Auditory agnosia is a form of agnosia that manifests itself primarily in the inability to recognize or differentiate between sounds. It is not a defect of the ear or "hearing", but a neurological inability of the brain to process sound meaning. It is a disruption of the "what" pathway in the brain. Persons with auditory agnosia can physically hear the sounds and describe them using unrelated terms, but are unable to recognize them. They might describe the sound of some environmental sounds, such as a motor starting, as resembling a lion roaring, but would not be able to associate the sound with "car" or "engine", nor would they say that it was a lion creating the noise. Auditory agnosia is caused by damage to the secondary and tertiary auditory cortex of the temporal lobe of the brain.
Types of auditory agnosia
There are three primary distinctions of auditory agnosia that fall into two categories.
This type of auditory agnosia is caused by lesions to the left hemisphere of the brain, specifically the temporal lobes and Wernicke's area.
Linguistic (or verbal information or Wernicke's) agnosia indicates that the subject can't comprehend words, although they can understand words using sign language and words from reading books, and are themselves capable of speech (and even of deriving meaning from non-linguistic communication e.g. body language) the particular sounds associated to each word are meaningless.
This type of auditory agnosia is caused by lesions to the right hemisphere of the brain.
Classical (or pure) auditory agnosia is an inability to process environmental sounds, such as animal noises, industrial noises, or the like. An airplane roaring overhead would not be understood to be related to the idea of "airplane"—indeed, the person would not even think to look up.
Interpretive or receptive agnosia (amusia) is an inability to understand music. The term "amusia" covers a broad spectrum: from those with a mere deficit of rhythmic ability (mild dysrhythmia), to those with heavy all-encompassing amusia, including the recently coined "distimbria"; sufferers regard music as simply "noise", often compared to drainpipes or drills, or other invasive forms of background noise. Vocal singing can be understood, but is simply seen as "odd tone of voice". The standard is considered to be that amusics with a "normal" intensity of amusia are cortically unable to distinguish pitch changes of less than three semitones (when it includes a pitch-deficit; it is contentious whether without this symptom it can be considered amusia). They may appreciate or enjoy listening to or performing music, but some can not tolerate it or find it irritating.
Testing for auditory agnosia
Meaningless Sounds Discrimination Test
In this test subjects are presented with two consecutive synthetic noises (usually with a two-second interval) and asked whether the noises are the same or different. Those with damage to the right hemisphere have significantly lower scores on this test than those of controls and those with left hemisphere damage.
Meaningful Sounds Identification Test
In this test subjects are presented a natural sound followed by four pictures. The subject is then asked to point to the picture that best represents the sound they just heard. Those with damage to the left hemisphere do significantly poorer on this test than controls and those with right hemisphere damage.
A seventy-four-year-old man named "M" had a special case of auditory agnosia. "M" suffered from, "unilateral left posterior temporal and parietal damage", (Saygin, Leech, & Dick, 2010, p. 107) including Wernicke's area. These areas of the brain are associated with language processing. He had a stroke when he was 62 and went through intensive speech therapy for twelve weeks to help him recover. He was able to regain his language capacity, but when tested at age 74, he had great difficulty in recognizing non-verbal environmental sounds. He did not have either verbal comprehension deficits nor peripheral hearing problems (p. 107). His condition was very rare because auditory agnosia for nonverbal sounds is usually associated with the right side of the brain. "M" was able to identify familiar Christmas songs and some animal sounds. When he heard music, he couldn't distinguish individual instruments or voices, but he knew it was music. fMRI scans show that after the stroke, his brain re-wired itself through neuronal compensation to account for the damage.
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