Humans can hear sounds more clearly and sharper than some other animals and that may have played a role in the evolutionary origins of speech, according to a new research paper co-authored by York University Associate Professor Christopher Bergevin.
The research focused on the frequency analysis of the cochlea, a part of the inner ear that resides deep in the base of the skull. By comparing various measures, the study addressed the highly debated question of whether the cochlea in humans allows us to hear differently.
“Our ears are both sensitive and selective,” said Bergevin of the Department of Physics and Astronomy in the Faculty of Science as well as the Centre for Vision Research at York.
“Think of a piano keyboard. How lightly can I tap a key before you cannot notice the sound anymore? That is a measure of sensitivity. But how easily can you discriminate between two different adjacent keys? That is selectivity. A piano is built to make that distinction obvious, for musical reasons. But you can envision the fretless neck of a violin to better see the point.”
The study, led by Professor Christian Sumner of the University of Nottingham, compared three different measures of frequency selectivity. Combining neurophysiology and psychophysics, along with otoacoustics, the researchers were able to utilize the fact that healthy ears emit sound.
Traditionally, otoacoustic methods served as scientific and clinical probes for sensitivity, often to screen the hearing ability of newborns. But this study helps cement their use as an objective and non-invasive measure of sound selectivity. The results, which compared human hearing to that of ferrets, also solidified the idea that humans are relatively good at discriminating between different frequencies. In other words, they are more sharply tuned.
“The research raises important questions, particularly about how human cochlear tuning evolved in humans and the role it played in the early development of speech,” said Bergevin. “It also begs the question of what mechanisms are responsible for our ability to separate and process natural sounds, especially in complex acoustic environments.”
The research was published in the October issue of the journal Proceedings of the National Academy of Sciences of the United States of America.