Amplitude
Intensity of sound waves, how tall a wave is
- Loudness
Cochlea
Long tubes filled with fluid, fluid moves in response to sound waves
- Contains auditory receptors (hair cells)
- Air on one side and fluid on other, must press hard on fluid to create movement
- Vibration of fluid within cochlea signals hair cells
- Membranes run almost the length of the cochlea which separates it into different chambers
- Different chambers = different ion concentrations
- Basilar membrane
Conductive Deafness (Middle-Ear Deafness)
Occurs if bones in middle ear fail to transmit sound waves properly to cochlea (bone growth issue)
- Can be caused by disease, infections, or tumorous bone growth
- Can’t hear external stimuli that must pass through ear
- Normal cochlea and auditory nerve allow people to hear own voice clearly
- Can be correct by surgery or hearing aids that amplify the stimulus
Frequency
Number of compressions per second (Hz), how far apart are peaks
- Pitch
- Humans hear 15-20,000 Hz
- Higher frequency perception decreases with age and loud noise exposure
Frequency Theory
How often a cell has an action potential encodes differences between low-pitched sounds
- Each peak of sound wave causes fluid in cochlea to move, causing hair cells to bend, and an AP
- Works best for low frequency sounds
- Maximum firing rate for neurons ~ 1000 Hz (1000 actions potentials)
- More frequent peaks = more action potentials
Hair Cells
Sensory receptors for auditory and vestibular systems
- Inside the cochlea, bend when moved
- Transform sound vibrations in fluid into electrical signals
- Located between basilar membrane and tectorial membrane
- Sound waves moving fluid in cochlea moves basilar membrane
- Ion channels open as they bend
- Potassium and calcium enter cell, triggers release of neurotransmitter
Nerve Deafness (Inner-Ear Deafness)
Results from damage to the cochlea, hair cells, or auditory nerve
- Can vary in degree (how much of cochlea is gone/what parts)
- Can be confined to certain part of cochlea (can only hear certain frequencies)
- Can be inherited or caused by prenatal problems or early childhood disorders
Oval Window
Where sound comes into the cochlea
Pinna
- Gathers sound waves
- Helps in sound localization
- Amplifies (concentrates) sound
- Fleshy part attached to head
- Outer ear
Pitch
Property related to frequency
- Higher frequency leads high pitch noises
Place Theory
Which cells have actions potentials encode the differences between high-pitched sounds
- Above 4000 Hz
- Specific localization
- Hair cells along basilar membrane resonate only for sound waves of particular frequency (higher frequency = nearer to base by oval window)
- Vibration spreads more easily in low frequency areas (near the apex) and which cells fire is less specific, making this best for high-frequency sounds
Primary Auditory Cortex (A1)
Active during auditory imagery (ex. river flowing, birds chirping)
- Asymmetrical
- Tonotopic map: tones near each other in frequency are near each other in brain
- Low pitch = anterolateral part of Heschl’s gyrus
- High pitch = more posterior; deeper into Sylvian fissure
Tympanic Membrane
Eardrum
- Vibrates in response to sound waves
- Each compression (peak) pushes the membrane in
- Vibrates at the same frequency as sound waves (air on either side allows free vibration)
- Transition from outer to inner ear
Volley Principle
Auditory nerve produces volleys of impulses (action potentials from different cells) up to 4000 Hz
- Encodes more peaks than a single cell can catch
- Auditory nerve sums up peaks caught by different neurons to create an action potential corresponding to every peak
Sound Localization
Determining direction + distance of sound
- Compare responses between two ears
- 3 different mechanisms, depending on type of sound being localized
- If sound isn’t directly in front/behind you, ears may differ in: time of arrival, sound shadow (intensity), phase difference
Organization of the Auditory Cortex
Primary auditory cortex = A1
“What” pathway: anterior temporal cortex
- Identifying sounds (ex. dog barking)
“Where” pathway: posterior temporal cortex; parietal cortex
- Locating sounds
- Where is that sound so I can reach it
How are Sound Waves Turned Into Electrical Activity in the Brain
Hair cells transform sound vibrations into electrical signals
Parts of the Ear
Inner ear, middle ear, and outer ear
Outer Ear
- Pinna
- External auditory canal
Middle Ear
- Tympanic membrane
- Ossicles
Inner Ear
- Cochlea
Ossicles
3 bones (hammer/anvil/stirrup or malleus/incus/stapes)
- Takes movement/vibration of tympanic membrane + concentrates it to create more pressure
- Goal is to move fluid around inner ear, which is harder to move than air
- Bones concentrate the force (same pressure, smaller area)
External Auditory Canal
Links pinna and eardrum (tympanic membrane)
- Approximately 1 inch long
- Protects tympanic membrane
Basilar Membrane
Separates scala media from scala tympanic (dividing point down middle of cochlea)
- Organ of Corti is anterior to basilar membrane (on top)
