We talk a lot about the cochlea, but in reality the cochlea is just one part in a complex series of steps that let you to hear sound. In this post, we’ll get into the fascinating details of how hearing works.
If you’re curious, here’s a quick overview of the whole chain and how it works in normal hearing:
That video ends with, “The brain interprets this information as sound?” But what does that last sentence mean?
That’s a question that doctors have been asking for over one hundred years, and we’re been getting closer to the answer with each and every day.
Tonotopy: Specific Nerves for Specific Sounds
We’ve said before how the cochlea is arranged in a specific way, so that different parts of the cochlea respond to different pitches of sound. There are about 20,000 nerve cells in a cochlea and they’re arranged like a piano keyboard: from the low pitches on one end to the high pitches on the other. But it’s not just the cochlea that’s arranged like this: your hearing nerve and even your brain also have specific sections for specific sounds.1
The hearing nerve isn’t just one single nerve, but rather a bundle of lots of little nerve fibers. There are about as many fibers in the hearing nerve as there are nerve cells in the cochlea, which means a normal hearing nerve has about 20,000 fibers. It’s like a superhighway that starts in the cochlea and ends in the brain. Each of the fibers, or lanes, carries the information for a different range of pitches.
Most hearing implants all get on that highway at the very beginning: they stimulate nerve cells in the cochlea, which are directly connected to the fibers in the hearing nerve and send sound signals on a one-way trip to the brain.
Hearing in the Brain
So, that’s how sound gets to the brain. But what happens once it gets there?
In 1878, Richard L. Heschl found that a very specific part of the brain is activated when the brain is processing sound. (Side note: Heschl started his research in Vienna, just like MED-EL’s founders!) Today, we call that general area the auditory cortex.
Further research has found that specific parts of the auditory cortex are responsible for different bits of sound, and researchers have even been able to make a map of this layout.2 Here’s what it looks like:
Electrical signals sent from the cochlea reach the brain and, kind of like a lightbulb, “light it up.” When that happens, you hear. Different sounds send different electrical signals, so depending on the sound a different part of the brain will be activated.
Not cool enough? Think about selective hearing: how you’re able to tune in to certain sounds and almost completely block out other sounds. Selective hearing works because you’re literally tuning in to certain physical areas of your brain, and ignoring others.3
At this point in the chain we’re on the cutting edge of research. How does activating these nerve cells in the brain cause someone to hear? That’s something scientists are trying to find out.
- Greenwood, D. (1961). Critical Bandwidth and the Frequency Coordinates of the Basilar Membrane. Journal of the Acoustical Society of America, 33: 1344–1356. doi:1121/1.1908437.
- Romani, G.L., Williamson, S.J., Kaufman, L. (1982). Tonotopic organization of the human auditory cortex. Science, 216(4552): 1339–40. doi:10.1126/science.7079770
- Da Costa et. al. (2013). Tuning In to Sound: Frequency-Selective Attentional Filter in Human Primary Auditory Cortex. The Journal of Neuroscience, 33(5): 1858–63. doi: 10.1523/JNEUROSCI.4405-12.2013