Researchers at MIT have published a new analysis of the underlying mechanisms of hearing that may provide insight to more effective machine hearing and better hearing aids. The new findings, led by MIT graduate student Jonathan Sellon, including research scientist Roozbeh Ghaffari, former graduate student Shirin Farrahi, and professor of electrical engineering Dennis Freeman, are reported in the Biophysical Journal. This group of researchers also collaborated with biologist Guy Richardson of University of Sussex.
Millions of years of evolution have brought about precise tuning of a small membrane inside the inner ear known as the tectorial membrane. The viscosity of this membrane is dependent on the size and distribution of tiny pores that are a few tens of nanometers in width. This allows for mechanical filtering that aids to sort out certain sounds.
According to Freeman, in terms of discriminating among competing sounds, the human ear is “extraordinary compared to conventional speech- and sound-recognition technologies.” The reasons for this have remained elusive until now. It appears that a flawed assumption contributed to the longstanding problem in understanding the importance of the tectorial membrane.” Freeman goes on further to say that our ability to differentiate among sounds is frequency-based. Thus, researchers had “assumed that the better we could resolve frequency, the better we could hear.” However, this longstanding assumption turns out not to be true all the time. Freeman and colleagues have previously found that tectorial membranes with a certain genetic defect are actually highly sensitive to variations in frequency. And, what that means is that hearing becomes worse not better.
Freeman explains, there is a “fundamental tradeoff between how well you can resolve different frequencies and how long it takes to do it.” Apparently, that makes the finer frequency discrimination too slow to be useful in real-world sound selectivity.
Earlier work by Freeman and colleagues has demonstrated that the tectorial membrane plays a basic role in discrimination of sound by carrying waves that activate a particular kind of sensory receptor. This turns out to be essential in deciphering competing sounds, however it takes place too quickly for neural processes to keep pace. Evolution seems to have produced a very effective electromechanical system that has the ability to keep up with the speed of these sound waves. The tectorial membrane’s structure determines how well it filters sound. Researchers looked at two genetic variants that cause nanopores within the membrane to be smaller or larger than normal. Pore size changes the viscosity of the membrane and it sensitivity to various frequencies.
The tectorial membrane has a sponge-like quality that is riddled with tiny pores. Researchers were able to see how viscosity varies with pore size. They observed pore size in mice and found they ran about 40 nanometers across. This represents an optimal size for combining frequency discrimination with overall sensitivity. They found that pores that were larger or smaller impaired hearing.
Ghaffair notes, “It really changes the way we think about this structure”. The research has demonstrated that fluid viscosity and pores are essential to tectorial membrane performance. And, in fact, changing the sizes of tectorial membrane nanopores by way of biochemical manipulation or by other means, can generate unique ways to alter hearing sensitivity and frequency discrimination.
According to William Brownell, professor of otolaryngology at Baylor College of Medicine,“This is the first study to suggest that porosity may affect cochlear tuning”. Brownell goes on further to say, this work “could provide insight” into the development of certain hearing impairments.