The precision of human hearing exceeds that found in the auditory cortices of all other land mammals.The human being is the only animal on earth that listens to and creates music out of sheer emotion. We play instruments, compose sonnets and sing our hearts out when we are in love or happy, or even down and depressed.
The auditory system of humans is similar to that of many other mammals, but it is clearly special in a unique way. It would seem to make sense, then, that our brains have developed a highly specific mechanism for processing the sounds we create.
In humans, as in other mammals, a complex grouping of neurons, in a part of the brain called auditory cortex, is working together to interpret sound received at the eardrum. Until now, however, scientists had no understanding of the mechanisms underlying the higher selectivity of the auditory system of humans.
But a new study, by an Israeli team of scientists, shows for the first time that single neurons in the brain, fire and respond to frequencies of different sounds with an unexpected precision, roughly down to about one-tenth of an octave. The team recently published its findings in the journal Nature.
In comparison, the sensitivity of neurons in the same part of the brain of a cat is about one octave, a third of an octave for a rat, and somewhere between half to one octave in a monkey.
“This precision of us humans, being able to hear and process sound, exceeds that found in the auditory cortices of all other land mammals,” Prof. Israel Nelken, from the Department of Neurobiology at Hebrew University in Jerusalem, tells ISRAEL21c.
The highly precise mechanism found in the human brain probably has some sort of evolutionary purpose, but this is not yet fully understood, he adds.
“The role of sound for human communication in evolution is obvious – whether these special properties of the neurons in the brain have anything to do with it, we have yet to investigate,” says Nelken, whose student Yael Bitterman gets credit as first author in the paper.
The study belongs to a multi-center collaboration between Israeli researchers at Hebrew University, the Tel-Aviv Medical Center, the Weizmann Institute of Science and the University of California, in Los Angeles (UCLA).
In the study, the researchers were able to take advantage of another ongoing study conducted by Prof. Itzhak Fried from UCLA and the Tel Aviv Medical Center, who traced abnormal neural activity in people with epilepsy.
The scientists had obtained recordings from single neurons in the auditory cortex, during an experiment where patients were presented with auditory stimuli. Research on brain mechanisms of human hearing until now was done non-invasively and left many unanswered questions about how the system works.
“The neurons can discriminate better than quarter tones – the Arabic scale – and we see this in people who have non-trained neurons,” says Nelken, who adds that the study found a response to sound is not only dependent on the physical characteristic of the sound, but also on its context.
Sounds from a natural source versus a computer synthesized sound, evoke a more complicated hearing response pattern, he added.
“This is remarkable selectivity,” said co-researcher Fried. “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.”