In a previous post we showed you all kinds of pictures of the cochlea and how there are as many as 20,000 hair cells within this wonderful little organ, but have you ever wondered why there are so many different cells? In a word, it’s called “tonotopy:” a Greek word meaning sound + place. But what is tonotopy, and what does it have to do with how the ear hears sound? Let’s find out!
How the Ear Hears
It all began in 1961. At the time, scientists knew that the cochlea contained thousands of nerve cells and that they were spread along the length of the cochlea. However, there was no conclusive proof as to what (if any) relationship there was between different nerve cells and different sounds.
The breakthrough came when Donald Greenwood, who worked in the Psycho-Acoustic Laboratory at Harvard University, published a study titled Critical Bandwidth and the Frequency Coordinates of the Basilar Membrane. In this study Greenwood found that specific frequencies of sound stimulate nerve cells at specific locations within the cochlea, and that a mathematical formula can be used to find exactly which nerve cells are stimulated by a given sound.1
Like the keys of a piano, the nerve cells inside the cochlea are arranged from the lowest to the highest pitch. Nerve cells from one end of the cochlea, the basal region, send high-pitched sound information to the brain while those in the other end, the apical region, send low-pitched information. Just like pressing a specific piano key, stimulating a specific area of the cochlea makes a specific sound. Here’s an easy graphic that shows where all of these different frequencies stimulate the cochlea:
