• Your Ear—The Great Communicator
    Awake!—1990 | January 22

    • Inner Ear​—The Business End of the Ear

      From the oval window, we come to the inner ear. The three mutually perpendicular loops, called the semicircular canals, enable us to maintain balance and coordination. It is in the cochlea, however, that the business of hearing really begins.

      The cochlea (from Greek ko·khliʹas, snail) is basically a bundle of three fluid-filled ducts, or canals, coiled up in a spiral like the shell of a snail. Two of the ducts are connected at the apex of the spiral. When the oval window, at the base of the spiral, is set in motion by the stirrup, it moves in and out like a piston, setting up hydraulic pressure waves in the fluid. As these waves travel to and from the apex, they cause the walls separating the ducts to undulate.

      Along one of these walls, known as the basilar membrane, is the highly sensitive organ of Corti, named after Alfonso Corti, who in 1851 discovered this true center of hearing. Its key part consists of rows of sensory hair cells, some 15,000 or more. From these hair cells, thousands of nerve fibers carry information about the frequency, intensity, and timbre of the sound to the brain, where the sensation of hearing occurs.

      The Mystery Unraveled

      How the organ of Corti communicates this complicated information to the brain remained a mystery for a long time. One thing scientists did know was that the brain does not respond to mechanical vibrations but only to electro-​chemical changes. The organ of Corti must in some way convert the undulating movement of the basilar membrane into matching electrical impulses and send these to the brain.

      It took the Hungarian scientist Georg von Békésy some 25 years to unravel the mystery of this tiny organ. One thing he discovered was that as the hydraulic pressure waves travel along the ducts in the cochlea, they reach a peak somewhere along the way and push on the basilar membrane. Waves generated by high-​frequency sounds push on the membrane near the base of the cochlea, and waves from low-​frequency sounds push on the membrane near the apex. Thus, Békésy concluded that sound of a specific frequency produces waves that flex the basilar membrane at a specific spot, causing the hair cells there to react and send signals to the brain. The location of hair cells would correspond to the frequency, and the number of hair cells triggered would correspond to the intensity.

      This explanation holds good for simple tones. Sounds occurring in nature, however, are rarely simple. A bullfrog’s croak sounds quite different from a drumbeat even though they may be of the same frequency. This is because each sound is made up of a fundamental tone and many overtones. The number of overtones and their relative strength give each sound its distinctive timbre, or character. This is how we recognize the sounds we hear.

      The basilar membrane can respond to all the overtones of a sound simultaneously and detect how many and what overtones are present, thus identifying the sound. Mathematicians call this process Fourier analysis, naming it after the brilliant 19th-​century French mathematician Jean-​Baptiste-​Joseph Fourier. Yet, the ear has been using this advanced mathematical technique all along to analyze the sounds heard and communicate the information to the brain.

      Even now, scientists are still not sure what sort of signals the inner ear sends to the brain. Investigations reveal that the signals sent by all the hair cells are about the same in duration and strength. Thus, scientists believe that it is not the content of the signals but the simple signals themselves that convey a message to the brain.

      To appreciate the significance of this, recall the children’s game in which a story is relayed from one child to another down the line. What the child at the other end hears often bears no resemblance to the original. If a code, such as a number, is passed along instead of the complicated story, it will likely not be distorted. And that, apparently, is what the inner ear does.