Lesson 1.15 · Unit 1 · CED topic 1.6 (part 3)
Hearing: sound, the ear, and theories of pitch
Hearing may seem effortless, but it turns a series of air-pressure waves into the voice of a friend or a favorite song through a chain of mechanical and neural steps. The exam asks about what the ear does with a sound, the competing and complementary explanations for how we tell high from low, how we know where a sound came from, and the two main kinds of hearing loss. This lesson follows a sound from the air to the brain and ends with problems that interrupt it.
Learning objective.
- 1.6.C. Explain how the structures and functions of the auditory sensory system relate to behavior and mental processes.
Essential knowledge.
- 1.6.C.1. Sound occurs through the movement of air molecules at different wavelengths (called pitch) and amplitudes (called loudness).
- 1.6.C.2. Theories that help explain pitch perception include place theory, volley theory, and frequency theory.
- 1.6.C.3. Sound localization describes how we identify where sounds in our environment are coming from.
- 1.6.C.4. Hearing difficulties can result from aging and various kinds of damage to auditory structures. Types of hearing loss include conduction deafness and sensorineural deafness.
What sound is
Sound occurs through the movement of air molecules, which produces waves of alternating pressure. Two physical features of the wave matter. The frequency of the wave, how many cycles occur per second (measured in hertz, Hz), is perceived as pitch: high frequencies sound high and low frequencies sound low. (The course description calls this feature "wavelength," because longer waves are lower in frequency.) The amplitude, or height of the wave, is perceived as loudness, measured in decibels (dB): the larger the amplitude, the louder the sound. Humans can hear frequencies from roughly 20 to 20,000 Hz, and are most sensitive in the range used for speech.
From the air to the brain
The ear has three sections, each of which does one job in the chain.
- The outer ear. The visible pinna collects sound waves and funnels them down the ear canal to the eardrum (tympanic membrane), which vibrates when sound strikes it.
- The middle ear. The eardrum's vibrations move three tiny bones, the hammer, anvil, and stirrup (together the ossicles). They amplify the vibrations and pass them to the oval window, a membrane at the entrance to the inner ear.
- The inner ear. The oval window vibrates the fluid inside the snail-shaped cochlea. The movement of the fluid causes a flexible strip called the basilar membrane to ripple. Resting on it are hair cells, the receptor cells for hearing, which bend as the membrane moves. The bending of the hair cells is the transduction: it triggers neural signals, which travel along the auditory nerve to the thalamus and then to the auditory cortex in the temporal lobes.
A key idea is that damage to hair cells is permanent in humans. They do not regrow, so exposure to loud sounds can cause lasting loss.
Theories of pitch perception
How does the ear tell a high note from a low one? Three theories explain different parts of the answer.
- Place theory says that different pitches stimulate different locations (places) along the basilar membrane. High-frequency sounds make the hair cells near the base (the start) of the cochlea respond, and low-frequency sounds stimulate cells farther along toward the tip. The brain reads pitch from which hair cells are active. Place theory explains the perception of high-pitched sounds well.
- Frequency theory says that the rate at which neurons fire matches the frequency of the sound wave: a 100-Hz tone makes the auditory nerve fire about 100 times per second. This explains low pitches, but a single neuron cannot fire fast enough for high frequencies (neurons are limited by the refractory period).
- Volley theory is an extension of frequency theory. Groups of neurons fire in alternation, in volleys, so that together they reach higher rates than any single neuron could. Volley theory helps explain medium-to-high pitches.
The current understanding is that the three combine: place theory for high pitches, frequency and volley for lower pitches, with overlap in the middle.
Georg von Békésy, a Hungarian-born scientist, studied the cochleas of human cadavers and other animals under a microscope, using tiny flecks of silver on the basilar membrane to see how it moved when sounds of different frequencies were played. He found that each frequency made a wave that traveled along the membrane and peaked at a different place: high frequencies near the base, low frequencies farther toward the tip. He received a Nobel Prize in 1961 for this work.
Reading the research: a controlled laboratory study in which the independent variable was the frequency of the sound and the dependent variable was the location of the peak movement. It provided strong support for place theory. A limitation is that the cochleas were not alive, so the live ear's responses can differ in some details.
Sound localization
You can tell where a sound is coming from because you have two ears. A sound from your right reaches the right ear slightly earlier than the left ear, and it is slightly louder in the right ear because your head blocks and weakens it on the way to the left. The brain compares the time and intensity differences between the ears to work out the location. This process is called sound localization. A sound directly in front or behind you reaches both ears at the same time and is harder to locate, so people often tilt their heads to get a better comparison.
