Hearing is also called audition. This sensory mechanism enables us to determine what is happening in our environments. It provides a medium for spoken language. It brings pleasure to our lives, such as through music. In this section, let’s begin by discussing how sound waves are transduced in the auditory system. Then we’ll look at how we perceive the richness of sound information.
Auditory Receptors in Our Ears Detect Sound Waves The world is full of sound. A song plays, a person speaks, the TV drones, an over- head light hums. But everything you hear is merely changes in air pressure produced within your hearing distance. Just as objects in the world have no essential color, these changes in air pressure have no sound. Instead, your ability to hear is based on the intricate interactions of various regions of the ear and on processing in the brain.
An age-old question asks, “If a tree falls in the woods and no one is there to hear it, does it make a sound?” The answer is no. The falling of the tree makes vibrations in the air. It is only the way our ears and brains process the vibrations in the air that creates the perception of sound.
FROM THE EAR TO THE BRAIN Suppose you hear music, such as the sound of a saxophone. The sound waves from the music are the sensory input (Figure 5.18). The process of hearing begins when sound waves arrive at the shell-shaped struc- ture of your outer ear (see Figure 5.18, Step 1). The odd shape of the outer ear actually is functional: The shell shape increases the ear’s ability to capture sound waves and then funnel the waves down the auditory canal. When you have trouble hearing something, it helps to cup or bend your outer ear because you funnel even more sound waves into the auditory canal.
Next, the sound waves from the music travel down the auditory canal to the eardrum. The eardrum is a membrane stretched tightly across the canal. This membrane marks the beginning of the middle ear. When the sound waves hit the eardrum, they make it vibrate. The vibrations of the eardrum are transferred to three tiny bones, which together are called the ossicles. The ossicles amplify the vibrations even more. If you have ever experienced an ear infection, you know how important the eardrum is to hearing. When fluid builds up behind the eardrum, the membrane cannot vibrate properly, so it seems as if you have cotton in your ears. You can’t really hear much of anything.
At the start of the inner ear, the amplified vibrations reach the oval window. Though its name makes it seem like an opening, the oval window is actually another membrane. That membrane vibrates in turn. The oval window is located within the cochlea, a fluid-filled tube that curls into a snail-like shape. Running through the center of the cochlea is the thin basilar membrane. The oval window’s vibrations create pressure waves in the cochlear fluid that make the basilar membrane move in a wave. Movement of the basilar membrane stimulates the bending of hair cells. These cells are the sensory receptors for detecting auditory input (see Figure 5.18, Step 2).
The bending of the hair cells then causes them to transduce the auditory infor- mation into signals (see Figure 5.18, Step 3). This transduction initiates the creation of action potentials in the auditory nerve. The auditory nerve sends the information to the sensory processing center of the thalamus and finally to the primary auditory cortex in the brain (see Figure 5.18, Step 4). This region of the cortex processes the information. As a result, you perceive the music as coming from a saxophone.