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Music and the Brain
Although congenital amusia refers to the lifelong deficit of musical abilities, acquired amusia refers to the loss of musi- cal ability resulting from brain damage. Lesion analyses show that disconnections in multiple pathways are common among those with acquired amusia. Those with lesions covering mul- tiple white matter pathways are least likely to recover from acquired amusia after a stroke, whereas those who recover from acquired amusia are more likely to have damage in one pathway while sparing others (Sihvonen et al., 2017). These findings provide insight into possible targets for neuroreha- bilitation after a stroke or other brain injury. Because stroke is a leading cause of long-term disability in older adults, rehabilitating musical functions in those suffering from the aftermath of a stroke will be key to improving the quality of life in these affected individuals (Norton et al., 2008).
Absolute Pitch
Whereas amusia is a deficit in pitch perception and produc- tion ability, absolute pitch (AP) seems ostensibly to reflect the opposite. People with AP have the ability to identify the pitch class of musical notes without an external reference (Ward, 1999). In addition to being more common in musi- cally trained individuals, especially those who started musical training before the age of seven, AP runs in families and is more common among those of East Asian descent. It is espe- cially common in those of East Asian heritage who speak tone languages fluently, suggesting that the ability is associated with early language experience (Deutsch et al., 2009). Even among people with AP, there is a range of pitch identifica- tion ability. Although some AP possessors are able to name any note in any timbre, other individuals have AP only for the instruments they play (Miyazaki, 1989). Because of these intriguing interactions between genetic and environmental factors, AP is an ideal model for understanding the influences of genes and the environment.
The neural substrates that enable automatic pitch categoriza- tion likely come from the planum temporale, a region within the aforementioned STG that is exceptionally larger in the left hemisphere of AP musicians, presenting as a more left- ward asymmetrical brain in MRIs (Schlaug et al., 1995). In addition to being larger in volume, the left STG is also better connected in AP musicians relative to their non-AP counter- parts, and pitch categorization accuracy is correlated with the white matter volume of connections identified from the STG (Loui et al., 2011). Functional MRI results point to a distrib- uted network of enhanced activity throughout the brain in AP musicians, albeit with results centering around the left
STG (Loui et al., 2012). Thus, it appears that both specific brain structure and general network-level brain functioning are special in AP possessors.
Although AP is presumed to be rare, occurring in less than 1% of the general population (Ward, 1999), most listen- ers possess some absolute memory for pitch as shown by being able to produce familiar songs at the right starting pitch after repeated listening (Levitin, 1994). However, the enhanced categorization ability seems relatively rare, and specific to a unique population. This has led some research- ers to ask whether the AP possessors’ tendency to categorize pitch might be thought of as a savant-like ability as seen in some individuals with autism (Mottron et al., 2013). In that regard, musicians with and without AP were tested on the subclinical traits of autism using the autism spectrum quotient (Dohn et al., 2012). Results showed that although
AP possessors scored higher than non-AP counterparts in the autism quotient, they were still well lower than those that would meet the criteria for autism spectrum disorders. Crucially, AP possessors showed some higher scores in imagination but no differences from controls in social and communicative subscales. Taken together, AP could be con- sidered an enhanced perceptual categorization ability, likely subserved by a network of regions centering around the structurally altered superior temporal lobe. Whether this brain network and its supported functions can be trained in the laboratory or in the practice room remains an active area of both psychological and pedagogical research.
Timbre
Although the sensation of pitch is correlated with the f0 of periodic sounds, the sensation of timbre is an emergent prop- erty of spectral and temporal characteristics of sounds. The temporal envelope of a sound, especially the time between its onset and its peak amplitude (“attack time”), is a strong deter- minant of the perceptual attribute of “bite.” Although attack time is a feature of the temporal envelope, spectral centroid is a feature of the spectral envelope and is computed as the weighted average of the frequency of all harmonics present, giving rise to its “brightness.” Classic studies have found that spectral centroid and attack time are two orthogonal dimen- sions that account for much of the variance in judgments in sound quality (Wessel, 1979). The third and most salient dimension that is both spectral and temporal in nature is spectral flux, which is the change in spectral centroid over time (McAdams, 2013). Because these dimensions of sound are clearly defined and orthogonal to each other, both per-
32 | Acoustics Today | Winter 2019