Auditory agnosia

Medical quality assurance by Dr. Albrecht Nonnenmacher, MD at October 15, 2016
StartDiseasesAuditory agnosia

Sometimes confused with hearing impairments or structural abnormalities of the ear, auditory agnosia is a person’s inability to recognize and differentiate between sounds, including voices due to brain damage. This condition has been medically recognized since 1877, but it is very rare, and variations in severity and specific subtypes of auditory agnosia remain understudied.

Contents

Definition & Facts

Individuals with auditory agnosia are unable to differentiate between sounds in their environments. However, these people do not have any defects in their ability to hear. The issue lies in the brains of those with auditory agnosia because although they can hear the sounds, they cannot identify the sources or differentiate those sounds from other sounds.

For example, a person with this condition can physically hear the sound of a bird singing. However, without seeing the source of that singing, the bird itself, the person would only be able to say that he or she hears a high-pitched sound in short spurts. This person would not recognize the sound as that of a bird. 

Symptoms & Complaints

There are different types of auditory agnosia. The main symptom of auditory verbal agnosia or pure word deafness is that the person is unable to comprehend auditory words. However, those with auditory verbal agnosia are often able to read, use sign language, and speak. They can understand language in these forms, but spoken words from others are typically unintelligible to people with this condition.

Nonverbal auditory agnosia describes individuals who are unable to process sounds from their environment, such as the example of the bird singing. They can understand speech, but they cannot differentiate between voices, even those of close family members and friends.

Another type of auditory agnosia is termed amusia or tone deafness, and this form of auditory agnosia includes those who cannot understand music. Amusia can be congenital in which case it affects approximately 4 percent of the population, and can include any of these complaints:

  • A deficit in rhythmic ability
  • A complete inability to understand music
  • Hearing music simply as noise, like that of heavy machinery
  • Hearing vocal singing as an odd tone of voice and not as music
  • A complete intolerance to music

Causes

Because tests and studies regarding auditory agnosia are few, the exact causes of this neurological issue are less than clear. Most medical professionals believe that the condition is caused by lesions on the bilateral or unilateral temporal lobe of the brain. Lesions to the transverse temporal gyral area of the temporal lobes seem to be particularly responsible for the condition.

There are also some cases of subcortical lesions appearing without cortical damage and leading to auditory agnosia. Linguistic agnosia is considered a semantic-associative form of the condition. Studies indicate that this form is associated with finding these lesions on the left hemisphere of the brain.

Linguistic or auditory verbal agnosia can sometimes be found directly on the Wernicke’s area of the brain in that hemisphere. Classical agnosia and amusia are associated with lesions on the right brain hemisphere. The damage to either side can be caused by normally occurring neurological disorders or developmental disorders or as a result of an event such as a stroke or the onset of dementia

Diagnosis and Tests

This basic understanding of the different varieties of auditory agnosia make it difficult to test and diagnose, especially for physicians who have never seen this extremely rare condition. First, the person must be tested for basic language abilities and intelligence to make sure that the issue is not in the intellect or communication capability.

Once it has been determined that the person has these abilities, other evaluations will begin looking for a sensory deficit. The meaningless sounds discrimination test may be given. In this evaluation, the person is presented with two consecutive noises at a two-second interval. These noises are synthetic in nature, meaning that they are man-made and not from a person or animal. When there is damage to the right hemisphere of the brain, the scores on this test are significantly lower, indicating either classical agnosia or amusia.

The meaningful sounds identification test is given to determine left hemisphere damage, or linguistic agnosia. In this test, the person is presented with a natural sound and four pictures that could represent that sound. The inability to select the correct image for the sound indicates left hemisphere damage. 

Various neuroimaging scans may also be conducted to determine the location of brain damage. These could include magnetic resonance imaging (MRI) scans and computed tomography (CT) scans.

Treatment & Therapy

At this time, there is no direct cure for auditory agnosia though occupational therapy and speech therapy may be utilized to improve the patient's quality of life. Certain therapies can help these individuals learn to utilize their other senses in order to overcome the auditory deficit. Because many people suffering from auditory agnosia do not realize that they have a neurological deficit, the first step in the treatment process is getting these individuals to recognize the problem and to want to undergo therapy.

This realization can be accomplished, for example, by giving an auditory stimulus to the person that will indicate his or her impairment. Once the person recognizes the issues, the most common strategy relies on improving the individual’s conscious auditory analysis. This can be done by practicing semantic categorization and association as well as by imitating the sound and analysis of auditory features with an occupational therapist. However, even with these therapy options, improvement is typically slow and minimal. 

Prevention & Prophylaxis

Most presentations of auditory agnosia cannot be prevented. This is partially because the causes are still relatively unknown, but it is also because the condition often comes as a result of a neurological or developmental issue. These issues could be based on genetic factors or on other health conditions that lead to these developmental delays. Lesions caused by the onset of dementia or by strokes can be prevented to an extent by proper nutrition and regular exercise, but there is typically still some aspect of genetics.