Photo by Franco Folini via Flickr

Anyone with normal hearing can distinguish between the musical tones in a scale: do, re, mi, fa, so, la, ti, do. We take this ability for granted, but among most mammals the feat is unparalleled.

This finding is one of many insights into the remarkable acuity of human hearing garnered by researchers at the University of California, Los Angeles, Hebrew University of Jerusalem and the Weizmann Institute of Science in Rehovot, Israel, reported in January in the journal Nature.

Izhak Fried of U.C.L.A. and his colleagues worked with epileptic patients who had electrodes implanted in their brain to pinpoint the source of their seizures. Some of the probes linked to the auditory cortex, providing the researchers with a detailed window into sound processing.

The study revealed that groups of exquisitely sensitive neurons exist along the auditory nerve on its way from the ear to the auditory cortex. In these neurons natural sounds, such as the human voice, elicit a completely different and far more complex set of responses than do artificial noises such as pure tones. In this mixed environ­ment humans can easily detect frequencies as fine as one twelfth of an octave—a half step in musical terminology.

The vexing question is: Why? Bats are the only mammal with a better ability to hear changes in pitch than humans do. Predatory species such as dogs are not nearly as sensitive—they can dis­criminate resolutions of one third of an octave. Even our primate relatives do not come close: macaques can resolve only half an octave. These results suggest the fine discrimination of sound is not a necessity for survival.

More likely, the researchers speculate, humans use their fine hearing to facilitate working memory and learning capa­bilities, but more research is needed to explore this puzzle.

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