Learning Objectives
By the end of this section, you should be able to:
- Describe the anatomy and function of the auditory (hearing) system.
- Explain how we perceive pitch.
- Discuss how we determine where sounds come from (sound localization).
What is Hearing?
Hearing (also called audition) is the process by which the brain converts sound waves into meaningful information, such as speech, music, and environmental sounds.
Sound is created when vibrating objects produce pressure waves that travel through the air. These waves enter the ear and are converted into electrical signals that the brain can understand.
Anatomy of the Ear
1. Outer Ear
The outer ear collects sound.
Main parts:
- Pinna (Auricle) – the visible part of the ear that collects sound.
- Auditory canal (ear canal) – carries sound waves inward.
- Tympanic membrane (eardrum) – vibrates when struck by sound waves.
2. Middle Ear
The middle ear contains three tiny bones called the ossicles.
These are:
- Malleus (Hammer)
- Incus (Anvil)
- Stapes (Stirrup)
These bones amplify vibrations before sending them into the inner ear. The stapes presses against the oval window of the cochlea.
3. Inner Ear
The inner ear contains two important structures:
Cochlea
- Fluid-filled
- Snail-shaped
- Contains hair cells, the sensory receptors for hearing
Semicircular Canals
- Help maintain balance
- Part of the vestibular system, not hearing itself.
How Hearing Works (Step-by-Step)
- Sound waves enter the pinna.
- They travel through the auditory canal.
- They strike the eardrum, causing it to vibrate.
- Vibrations move the malleus, incus, and stapes.
- The stapes pushes on the oval window.
- Fluid inside the cochlea begins moving.
- Movement bends the hair cells on the basilar membrane.
- Hair cells convert mechanical energy into neural impulses (transduction).
- Signals travel along the auditory nerve.
- The brain processes the information in the auditory cortex of the temporal lobe.
The Brain Pathway for Hearing
Information travels through several brain structures:
- Auditory nerve
- Inferior colliculus
- Medial geniculate nucleus of the thalamus
- Auditory cortex (temporal lobe)
This is where sounds become meaningful.
Pitch Perception
Pitch is how high or low a sound seems.
Low Frequency
- Low pitch
- Bass drum
- Thunder
High Frequency
- High pitch
- Birdsong
- Whistle
Frequency is measured in Hertz (Hz).
Humans generally hear frequencies from about 20 Hz to 20,000 Hz (20 kHz), although this range often decreases with age.
Two Theories of Pitch Perception
1. Temporal Theory
Also called the frequency theory.
This theory says:
- Hair cells fire action potentials at the same rate as the sound frequency.
- Works well for lower-frequency sounds.
Limitation:
Neurons cannot fire fast enough to represent all audible frequencies.
2. Place Theory
This theory says:
Different parts of the basilar membrane respond to different frequencies.
- Base of cochlea → High-frequency sounds
- Tip (apex) of cochlea → Low-frequency sounds
This explains the perception of higher-frequency sounds.
Which Theory Is Correct?
Both contribute:
- Temporal theory helps explain lower frequencies.
- Place theory is especially important for higher frequencies.
- Together, they explain how we perceive pitch.
Sound Localization
Sound localization is the ability to determine where a sound originates.
The brain compares information from both ears.
Monaural Cues
Monaural = one ear
These cues help judge whether a sound comes from:
- Above
- Below
- In front
- Behind
The shape of the pinna changes the incoming sound slightly, helping the brain identify these locations.
Binaural Cues
Binaural = two ears
These help determine whether sounds come from the left or right.
The brain uses:
Interaural Level Difference (ILD)
The sound is louder in the ear closer to the source.
Example:
A barking dog on your right sounds louder in your right ear.
Interaural Timing Difference (ITD)
The sound reaches one ear a tiny fraction of a second before the other.
The brain uses this timing difference to locate the sound.
Hearing Loss
Deafness means partial or complete loss of hearing.
Congenital Deafness
Present at birth.
Conductive Hearing Loss
Problem transmitting sound through the outer or middle ear.
Possible causes:
- Earwax blockage
- Hole in the eardrum
- Damage to the ossicles
- Fluid behind the eardrum
Often helped by hearing aids, which amplify sound.
Sensorineural Hearing Loss
Damage to:
- Hair cells
- Cochlea
- Auditory nerve
Common causes:
- Aging
- Loud noise exposure
- Head injury
- Disease
- Certain medications
- Tumors
- Toxins
This is the most common type of hearing loss.
Ménière's Disease
A disorder of the inner ear that can cause:
- Hearing loss
- Tinnitus (ringing in the ears)
- Vertigo (spinning sensation)
- Increased pressure inside the inner ear
Some people with severe sensorineural hearing loss may benefit from a cochlear implant, which directly stimulates the auditory nerve.
Deaf Culture
The OpenStax chapter also discusses Deaf culture, emphasizing that many deaf individuals identify with a rich cultural and linguistic community. In the United States, many people communicate using American Sign Language (ASL), and families may make different choices about education, communication methods, and cochlear implants based on their values and experiences.
Key Terms to Remember
| Term | Meaning |
|---|---|
| Audition | Hearing |
| Pinna | Collects sound |
| Auditory canal | Carries sound to the eardrum |
| Tympanic membrane | Eardrum |
| Ossicles | Three middle ear bones |
| Malleus | Hammer |
| Incus | Anvil |
| Stapes | Stirrup |
| Cochlea | Hearing organ |
| Hair cells | Hearing receptors |
| Basilar membrane | Supports hair cells |
| Oval window | Membrane pressed by the stapes |
| Auditory nerve | Carries hearing information to the brain |
| Pitch | Perceived highness or lowness of sound |
| Frequency | Number of sound wave cycles per second (Hz) |
| Temporal theory | Pitch based on firing rate |
| Place theory | Pitch based on location along the basilar membrane |
| Monaural cues | One-ear localization cues |
| Binaural cues | Two-ear localization cues |
| Interaural level difference | Difference in loudness between ears |
| Interaural timing difference | Difference in arrival time between ears |
| Conductive hearing loss | Problem transmitting sound through the outer/middle ear |
| Sensorineural hearing loss | Damage to the cochlea, hair cells, or auditory nerve |
| Cochlear implant | Device that stimulates the auditory nerve |
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