Human auditory neurons more sensitive than those of other mammals

January 16, 2008

The human ear is exquisitely tuned to discern different sound frequencies, whether such tones are high or low, near or far. But the ability of our ears pales in comparison to the remarkable knack of single neurons in our brains to distinguish between the very subtlest of frequency differences.

Reporting in the Jan. 10 issue of the journal Nature, Dr. Itzhak Fried, professor of neurosurgery and director of the UCLA Epilepsy Surgery Program, and colleagues from Hebrew University and the Weizmann Institute of Science in Israel, show that in humans, a single auditory neuron in the brain exhibits an amazing selectivity to a very narrow sound-frequency range, roughly down to a tenth of an octave.

In fact, the ability of such neurons to detect the slightest of differences in sound frequency far surpasses that of the human auditory nerve, which carries information from the hair cells of the inner ear to the brain's auditory cortex — by as much as 30 times greater sensitivity. Indeed, such frequency tuning in the human auditory cortex is substantially superior to that typically found in the cortex of nonhuman mammals, with the exception of bats.

It is a paradox, the researchers note, that even the auditory neurons of musically untrained people can detect very small differences in frequency much better than their peripheral auditory nerve. With other peripheral nerves, such as those in the skin, the human ability to detect differences between two points — say from the prick of a needle — is limited by the receptors in the skin; the neurons associated with those peripheral nerves display no greater sensitivity. With hearing, however, the sensitivity of the neuron actually exceeds that of the peripheral nerve.

The researchers, including senior author Israel Nelken and first author Yael Bitterman from Hebrew University, determined how neurons in the human auditory cortex responded to various sounds by taking recordings of brain activity from four consenting clinical patients at UCLA Medical Center. These patients had intractable epilepsy and were being monitored with intracranial depth electrodes to identify the focal point of their seizures for potential surgical treatment.

Using clinical criteria, electrodes were implanted bilaterally at various brain sites that were suspected to be involved in the seizures, including the auditory cortex. The recording of brain activity was carried out while patients listened to artificial random chords at different tones per octave and to segments from the film "The Good, the Bad and the Ugly.'' Thus, the sounds the patients heard were both artificial (the random chords) and more natural (the voices and noise from the movie soundtrack).

The results surprised the researchers. A single auditory neuron from humans showed an amazing ability to distinguish between very subtle frequency differences, down to a tenth of an octave. This, compared to a sensitivity of about one octave in the cat, about a third of an octave in rats and a half to a full octave in the macaque.

"This is remarkable selectivity," said Fried, who is also co-director of UCLA's Seizure Disorder Center. "It is indeed a mystery why such resolution in humans came to be. Why did we develop this? Such selectivity is not needed for speech comprehension, but it may have a role in musical skill. The 3 percent frequency differences that can be detected by single neurons may explain the fact that even musically untrained people can detect such frequency differences.

"There is also evidence that frequency discrimination in humans correlates with various cognitive skills, including working memory and the capability to learn, but more research is needed to clarify this puzzle," he said.

This study, Fried noted, is the latest example of the power of neurobiological research that uses data drawn directly from inside a living human brain at the single-neuron level. Previous studies from Fried's lab have identified single cells in the human hippocampus specific to place in human navigation, and single cells that can translate varied visual images of the same item — such as the identity of an individual — into a single concept that is instantly and consistently recognizable.

Source: UCLA

Explore further: Is human noise pollution affecting our sharks?

Related Stories

Is human noise pollution affecting our sharks?

August 6, 2015

Human made noise, also called anthropogenic noise, is rising in many environments due to the increase in transportation and the exploration for and exploitation of energy sources.

Innovations from the wild world of optics and photonics

August 2, 2015

Traditional computers manipulate electrons to turn our keystrokes and Google searches into meaningful actions. But as components of the computer processor shrink to only a few atoms across, those same electrons become unpredictable ...

Graphene sheets enable ultrasound transmitters

July 7, 2015

University of California, Berkeley, physicists have used graphene to build lightweight ultrasonic loudspeakers and microphones, enabling people to mimic bats or dolphins' ability to use sound to communicate and gauge the ...

Songbirds have a thing for patterns

June 25, 2015

You might think that young children would first learn to recognize sounds and then learn how those categories of sounds fit together into words. But that isn't how it works. Rather, kids learn sounds and words at the same ...

Method to reconstruct overt and covert speech

October 31, 2014

Can scientists read the mind, picking up inner thoughts? Interesting research has emerged in that direction. According to a report from New Scientist, researchers discuss their findings in converting brain activity into sounds ...

Recommended for you

How the finch changes its tune

August 3, 2015

Like top musicians, songbirds train from a young age to weed out errors and trim variability from their songs, ultimately becoming consistent and reliable performers. But as with human musicians, even the best are not machines. ...

Machine Translates Thoughts into Speech in Real Time

December 21, 2009

(PhysOrg.com) -- By implanting an electrode into the brain of a person with locked-in syndrome, scientists have demonstrated how to wirelessly transmit neural signals to a speech synthesizer. The "thought-to-speech" process ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

KB6
not rated yet Jan 17, 2008
"Such selectivity is not needed for speech comprehension..."
---
It might not be needed to just comprehend words. But even very subtle tonal differences in speech can change the entire meaning and emotional content of what is being said, like:
"You aren't going to wear THAT to the party."
"YOU aren't going to wear that to the party."
barakn
1 / 5 (1) Feb 21, 2008
And it would be very much necessary for tonal languages like the various Chinese languages.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.