Hearing loss
Hearing difficulties can result from aging and from various kinds of damage to the structures of the auditory system. The two types that you need to know are:
- Conduction deafness (conductive hearing loss) is caused by damage to the mechanical system that conducts sound waves to the cochlea, such as a punctured eardrum, fluid in the middle ear, or damaged ossicles. The hair cells and nerves are fine. It is often treatable with hearing aids that amplify the sound, or surgery.
- Sensorineural deafness is caused by damage to the cochlea's hair cells or to the auditory nerve. It is the more common type, caused by aging and by loud noise, and it is usually permanent. Hearing aids help less because the damage is to the receptors, but a cochlear implant, which converts sounds into electrical signals and stimulates the auditory nerve directly, can restore some hearing.
A friend calls your name. List, in order, the structures that carry the sound and the type of energy at each step.
Step 1, sound waves (air pressure changes) are collected by the pinna and funneled down the ear canal.
Step 2, the waves vibrate the eardrum, converting air pressure waves to mechanical vibration.
Step 3, the vibrations pass through the hammer, anvil, and stirrup, which amplify them, to the oval window.
Step 4, the oval window vibrates the fluid in the cochlea, which moves the basilar membrane and bends the hair cells.
Step 5, the hair cells transduce the movement into neural impulses, which go to the thalamus and the auditory cortex in the temporal lobes.
For each, name the best theory. (a) A very high-pitched whistle activates hair cells near the base of the cochlea. (b) A 150-Hz tone makes auditory neurons fire about 150 times per second. (c) A 2,000-Hz tone is above the firing rate of any single neuron, yet the auditory nerve represents it.
Step 1, (a) pitch is represented by location on the basilar membrane, so place theory.
Step 2, (b) firing rate matches frequency, so frequency theory.
Step 3, (c) groups of neurons taking turns firing to reach a combined rate that is higher than any one can fire alone is volley theory.
Case 1: A boy has an ear infection, and fluid builds up behind his eardrum. He hears muffled sounds, but a hearing aid helps. Case 2: A worker exposed to loud machinery for 30 years has lost most hearing for high pitches. A hearing aid helps only a little. Classify each.
Step 1, case 1: the problem is in the middle ear (fluid prevents the eardrum and ossicles from vibrating normally), so it is conduction deafness. The hearing aid helps because it amplifies the sound to overcome the blockage.
Step 2, case 2: years of loud noise destroy hair cells in the cochlea, so it is sensorineural deafness. The damage is to the receptors, so amplification helps less.
Step 3, high-pitched sounds are often the first lost because of the hair cells at the base of the cochlea, which are most vulnerable to damage (consistent with place theory).
- Frequency (wavelength) = pitch; amplitude = loudness.
- Path: pinna, eardrum, hammer-anvil-stirrup, oval window, cochlea (basilar membrane, hair cells), auditory nerve, thalamus, temporal lobe.
- Place theory (location on the membrane), frequency theory (firing rate), volley theory (groups of neurons firing in turn).
- Sound localization uses time and loudness differences between the ears.
- Conduction deafness (outer or middle ear) versus sensorineural deafness (hair cells or nerve).
- What physical feature of a sound wave determines pitch, and which determines loudness?
Show answer
Frequency (wavelength) determines pitch; amplitude determines loudness.
- What do the hair cells in the cochlea do, and why is damage to them serious?
Show answer
They transduce the movement of the basilar membrane into neural signals. In humans, hair cells do not regrow, so damage causes permanent hearing loss.
- Why is it harder to locate a sound directly in front of you than one to your side?
Show answer
A sound at the side reaches one ear earlier and louder than the other, giving the brain differences to compare. A sound straight ahead arrives at both ears at the same time and loudness.
- Which pitch theory explains how we hear a very low sound, and what is its limit?
Show answer
Frequency theory: neurons fire at the same rate as the sound waves. Its limit is that neurons cannot fire fast enough to match high frequencies.
- Which type of hearing loss would a cochlear implant most likely treat?
Show answer
Sensorineural deafness, because the implant bypasses damaged hair cells by stimulating the auditory nerve directly.
- Explain how von Békésy's research supported place theory.
Show answer
He saw that different frequencies produce traveling waves that peak at different locations on the basilar membrane, so pitch is related to where on the membrane the hair cells are stimulated.