Do humans hear better than
animals? It is known that various species of land and
water-based living creatures are capable of hearing some
lower and higher frequencies than humans are capable of
detecting. However, scientists from the Hebrew University
of Jerusalem and elsewhere have now for the first time
demonstrated how the reactions of single neurons give
humans the capability of detecting fine differences in
frequencies better than animals.
IThey did this by utilizing a technique for recording
the activity of single neurons in the auditory cortex
while subjects were exposed to sound stimuli. The auditory
cortex has a central role in the perception of sounds
by the brain.
Current knowledge on the auditory cortex was largely based
on earlier studies that traced neural activity in animals
while they were exposed to sounds. And while such studies
have supplied invaluable information regarding sound processing
in the auditory system, they could not shed light on the
human auditory system's own distinctive attributes.
Experimental study of neural activity in the human auditory
cortex has been limited until now to non-invasive techniques
that gave only a crude picture of how the brain responds
to sounds. But recently, investigators from the Hebrew
University, the University of California, Los Angeles
(UCLA), the Tel Aviv Sourasky Medical Center and the Weizmann
Institute of Science were successful in recording activity
of single neurons in the auditory cortex while the subjects
were presented with auditory stimuli. They did this by
utilizing an opportunity provided during an innovative
and complicated clinical procedure, which traces abnormal
neural activity in order to improve the success of surgical
treatment of intractable epilepsy,
The researchers included Prof. Israel Nelken of the Department
of Neurobiology at the Alexander Silberman Institute of
Life Sciences at the Hebrew University of Jerusalem, Prof.
Itzhak Fried from UCLA and Tel Aviv Medical Center, and
Prof. Rafi Malach of the Weizmann Institute of Science,
together with their students Roy Mukamel and Yael Bitterman.
Their work was described in an article appearing in the
journal Nature.
In tests measuring response to artificial sounds, the
researchers found that neurons in the human auditory cortex
responded to specific frequencies with unexpected precision.
Frequency differences as small as a quarter of a tone
(in western music, the smallest interval is half a tone)
could be reliably detected from individual responses of
single neurons.
Such resolution exceeds that typically found in the auditory
cortex of other mammalian species (besides, perhaps, bats,
which make unique use of their auditory system), serving
as a possible correlate of the finding that the human
auditory system can discriminate between frequencies better
than animals. The result suggests that the neural representation
of frequency in the human brain has unique features.
Interestingly, when the patients in the study were presented
with "real-world" sounds -- including dialogues,
music (from "The Good, the Bad and the Ugly"
soundtrack) and background noise -- the neurons exhibited
complex activity patterns which could not be explained
based solely on the frequency selectivity of the same
neurons. This phenomenon has been shown in animal studies
but never before in humans.
Thus, it can be seen that in contrast to the artificial
sounds, behaviorally relevant sounds such as speech and
music engage additional, context-dependant processing
mechanisms in the human auditory cortex.
MORE
INFORMATION
This article first appeared
in Science Daily News with materials adapted from The
Hebrew University of Jerusalem. The article can be found
at: www.sciencedaily.com
Music without words means leaving behind the mind. And leaving behind the mind is meditation.
Meditation returns you to the source. And the source of all is sound. — Kabir
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