Have you ever had a conversation that was so boring that you found yourself tuning out? Did you instead find yourself focusing on some other noise be it a conversation nearby or the chirping of a bird?
The human ear has the incredible ability to focus on sounds of specific frequencies while simultaneously filtering out background noise. This is why we can sustain conversations in loud atmospheres. How the human ear carries out this incredible feat has been unknown until recently.
Ear: Source Wikimedia Commons – the Ear
On March 18, 2014, a research team led by MIT graduate student Jonathan Sellon published a paper that uncovered this mystery. They found that the size of the tiny nanopores in the tectorial membrane (a small, viscous inner-ear structure) played a key role in sound filtration. The researchers studied genetically mutated mice that had different sized pores in their tectorial membranes.
Before we go on to discuss the findings of this study lets take a brief look at how sound travels in the ear and the function of the tectorial membrane. When sound waves travel in the air they compress air molecules into compressions. When these compressions enter the ear canal and encounter the ear drum they cause it to vibrate. These vibrations in turn cause the 3 small bones of the middle-ear (the malleus, the incus and the stapes) to jiggle and push upon the cochlea, a fluid-filled spiral structure that looks like a snail shell.
Tectorial Membrane: Source Wikimedia Commons – the tectorial membrane
Lining the inside of the cochlea are small hair cells that are covered by the tectorial membrane. The tectorial membrane has small pores known as nanopores (on average 40nm in diameter in mice). When the vibrations reach the cochlea, the tectorial membrane within slides back and forth over the layer of hair cells. This induces electrical signals to be sent to a special part of the brain that processes sound. You can think of the tectorial membrane as a carpet sliding across a wooden floor and the friction that arises as the electrical signals triggered.
So, what did the researchers find?
The researchers had 2 main findings. Firstly, they found that mice with smaller pores in their tectorial membranes could focus on sounds over smaller frequency ranges while those with larger pores could not focus on sounds as well. Secondly, they found that mice with larger pores could hear sounds over a greater range of frequencies (they have a greater overall sound sensitivity) as compared to mice with smaller pores. Therefore, optimal hearing is achieved by intermediate sized pores.
What makes this study particularly exciting is that Scientists have yet to make hearing aids that can select frequencies like the natural ear does. With these new findings better hearing aids can be produced.
So, the next time someone catches you not listening to them you can always blame your tectoid membrane!
Fardowsa Yusuf
Jonathan B. Sellon, Roozbeh Ghaffari, Shirin Farrahi, Guy P. Richardson, Dennis M. Freeman, Porosity Controls Spread of Excitation in Tectorial Membrane Traveling Waves, Biophysical Journal, Volume 106, Issue 6, 18 March 2014, Pages 1406-1413, ISSN 0006-3495, http://dx.doi.org/10.1016/j.bpj.2014.02.012. (http://www.sciencedirect.com/science/article/pii/S0006349514001891) http://www.dailymail.co.uk/sciencetech/article-2585686/What-dear-Researchers-discover-tiny-nanotunnels-ear-let-tune-conversations.html http://www.wired.com/wiredscience/2014/03/ear-nanopores-hearing
