Amusia, also called being tone-deaf, is a musical disorder that appears mainly as a defect in processing pitch but also encompasses musical memory and recognition. Two main classifications of amusia exist: acquired amusia, which occurs as a result of brain damage, and congenital amusia, which results from a music-processing anomaly present since birth.

Studies have shown that congenital amusia is a deficit in fine-grained pitch discrimination. Early estimates suggested that 4% of the population has this disorder. More recent direct counts based on a sample of 20,000 people indicate a true rate closer to 1.5%. Acquired amusia may take several forms. Patients with brain damage may experience the loss of ability to produce musical sounds while sparing speech, much like aphasics lose speech selectively but can sometimes still sing. Other forms of amusia may affect specific sub-processes of music processing. Current research has demonstrated dissociations between rhythm, melody, and emotional processing of music. Amusia may include impairment of any combination of these skill sets.

Signs and symptoms

Symptoms of amusia are generally categorized as receptive, clinical, or mixed. Symptoms of receptive amusia, sometimes referred to as "musical deafness" or "tone deafness", Clinical, or expressive, symptoms include the loss of ability to sing, write musical notation, and/or play an instrument. A mixed disorder is a combination of expressive and receptive impairment.

Clinical symptoms of acquired amusia are much more variable than those of congenital amusia and are determined by the location and nature of the lesion.

Social and emotional

Amusic individuals have a remarkable sparing of emotional responses to music in the context of severe and lifelong deficits in processing music. Some individuals with amusia describe music as unpleasant. Others simply refer to it as noise and find it annoying. This can have social implications because amusics often try to avoid music, which in many social situations is not an option.

In China and other countries where tonal languages are spoken, amusia may have the more pronounced social and emotional impact of experiencing difficulty in speaking and understanding the language. However, context clues are often strong enough to determine the correct meaning, similarly to how homophones can be understood.

Amusia has been classified as a learning disability that affects musical abilities. Research suggests that in congenital amusia, younger subjects can be taught tone differentiation techniques. This finding leads researchers to believe that amusia is related to dyslexia and other similar disorders. Research has been shown that amusia may be related to an increase in size of the cerebral cortex, which may be a result of a malformation in cortical development. Conditions such as dyslexia and epilepsy are due to a malformation in cortical development and also lead to an increase in cortical thickness, which leads researchers to believe that congenital amusia may be caused by the identical phenomenon in a different area of the brain.

Amusia is also similar to aphasia in that they affect similar areas of the brain near the temporal lobe. Most cases of those with amusia do not show any symptoms of aphasia. However, a number of cases have shown that those who have aphasia can exhibit symptoms of amusia, especially in acquired aphasia. The two are not mutually exclusive, nor does having one imply possession of the other.

Diagnosis

The diagnosis of amusia requires multiple investigative tools all described in the Montreal Protocol for Identification of Amusia. This protocol has at its center the Montreal Battery of Evaluation of Amusia (MBEA), which involves a series of tests that evaluate the use of musical characteristics known to contribute to the memory and perception of conventional music, but the protocol also allows for the ruling out of other conditions that can explain the clinical signs observed. The battery comprises six subtests which assess the ability to discriminate pitch contour, musical scales, pitch intervals, rhythm, meter, and memory. Both MRI-based brain structural analyses and electroencephalography (EEG) are common methods employed to uncover brain anomalies associated with amusia (See Neuroanatomy). Additionally, voxel-based morphometry (VBM) is used to detect anatomical differences between the MRIs of amusic brains and musically intact brains, specifically with respect increased and/or decreased amounts of white and grey matter. They are unable to recognize or hum familiar tunes even if they have normal audiometry and above-average intellectual and memory skills. Also, they do not show sensitivity to dissonant chords in a melodic context, which, as discussed earlier, is one of the musical predispositions exhibited by infants. The hallmark of congenital amusia is a deficit in fine-grained pitch discrimination, and this deficit is most apparent when congenital amusics are asked to pick out a wrong note in a given melody.

Tone deafness is also associated with other musical-specific impairments such as the inability to keep time with music (beat deafness, or the lack of rhythm), or the inability to remember or recognize a song. These disabilities can appear separately, but some research shows that they are more likely to appear in tone-deaf people. Experienced musicians, such as W. A. Mathieu, have addressed tone deafness in adults as correctable with training.

