WITHOUT OUR SENSE OF HEARING WE WOULD NOT HAVE LANGUAGE OR MANY OTHER HUMAN FACILITIES

So what is this sense we call hearing? Our ears alert us to events in the environment, and they detect that special human form of communication... speech. Our hearing mechanisms accomplish these tasks by sensing sound waves, which are changes in air pressure, and converting these changes into electrical signals that the brain can analyze and interpret. This sense is not perfect as many know. About 28 million people in the United States have hearing loss significant enough to interfere with understanding conversations, and about one third of people over the age of 75 have difficulty hearing. Just as we do not actually smell with the bumps on our faces-- called noses-- neither do we perceive sound solely with the flaps we call ears. Although hearing begins with the ear flap or pinna, the receptor cells that change sound energy into the electrical current of the nervous system lie deep inside the temporal bone of the skull. Unlike olfactory or taste receptors however, hair cells are not renewed when they die or are damaged.

Simply put here’s how hearing takes place: The ear with its unique folds captures sound waves and channels them through the ear canal to the eardrum. Vibrations of the eardrum pass along the three bones of the middle ear, with the base of the stapes then rocking the oval window in and out. The membranous oval window acts something like a piston in a hydraulic system: it pushes and pulls on the enclosed fluid of the cochlea. The fluid vibrations move the basilar membrane, and this motion activates auditory receptor cells (hair cells) sitting on the membrane, which send signals to the brain. The inner ear is connected to the common mouth and nasal cavity, or pharynx, by the Eustachian tube. The Eustachian tube allows air pressure to equalize between the outside of the eardrum (surrounding atmosphere) and the inside of the eardrum (the middle ear). This is also the pathway that allows infections from the mouth and nose cavities to enter the middle ear, causing the common ear infections of childhood. The middle ear houses the three smallest bones in the body, the malleus, incus, and stapes (hammer, anvil, and stirrup), which form a chain of levers connected by joints.

We hear in stereo. When a sound occurs at the extreme left of a subject, the arrival of the sound at the left ear is about 600 to 700 microseconds (millionths of a second) earlier than at the right ear. Further, the head acts as a sound barrier, so the sound is a little louder in the left ear. The timing and intensity of sound in the two ears tell the brain where the sound source is? A composite sound such as a vowel in human speech usually has three dominant frequency components. Each of the frequency components sets off a separate traveling wave and each wave produces its peak at the position on the membrane that responds best to that frequency. Next, the hair cells on the membrane at each peak send signals indicating to higher centers that a certain frequency of sound has been detected. Researchers are now also using positron emission tomography (PET) to study brain function during language processing in normal individuals. Maybe someday we will know how we humans REALLY interpret speech.