Tune-Deaf People May Hear a Sour Note
Findings Could Help Scientists Study Consciousness
People with tune deafness aren't able to tell when a musician
accidentally strikes the wrong note in a song, but their brains
know the difference. Researchers from the National Institute on
Deafness and Other Communication Disorders (NIDCD), part of the
National Institutes of Health, have found that people with tune
deafness, an auditory processing disorder in which a person with
normal hearing has trouble distinguishing notes in a melody, are
able to detect a wrong note unconsciously. The study is published
in the June 11, 2008, issue of the online journal PLoS ONE (http://www.plosone.org/doi/pone.0002349).
Because tune deafness is a commonly occurring phenomenon that
is largely inherited, the study of this disorder could enable scientists
to use the tools of genetic research to better understand the differences
between conscious and unconscious thought.
Neuroscientists have long been baffled by what separates the state
of consciousness from unconsciousness. Other sensory disorders
have been identified in which the brain perceives a stimulus outside
of conscious awareness. However, because these disorders are typically
caused by damage to the brain, there is an inconsistency in data
from one patient to the next and researchers have difficulty finding
a sufficient number of volunteer patients for clinical trials.
"The prevalence of tune deafness is surprisingly high—perhaps
as much as 2 percent of the population is tune deaf—and
it exists in an otherwise normal, uninjured brain," said James
F. Battey, Jr., M.D., Ph.D., director of the NIDCD. "These
factors, combined with the fact that tune deafness is largely genetic
in origin, now raises the possibility of using tune deafness as
a new way to study consciousness."
A person who is tune deaf is unable to perceive pitch, reproduce
melodies, or identify deviations in a melody. According to geneticist
Dennis Drayna, Ph.D., one of the study authors, not only is music
not enjoyable for people with tune deafness, many of them don't
fully understand what music is. "For severely affected tune-deaf
people, Yankee Doodle is no different than traffic noise in Manhattan.
It's fairly meaningless to them," he said.
Dr. Drayna worked closely with neuroimaging scientist Allen Braun,
M.D., and others in NIDCD's Division of Intramural Research to
randomly screen 1,218 individuals using an online version of the
Distorted Tunes Test. The Distorted Tunes Test is a standardized
survey that tests a person's ability to identify whether or not
a short melody is played correctly. (The online version, which
was created by Dr. Drayna, can be found on the NIDCD Web site: http://www.nidcd.nih.gov/tunetest/.)
The researchers then selected those volunteers who scored in the
bottom 10 percent, screened them for hearing loss and other
factors, and arrived at seven subjects with severe tune deafness
who were otherwise medically normal and were willing to take part
in the study. Ten healthy control subjects who performed normally
on the Distorted Tunes Test also took part in the study.
Dr. Braun, Joseph McArdle, Ph.D., and others then used electroencephalography
(EEG), a brain imaging technique that places electrodes around
a person's head and measures the electrical impulses of millions
of neurons in the brain, to study these subjects further. The researchers
measured the volunteers' responses as they listened to an altered
version of the Distorted Tunes Test in which the incorrect melodies
had a single wrong note at the end. Volunteers listened to 102
familiar melodies, roughly half of which were correct, and half
of which contained the errant last note. The researchers then sifted
through the EEG data to isolate the brain's response to a specific
stimulus—in this case, the right or wrong note.
Of principal interest were two signals that brains normally generate
when they are presented with a stimulus that doesn't match what
the brain expects to hear, such as the wrong note in a song. The
first, the mismatch negativity (MMN), is a large negative signal
that occurs roughly 200 milliseconds after the unexpected stimulus
is heard; the second signal, the P300, is a large positive signal
occurring roughly 300 milliseconds after the unexpected stimulus.
Because tune-deaf people consistently don't recognize when a wrong
note is played or sung, the researchers hypothesized that their
brains would not generate the MMN or P300 signals, and as expected,
this was true for the MMN signal. However, in the case of the P300
signal, tune-deaf volunteers were processing the wrong note in
the same way as the normal participants, even though they weren't
consciously aware of the deviation. Other brain signals demonstrated
that correct notes were being processed equally well for both tune-deaf
and normal volunteers.
As for how a brain can register a wrong note without the person
being aware of it, the researchers explain that the MMN and P300
signals are generated in different parts of the brain. The MMN
is generated near the primary auditory cortex, in the brain's temporal
lobe, while the P300 is generated in the frontoparietal cortex,
downstream from the auditory cortex. Normal brains process sounds
in a series, with the frontal and parietal cortices receiving signals
that have already been processed in the auditory cortex. In someone
with tune deafness, however, the direct route for processing the
wrong note may be disrupted, and signals are possibly being routed
to the two regions through parallel pathways independent of each
other. In this way, information about a wrong note may not be reaching
the auditory cortex at all, while information reaching the frontoparietal
cortex is not consciously perceived.
The researchers hope to conduct studies to better pinpoint the
locations from which the MMN and P300 signals originate in the
brain. In addition, the researchers will continue to pursue genetic
studies on the causes of tune deafness which, if found, could help
them and others grapple with the very puzzling notion of consciousness
at the cellular and molecular level.
NIDCD supports and conducts research and research training on
the normal and disordered processes of hearing, balance, smell,
taste, voice, speech and language and provides health information,
based upon scientific discovery, to the public. For more information
about NIDCD programs, see the Web site at www.nidcd.nih.gov.
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit www.nih.gov.