Abstract
In this article, we report the first documented case of congenital amusia in dizygotic twins. The female twin pair was 27 years old at the time of testing, with normal hearing and above average intelligence. Both had formal music lesson from the age of 8–12 and were exposed to music in their childhood. Using the Montreal Battery of Evaluation of Amusia (Peretz et al., 2003), one twin was diagnosed as amusic, with a pitch perception as well as a rhythm perception deficit, while the other twin had normal pitch and rhythm perception. We conducted a large battery of tests assessing the performance of the twins in music, pitch perception and memory, language perception and spatial processing. Both showed an identical albeit low pitch memory span of 3.5 tones and an impaired performance on a beat alignment task, yet the non-amusic twin outperformed the amusic twin in three other musical and all language related tasks. The twins also differed significantly in their performance on one of two spatial tasks (visualization), with the non-amusic twin outperforming the amusic twin (83% vs. 20% correct). The performance of the twins is also compared to normative samples of normal and amusic participants from other studies. This twin case study highlights that congenital amusia is not due to insufficient exposure to music in childhood: The exposure to music of the twin pair was as comparable as it can be for two individuals. This study also indicates that there is an association between amusia and a spatial processing deficit (see Douglas and Bilkey, 2007; contra Tillmann et al., 2010; Williamson et al., 2011) and that more research is needed in this area.
Highlights
Congenital amusia is an innate disorder that has been shown to have a negative influence on pitch perception (Peretz et al, 2002; Foxton et al, 2004; Stewart, 2008), with a co-occurring deficit in rhythm perception in about 50% of the cases (Pfeifer and Hamann, 2015)
The pitch perception and memory tasks as well as the language perception tasks have previously been used with amusics, and the performance of the twin pair is compared to those samples
The results indicate that the amusic twin can perform egocentric spatial transformations, as shown by the Object Perspective Taking Test, but struggles with objectbased spatial transformations that were required in the Santa Barbara Solids Test
Summary
Congenital amusia is an innate disorder that has been shown to have a negative influence on pitch perception (Peretz et al, 2002; Foxton et al, 2004; Stewart, 2008), with a co-occurring deficit in rhythm perception in about 50% of the cases (Pfeifer and Hamann, 2015). Peretz et al (2007) studied 13 amusics from nine families and calculated a sibling recurrence risk ratio (the ratio of manifestation, given that a sibling is affected, compared with the prevalence in the general population; Risch, 1990) of λs = 10.8 This ratio is in the same order of magnitude as the heritability of specific language impairments and of absolute pitch. Familial aggregation could be due to shared family environment (in the case of congenital amusia, e.g., non-exposure to music within a family) Such environmental factors can only be reliably separated from genetic effects in twin studies, which have been employed successfully to test the heritability of pitch processing in general. They found high correlations of 0.57 for melody and 0.48 for pitch in monozygotic twins but lower correlations of 0.32 for melody and 0.29 for pitch in monozygotic twins
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