March 19, 2014
Miniature Pores In Tectonic Membrane Help People Distinguish Sound Frequency
redOrbit Staff & Wire Reports - Your Universe Online
Computers have been largely unable to duplicate a person’s ability to tune into a single voice, even in the midst of background noise, but new research into the biological mechanisms of the human ear could soon change that.
According to researchers from the Massachusetts Institute of Technology (MIT), the ear’s selective hearing is due to the evolution of the tectorial membrane, a tiny membrane located inside the inner ear. The viscosity of this membrane, the study authors said, is reliant upon the size and distribution of nanopores.
These nanopores are only a few tens of nanometers wide, they explain, and this feature helps provide the mechanical filtering that allows men and women to sort through various types of sound. This research, which appears in the March 18 edition of the Biophysical Journal, could ultimately lead to improved machine hearing capabilities, as well as better hearing aids.
In the study, MIT graduate student Jonathan Sellon and his colleagues examined the tectorial membrane and its role in helping the human ear consistently outperform traditional speech-recognition technologies. While it had long been assumed that this was linked to a person’s ability of resolving frequency, recent research by the MIT team indicates that this is not necessarily the case.
Actually, Sellon, electrical engineering professor Dennis Freeman and their associates have found that some individuals who have genetic defects in their tectorial membranes are more sensitive to frequency variations – and as a result, their hearing is actually worse.
They discovered a tradeoff between the ability to resolve different frequencies and the length that it takes to actually do so. As a result, the improved frequency discrimination is not fast enough to actually be useful in a real-world situation that requires sound selectivity, the institute explained in a statement Tuesday.
Previously, the MIT team has demonstrated that the tectorial membrane is instrumental in sound discrimination by carrying waves that stimulate a certain type of sensory receptor. This is vital in the process of deciphering a cacophony of sounds, but it occurs so quickly that neural processes are unable to keep up with the pace.
Thus, through evolution, nature has crafted what Freeman describes as an extremely effective electromechanical system capable to equaling the speed of those sound waves. The new study helps explain how the tectonic membrane’s structure is the determining factor in its ability to filter sound in a noisy environment.
They authors analyzed two genetic variants that cause nanopores in the membrane to be larger or smaller than usual. The pore size impacts the membrane’s viscosity and its sensitivity to various different frequencies, the National Institutes of Health (NIH), National Science Foundation (NSF) and Wellcome Trust-funded research uncovered.
“The tectorial membrane is spongelike, riddled with tiny pores. By studying how its viscosity varies with pore size, the team was able to determine that the typical pore size observed in mice – about 40 nanometers across – represents an optimal size for combining frequency discrimination with overall sensitivity,” the institute said.
“Pores that are larger or smaller impair hearing,” it added. “The new findings show that fluid viscosity and pores are actually essential to its performance. Changing the sizes of tectorial membrane nanopores, via biochemical manipulation or other means, can provide unique ways to alter hearing sensitivity and frequency discrimination.”
Image 2 (below): This optical microscope image depicts wave motion in a cross-section of the tectorial membrane, part of the inner ear. This membrane is a microscale gel, smaller in width than a single human hair, and it plays a key role in stimulating sensory receptors of the inner ear. Waves traveling on this membrane control our ability to separate sounds of varying pitch and intensity. Credit: MIT's Micromechanics Group