What is amusia

Amusia is a neural disorder characterized by severe impairment to perceive music caused by either abnormal brain development (congenital amusia) or brain tissue damage (acquired amusia) ¹. Deficit in perceiving fine-grained pitch changes (tone deafness) is the hallmark symptom of amusia, but also other domains of music, such as rhythm, can be affected ². Congenital amusia is a neurodevelopmental auditory disorder characterized by deficits in pitch perception and production that cannot be attributed to hearing loss or neurophysiological causes ³. Congenital amusia has been estimated to affect 1.5%–4% of the general population with slightly higher rates in females ⁴, has been shown to be hereditary ⁵, and does not appear to co-occur with other cognitive disorders ⁴. Besides having pitch discrimination difficulties ⁶, individuals with congenital amusia are impaired in pitch contour identification ⁷, melodic sequence recognition ⁸, singing in tune ⁹, as well as in memorizing pitch-based materials ¹⁰. However, amusics show normal processing on some other music related attributes such as musical emotion based on temporal or timbral cues ¹¹, but not when emotion was elicited by the tonality of musical excerpts ¹². Recent theories have suggested that amusic individuals’ pronounced difficulties in pitch processing may stem from an inability to use the fine spectro-temporal cues in the resolved harmonics in complex tones without peripheral basis ¹³. Consistent with this view, one recent meta analysis has shown that pitch change is an effective moderator of the effect size of performance gap between amusic and control across studies, supporting the hypothesis that amusia stems from a broad disorder of acoustic pitch processing ¹⁴.

The perception and experience of music in the healthy brain is based on the functioning of a large-scale bilateral neural network comprising temporal, frontal, parietal, cerebellar, and subcortical areas ¹⁵. The amusic brain shows abnormalities in neural transmission between the auditory cortex and the inferior frontal gyrus in the right cerebral hemisphere.

The two types of amusia, acquired and congenital amusia, may have partly distinct neural basis. Congenital amusia is a developmental deficit and thus impedes acquiring musical syntax ¹⁶, whereas acquired amusia represents a shift from a normal to deficiently functioning music processing system caused by a brain lesion. Studying brain lesions and associated cognitive deficits is essential in uncovering crucial brain regions that are causally connected ¹⁷. This can be achieved by voxel-based lesion-symptom mapping (VLSM), which is an advanced MRI analysis method investigating the relationship between focal brain damage and behavioral data on a voxel-by-voxel basis ¹⁸. Compared to the traditional lesion-led or symptom-led approaches, voxel-based lesion-symptom mapping (VLSM) allows both binary and continuous analyses and does not require patient grouping by lesion site. Voxel-based lesion-symptom mapping (VLSM) utilizes three-dimensional lesion maps formed from MRI images, and evaluates the presence or absence of lesion in each voxel to finally associate this information with the behavioral data.

While congenital amusia is generally described as a deficit in processing pitch, arguably due to an impairment of pitch perception and/or pitch-specific short-term or working memory—the processing of musical rhythm, timbre, and emotions can be affected as well ¹⁹.

The majority of the neuroimaging studies examining defective music processing in the brain have been carried out on congenital amusia, a condition affecting 2–4% of the population ²⁰. In contrast, acquired amusia is a relatively common disorder after a middle cerebral artery stroke, with incidence ranging from 35 to 69% ²¹. Evidence derived from MRI morphometry studies, utilizing e.g., voxel-based morphometry (VBM) ²², an automated method for analyzing gray matter and white matter differences between groups or across time, has implicated reduced white matter concentration in the right inferior frontal gyrus ²³ and right superior temporal gyrus ²³ in congenital amusics. Additionally, the cortex in both of these areas have been shown to be thicker in congenital amusic subjects than in controls ²⁴. However, the reported results have been contradictory regarding laterality of the observed effect: a recent study showed that congenital amusics had decreased gray matter volume in the left inferior frontal gyrus and superior temporal gyrus with no differences observed in the right homologous areas ²⁵. Taken together, these findings suggest that congenital amusia may be a somewhat heterogeneous condition ²⁶.

Figure 1. Amusia neural basis

Footnote: Anomalous Recurrent Processing in the Right Frontotemporal Network. Schematic (left panel) and anatomical representation (right panel) of the anomalies (in red) found in functional and structural connectivity between the right and left superior temporal gyrus (STG) and right inferior frontal gyrus (IFG) in the amusic brain as compared to the normal brain. Current evidence suggests that all other connections from the cochlea up to the STG, including the primary auditory cortex (A1), are normal. The proposed altered recurrent processing as a result of poor feedback control between the IFG and STG is represented by dashed lines.

[Source ²⁷ ]

Amusia treatment

Amusia treatment involves treating the underlying cause of amusia. For example, all stroke patients should receive standard medical treatment and rehabilitation for their stroke. Congenital amusia is a lifelong condition.

References

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