“Tone deaf” people may lack a major nerve pathway linking brain regions for sound-perception and sound-production. Using a special imaging method that maps connections within the brain, researchers at Harvard Medical School and Beth Israel Deaconess Medical Center in Boston found evidence that nearly all the tone-deaf people they studied were missing a nerve-fiber bundle known as the superior branch of the arcuate fasciculus in the right cerebral hemisphere of their brains.
“In the past, we knew that a connection disorder in the arcuate fasciculus led to language difficulties. Now that we know that tone-deafness affects similar branches of connectivity, we might start thinking about tone-deafness as a previously undiscovered connection disorder,” says Psyche Loui, the researcher who was first author on the study.
Loui and her colleagues studied 20 right-handed people, half of whom had tone deafness—formally known as congenital amusia—according to a standard test of their ability to discriminate musical frequencies or pitches. Building upon previous work in this area, which had found subtle abnormalities in specific brain regions, the researchers used the nerve-mapping technique diffusion-tensor imaging (DTI) to look for problems in the connections between these regions.
They found that in nine of the 10 tone-deaf subjects, they could not find a trace of the superior branch of the arcuate fasciculus in the brain’s right hemisphere. Apparently as a result, one of the key right-hemisphere regions to which the arcuate fasciculus connects, a temporal lobe region called the posterior superior temporal gyrus, was missing its usual functional links to the frontal lobe and had been rewired with connections to other brain regions.
“The work is entirely congruent with emerging views that a disrupted network between superior temporal lobe and frontal lobe underlies the condition,” says Timothy Griffiths, a professor of cognitive neurology at the University of Newcastle Medical School in the United Kingdom.
Division of aural labor?
Loui and her colleagues also compared the overall, DTI-measured volume of the arcuate fasciculus in each subject to the person’s abilities to perceive and produce musical pitches. Smaller volumes of the superior branch corresponded to a reduced ability to discriminate one pitch from another. Analyses of inferior branch volumes, by contrast, suggested that they were more associated with sound reproduction.
“The superior branch might be more responsible for conscious, fine-grained tuning of perception whereas the inferior branch might be more for involuntary, coarse-grained production,” Loui says. The retention of the second of these pathways might partly explain why tone-deaf people typically have little or no difficulty in automatically recognizing and producing the coarser pitch differences of language. Loui and colleagues have shown in previous work that such subjects also have moderately better pitch reproduction abilities than they are consciously aware of, implying that separate conscious and non-conscious neural pathways are involved.
Similar “dual-stream” processing systems, in which one pathway correlates more with conscious perception and the other correlates more with perception-related action, have been found for human vision and hearing, Loui says, and may prove to be a general design feature of human sensory-motor systems.
The findings highlight the perennial question of how to separate musical from language processing in the brain. Stroke damage to the left hemisphere arcuate fasciculus has been known to cause conduction aphasia, a language disorder in which people cannot repeat words or phrases they hear. The condition might seem to be the left-brain, verbal version of tone deafness, but there is evidence that musical and language abilities typically engage both hemispheres, and that may be true of tone-deafness too.
“We still see some differences in the left hemisphere arcuate fasciculus in our tone deaf group. The difference is more robust in the right hemisphere, but I’m reluctant to say that it’s just left or just right,” Loui says.
The study of tone deafness is somewhat complicated by the fact that different research groups use different diagnostic tests. Performance on these tests may involve different brain systems that relate to subtly different skills. “There are studies suggesting that pitch memory can be a contributing factor to this problem, in other words that it’s not solely a perceptual deficit,” says Lauren Stewart, a researcher into the neuroscience of music at the University of London.
One recent study of tone-deaf people randomly screened from the general population found that about one-third showed evidence of attention deficit/hyperactivity disorder. Thus in ordinary life, a person with memory or attention deficits might seem tone deaf even if she has more or less normal pitch-perception abilities. “I think there’s more than one kind of tone deafness,” Loui says.