Our Auditory Neurons - 10 Times More Sensitive than a Cat's: One Tenth of an Octave

There must have been something about our hearing that enabled us to differentiate speech and music from other sounds. The human ear can detect sound frequencies of 16Hz to 20 kHz, no matter if tones were high or low, near or far.

But our ears are simple compared to the remarkable ability of single brain neurons to make the difference between the very subtlest frequencies, down to a tenth of an octave. This is the result of a research published in the journal "Nature" by a team led by Dr. Itzhak Fried, a professor of neurosurgery and director of the epilepsy surgery program, at the Hebrew University in Jerusalem and the Weizmann Institute of Science. This sensitivity exceeds the ability of the auditory nerve to transport data from the auditory hair cells of the inner ear to the auditory cortex, by 30 times.

Such brain auditory tuning in the human auditory cortex is matched amongst mammals only by bats.

"It is quite a paradox in that even musically untrained people can detect very small sound frequency differences, much better than the resolution of the peripheral auditory nerves", wrote the authors.

In other senses, such as touch, human sensitivity is limited by the number of receptors in the skin. In hearing, the number of receptors seems to be less important.

The research was made on four UCLA Medical Center patients, who had intractable epilepsy, and were being monitored with intracranial depth electrodes to detect the focal point of their seizures for a subsequent surgery. Electrodes were implanted bilaterally in different brain areas that could be connected to the seizures, including the auditory cortex.

The brain activity was registered while the subjects listened artificial random chords at various tones per octave, and clips from the movie "The Good, the Bad and the Ugly". This way, the subjects listened both artificial sounds (the random chords) and natural sounds (voices and noise from the movie soundtrack).

The team found that just one auditory neuron from humans had an amazing sensitivity to detect subtle frequency differences of just a tenth of an octave, amazing compared to one octave in the case of cats, a third of an octave in rats, and half to one octave in the macaque monkeys.

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

"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", said Fried.

Fried's team had already detected single neurons in the human hippocampus, connected to human navigation, and single neurons decoding varied visual images of the same thing, like the identity of an individual.