Acquired amusia

Acquired amusia is a musical disability that shares the same characteristics as congenital amusia, but rather than being inherited, it is the result of brain damage. It is also more common than congenital amusia. While it has been suggested that music is processed by music-specific neural networks in the brain, this view has been broadened to show that music processing also encompasses generic cognitive functions, such as memory, attention, and executive processes. Furthermore, brain damage can disrupt an individual's ability to tell the difference between tonal and atonal music and detect the presence of wrong notes, but can preserve the individual's ability to assess the distance between pitches and the direction of the pitch. The right superior temporal gyrus recruits and evaluates contour information, while both right and left temporal regions recruit and evaluate interval information. In addition, the right anterolateral part of Heschl's gyrus (primary auditory cortex) is also concerned with processing pitch information.

Temporal relations

The brain analyzes the temporal (rhythmic) components of music in two ways: (1) it segments the ongoing sequences of music into temporal events based on duration, and (2) it groups those temporal events to understand the underlying beat to music. Studies on rhythmic discrimination reveal that the right temporal auditory cortex is responsible for temporal segmenting, and the left temporal auditory cortex is responsible for temporal grouping. Other studies suggest the participation of motor cortical areas in rhythm perception and production. Therefore, a lack of involvement and networking between bilateral temporal cortices and neural motor centers may contribute to both congenital and acquired amusia. This connection between the temporal and frontal regions of the brain is extremely important since these regions play critical roles in music processing. Changes in the temporal areas of the amusic brain are most likely associated with deficits in pitch perception and other musical characteristics, while changes in the frontal areas are potentially related to deficits in cognitive processing aspects, such as memory, that are needed for musical discrimination tasks. These findings suggest that any abnormalities and/or injuries to these regions of the brain could facilitate amusia.

Other regions of the brain possibly linked to amusia

  • Lesions in (or the absence of) associations between the right temporal lobe and inferior frontal lobe. In nine of ten tone-deaf people, the superior arcuate fasciculus in the right hemisphere could not be detected, suggesting a disconnection between the posterior superior temporal gyrus and the posterior inferior frontal gyrus. Researchers suggested the posterior superior temporal gyrus was the origin of the disorder.
  • Cortical thickness and reduced white matter – in a recent study, voxel-based morphometry, an imaging technique used to explore structural differences in the brain, revealed a decrease in white matter concentration in the right inferior frontal gyrus of amusic individuals as compared to controls. Lack of extensive exposure to music could be a contributing factor to this white matter reduction. In 1865, Jean-Baptiste Bouillaud described the first series of cases that involved the loss of music abilities that were due to brain injury. In 1878, Grant Allen was the first to describe in the medical literature what would later be termed congenital amusia, calling it "note-deafness". Later, during the late nineteenth century, several influential neurologists studied language in an attempt to construct a theory of cognition. While not studied as thoroughly as language, music and visual processing were also studied. In 1888–1890, August Knoblauch produced a cognitive model for music processing and termed it amusia. This model for music processing was the earliest produced.

While the possibility that certain individuals may be born with musical deficits is not a new notion, the first documented case of congenital amusia was published only in 2002.

This finding led to another test that was designed to assess the presence of a deficiency in pitch perception.

  • Franz Boas
  • William Lawrence Bragg
  • Alfred Duff Cooper
  • Charles Darwin
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  • Pope Francis
  • Ulysses S. Grant
  • W. D. Hamilton
  • Prince Henry, Duke of Gloucester
  • Isabel Paterson
  • Theodore Roosevelt
  • William Butler Yeats

In fiction

  • Horatio Hornblower
  • Trilby O'Ferrall from Trilby
  • Grace from Home on the Range
  • James Fraser from Outlander by Diana Gabaldon
  • Rodrigo De Souza from Mozart in the Jungle
  • Verity Auger and Susan White in Century Rain
  • Arima Kousei from Your Lie in April
  • Regulus from Reverse: 1999

See also

  • Absolute pitch, the human ability to name a musical note when played or sung (less common than relative pitch)
  • Auditory agnosia
  • Cognitive neuroscience of music
  • Musical aptitude
  • Music-specific disorders
  • Relative pitch, the human ability to accurately distinguish pitch intervals (more common than absolute pitch)
  • Synesthesia
  • Tonal memory

References

Further reading

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  • MedicineNet: Amusia
  • NIH: Distorted Tunes Test
  • The Listening Book: Tone-Deaf Choir audio description by W. A. Mathieu