Page 29 - Summer 2010

NEUROLOGICAL BASES OF MUSICAL DISORDERS AND THEIR IMPLICATIONS FOR STROKE RECOVERY
Psyche Loui, Catherine Y. Wan, and Gottfried Schlaug
Music, Neuroimaging and Stroke Recovery Laboratories
Beth Israel Deaconess Medical Center and Harvard Medical School Boston, Massachusetts 02215
“Singing, or the act of producing musical sounds with the voice, is celebrated in every culture around the world.”
Singing as an exercise of the auditory-motor system
Singing, or the act of producing
musical sounds with the voice, is
celebrated in every culture around
the world. From the earliest point in
infancy, humans have some knowledge
of musical sounds, broadly defined—
evidence for knowledge of musical
attributes has been observed even in
newborn infants (Winkler et al., 2009). Sensitivities to pitch, key, and harmony are known to emerge from infancy to childhood (Olsho et al., 1982; Trainor and Trehub, 1994) and have been reported in many other cultures (Castellano et al., 1984; Krumhansl et al., 2000; Trehub et al., 2008). Because of its prevalence among humans, the ability to make music has been posited as an innate human ability (Peretz, 2006).
Singing requires the coordination of auditory and motor networks and involves the perception and production of pitch and rhythm. People who have problems with singing, i.e., tone-deaf people, provide an interesting model for study- ing brain networks involved in singing and how they might overlap with networks for speaking abilities. This overlap of neural resources recruited in singing and speaking is useful in therapeutic applications, where the loss of language func- tion (a condition known as aphasia) can be rehabilitated using a treatment program known as Melodic Intonation Therapy (Schlaug et al., 2008; Norton et al., 2009; Schlaug et al., 2009). In this article, we will review current research in our lab on singing: what the ability entails, why some indi- viduals lack singing ability, and how singing can be used to improve the well-being of those afflicted by neurological dis- orders.
Tone-deafness as a musical disorder
Despite the pervasive evidence for musical ability (espe- cially singing ability) in all cultures, some individuals have an unusual lack of musical skill, especially in the perception and production of pitch: a condition known as congenital amusia, also known as tone-deafness. Tone-deaf individuals can be identified among others by using the standardized perceptu- al test known as Montreal Battery for the Evaluation of Amusia (MBEA), (Peretz et al., 2003). The MBEA consists of pairs of melodies that are either the same or different in melodic or rhythmic content; the test taker’s task is to decide whether the pairs of melodies are the same or different, and individuals who score below a cutoff are labeled as amusic. In addition to the MBEA, we employ a psychophysical listening test that finds the threshold at which pitch differences can
reliably be detected. A version of this test can now be taken online at www.musicianbrain.com. Using this test, individuals with a pitch-discrimi- nation threshold of more than a semi- tone are considered tone-deaf (Foxton et al., 2004; Loui et al., 2008). In addition to these perceptual tests, tone-deaf indi- viduals are identifiable by their inability to sing in tune.
The majority of tone-deaf individuals are unaware of their inability to perceive pitches, but only of their inability to sing—perhaps resulting from being discouraged from singing by those around them (Cuddy et al., 2005). While conventional wisdom might suggest that hearing pitches and producing them are intricately linked abilities, in previous studies we had identified that the abilities to perceive and to produce pitches are not necessarily the same amongst all individuals: in a study in published in 2008, we observed that tone-deaf individuals, who cannot consciously perceive pitch differences smaller than a semitone, can paradoxically pro- duce these pitch intervals in the right direction (Loui et al., 2008). Furthermore, the threshold difference at which tone- deaf subjects can reliably produce pitch differences is smaller than the threshold difference at which they can reliably per- ceive pitch differences, suggesting that tone-deaf individuals have the unconscious ability to produce pitches even in the absence of conscious pitch perception, resulting in a mis- match between pitch perception and production abilities. Perhaps because of these residual pitch production abilities even in the absence of intact pitch perception, tone-deaf indi- viduals are generally able to speak normally. There are, how- ever, several reports that the perception of intonation in speech is disrupted in tone-deaf individuals (Patel et al., 2005; Patel et al., 2008), which raises two interesting ques- tions. First, how do tone-deaf individuals speak and under- stand speech in tonal language cultures? And a second relat- ed question—what, if any, compensatory neural mechanisms might the tone-deaf individuals be using to produce normal speech? Several research projects in our lab and others are currently trying to answer these questions.
Neuroimaging of tone-deafness implicates auditory- motor regions in frontal and temporal lobes
In addition to the behavioral studies described above, neuroimaging research is now underway to examine the neu- ral underpinnings of pitch perception and production, specifically in tone-deaf individuals. Magnetic resonance imaging (MRI) studies have explored the neuroanatomical basis of tone-deafness. Studies looking at voxel-based mor-
28 Acoustics Today, July 2010