Амузия: Нарушения музыкального восприятия и обработки.

Amusia 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 and that 4% of the population has this disorder. 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.

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. Amusics performed more quickly than normal individuals on a combined task of both spatial and musical processing tasks, which is most likely due to their deficit. Normal individuals experience interference due to their intact processing of both musical and spatial tasks, while amusics do not.

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 a more pronounced social and emotional impact: 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 and having one does not imply possession of the other.

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 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. 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.

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.