Hearing When we hear, we are processing sound waves that are made up of compressions of air or water. We hear vibrations in the air as they strike a part of the ear called the eardrum and make it vibrate. These vibrations are sent through other parts of the ear and finally sent as action potentials to the brain. Sound waves have both amplitude and frequency. Amplitude is a sound’s intensity, and loudness is the perception of that intensity. Frequency of a sound is the number of compressions per second. Pitch is closely related to frequency.
Hearing – Outer Ear The part of the hearing system that we see on the outside of the head is called the pinna (the ear). It is designed to capture sound. When a sound reaches the ear, it passes through the tube called the external auditory canal until it reaches the tympanic membrane or eardrum.
Hearing – Middle Ear The eardrum vibrates at the same frequency as the sound waves that hit it. Attached to the eardrum are three very small bones (the smallest bones in the body!) that also vibrate to the frequency of the sound. These bones are known as the hammer, anvil, and stirrup because of their shapes. Together, they are known as the ossicles. The three bones are attached to the oval window. The oval window is the beginning of the inner ear.
Hearing – Inner Ear The inner ear has the cochlea, a snail-shaped fluid-filled structure. Vibrations from sound in the fluid in the cochlea displace hair cells that are the neuron receptors for sound at the bottom of the cochlea in the basilar membrane. The tectorial membrane covers the hair cells and protects them. The hair cells send signals to the auditory nerve, which sends a signal about sound to the temporal lobe of the brain.
Visualization of the Ear
Theories about Hearing There are three theories about hearing. The first theory is known as the frequency theory. This theory says that the basilar membrane that holds the hair cells vibrates at the same frequency as sound. This causes the auditory nerve axons to produce action potentials at the same frequency. However, the maximum firing rate of a neuron is short of the highest frequencies we can hear.
Theories about Hearing The second theory is known as the place theory. This theory suggests that the basilar membrane is similar to the strings of a piano and that each area along the membrane is tuned to a specific frequency and vibrates to that frequency. The nervous system would have to decide among the frequencies based on which neurons are active. However, the problem with this theory is that some parts of the basilar membrane are bound together too tightly for any part to vibrate like a piano string.
Theories about Hearing The final theory is known as the volley principle. This theory suggests we use methods that combine aspects of the frequency theory and the place theory. The basilar membrane is stiff at its base where the stirrup connects with the cochlea and floppy at the other end of the cochlea. Hair cells along the basilar membrane would act differently depending on their location. When we hear sounds at a very high frequency, we use something like the place theory. When we hear lower pitched sounds, we use something like the frequency theory. So combining parts of the first two theories explains how we hear better than using either one of the first two theories separately.
Hearing and the Brain Information about hearing, just like information about vision, is first routed through the thalamus and other brain areas below the cortex before reaching the primary auditory cortex, located in the temporal lobe of the cerebral cortex. Different areas of the auditory cortex, just like is true in the visual cortex, process information in different ways, including about the location of a sound and the motion of sound. And just like vision, hearing requires a certain amount of experience with sounds for our hearing to be fully developed.
Areas of the brain associated with vision and hearing