London, January 11 : Researchers at the Hebrew University in Jerusalem and the Weizmann Institute of Science have found that auditory neurons in humans are far more sensitive to fine sound frequencies than most non-human mammals, with the exception of bats.
Dr. Itzhak Fried, a professor of neurosurgery and director of the epilepsy surgery program at the university, has revealed that his team determined how neurons in the human auditory cortex respond to various sounds by taking recordings from four consenting clinical patients at the UCLA Medical Center.
The participants had intractable epilepsy, and were being monitored with intracranial depth electrodes to identify the focal point of their seizures for potential surgical treatment.
At various sites in their brains that were suspected to be involved in the seizures, including the auditory cortex, the researchers implanted electrodes bilaterally.
The recording of brain activity was carried out while the patients listened to artificial random chords at different tones per octave, and to segments from the film "The Good, the Bad and the Ugly''.
It was one such experiment where the patients heard artificial sounds like the random chords as well as more natural sounds such as the voices and noise from the movie soundtrack.
The researchers found that a single auditory neuron from humans showed an amazing sensitivity to distinguish between very subtle frequency differences, down to a tenth of an octave.
They also observed that cat showed a sensitivity of about one octave, rats about a third of an octave on average, and the macaque about half to one octave.
"This is remarkable selectivity. 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 three percent frequency differences that can be detected by single neurons may explain the fact that even musically untrained people can detect such frequency differences," Nature magazine quoted Fried as saying.
"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 added.
The researcher further said that his study was 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.