appears to be the cause of cortical deafness, which leads to not being able to perceive any auditory signals despite normal peripheral hearing. This is most likely due to the fact that many of the specialised acoustic feature analysers are inoperable. Since each ear projects auditory fibres to the ipsilateral as well as the contra-lateral auditory cortex, one-sided harm to the auditory cortex is not likely to cause central hearing loss (Ingram, 2007).

3.1.3. Treatment

Cortical deafness rarely persists for longer than a few weeks or evolves into another cortical auditory syndrome. Depending on the size and type of the cerebral lesion, patients recover to a variable degree. Eventually cochlear or auditory brainstem implantation could additionally be counted as treatment options. Electrical stimulation of the peripheral auditory system may also result in improved sound perception or cortical remapping in patients with cortical deafness (Graham, Greenwood, Lecky, 1980) . However, as even amplified auditory signals are not being able to be processed, hearing aids are useless. The best option would still be to train the patients in lip-reading, sign language and speech therapies.

3.2 Auditory agnosia

3.2.1. Symptoms

The term auditory agnosia was originally coined by Siegmund Freud (1891) to describe patients with selective impairment of environmental sounds. It refers to an inability to understand familiar day-to-day sounds, such as water dripping from a tap, a door closing or the ring of a telephone. Unlike people suffering from cortical deafness, individuals with auditory agnosia are consciously aware of auditory stimuli. The ability to discriminate between similar sounds may also be relatively intact, however sound identification is severely compromised. Generally patients also show difficulties in speech sound identification, but in rare cases even a selective impairment of non-speech auditory objects may also occur. Speech agnosia or word blindness constitutes a special case of specific auditory agnosia, which will be discussed below. Before that, it is necessary to inquire into the probable neural substrate for generalized auditory agnosia. Auditory object recognition relies, in the first instance, upon the integrity of complex feature analysers that respond to aspects of the spectral and temporal composition of complex acoustic signals (i.e. sounds with more than one frequency component).

[...]