Auditory System

Midterm 2

Sound

audible sinusoidal fluctuations in air pressure : speed, frequency, amplitude, timbre

frequency

#cycles of air pressure per second, measured in Hertz (Hz)

pitch: perception of frequency

timbre

distinctive character of a sound based on relative intensities of different frequencies

each note has a “fundamental frequency” - lowest frequency produced → determines pitch we hear

harmonics - higher frequencies (harmonics & their timing determine the unique qualities of sound)

amplitude 

magnitude of air pressure changes (decibels dB)

loudness: perception of amplitude 

Outer ear anatomy

• Pinna

• Auditory canal

• Tympanic membrane

Middle ear Anatomy

• ossicles

• eustachian tube

• oval window

inner ear anatomy

cochlea

Functions of Outer Ear

Pinna ● Collects and funnels sound into auditory canal ● Vertical sound localization

Auditory canal ● Channels sound waves to the tympanic membrane

Tympanic membrane (eardrum) ● Vibrates in sync and at the same frequency as sound waves entering the ear

Which of the following would Picasso have trouble with if he lost his pinna?

Localizing sound in the vertical plane

Middle Ear Functions

Ossicles ● Amplify and transfer vibrations from the tympanic membrane to the oval window of the cochlea

Eustachian tube ● Equilibrates air pressure between both sides of the tympanic membrane

Oval window ● Transfers vibrations from the middle ear to the inner ear

Ossicles

bones in middle ear

Malleus - hammer

Incus - anvil

Stapes - stirrup →  Acts as a piston that moves in and out of the oval window to displace cochlear fluid

Fluid filled cochlea

the fluid is much harder to displace than air

Fluid in cochlea solution

To make sure sound waves don’t die out, need to amplify them (AKA impedance matching) 

Impedance of air is low: easy to displace

Impedance of fluid is high: hard to displace How? → Ossicles!

Impedance Matching

We concentrate the force of the vibrations onto a small area on the footplate of the stapes (oval window) in comparison to the large area of the tympanic membrane → higher pressure

• Lever action of ossicles increases force on oval window

Which of the following is NOT true about sound?

Sound waves will directly move the oval window

Attenuation Reflex

Loud sounds cause a reflexive contraction of two muscles

○ Tensor tympani: attached to the malleus

○ Stapedius muscle: attached to the stapes

Contraction of muscles → ossicles become more rigid (lose power to act as lever) → reduced sound conduction to cochlea

Useful bc→ Adapts ear to continuous loud noise: protects delicate ear machinery (hair cells)

Limits→ delay of 50-100ms: cannot protect against sudden loud sounds → effective at lower frequencies

Cochlea function

Auditory transduction: Sound waves → electrical signals

Cochlea anatomy

• Scala vestibuli (assoc. w/ oval window) 

• Reissner’s membrane separates scala vestibuli and scala media 

• Scala media 

• Basilar membrane separates scala media and scala tympani 

• Scala tympani (assoc. w/ round window)

Cochlea flow of fluid

Oval window moves → fluid in scala vestibuli moves → goes down thru helicotrema → fluid in scala tympani moves → fluid pushes against round window

Cochlear Fluids

• Perilymph - in scala vestibuli and tympani (low [K+], hi [Na+])

• Endolymph - in scala media (hi [K+], low [Na+])

• Endocochlear potential = +80 mV → Endolymph is 80 mV more positive than perilymph

• Concentration gradients are maintained by stria vascularis → Absorbs Na+ from and secretes K+ into endolymph

Basilar Membrane structure 

Base: narrow, stiff

Apex: wide, floppy 

organ of corti: sits on top of BM, contains auditory receptors (hair cells)

tectorial membrane: hangs over Organ of corti 

Movement of Basilar Membrane 

Stapes movement evokes a traveling wave on the basilar membrane.

• Low frequency: vibrations greatest at apex

• High frequency: vibrations greatest at base (tonotopy more accurate)

If frequency too low → membranes don’t move & fluid flows from SV to ST, goes thru helicotrema

Path of Basilar Membrane movement

Oval window displaces perilymph in scala vestibuli → displaces flexible Reissner’s membrane → displaces endolymph in scala media → displaces BM

tonotopy - tones (frequencies) are represented by their location on the BM

What does the Organ of Corti contain?

auditory receptors called hair cells

Hair cells

each hair cell has 10-300 stereocilia

• sound waves → BM moves → stereocilia bend → neural signal 

hair cells are NOT neurons (no axons, no AP) → they are epithelial cells 

release excitatory transmitter (glutamate) onto spiral ganglion neurites 

Inner hair cells

1 row

between modiolus & rods of Corti 

stereocilia move in endolymph 

provide 90% of input to SGC

outer hair cells

3 rows

further out from rods of corti

stereocilia stick to tectorial membrane 

provide 10% of input to SGC

Hair cells release glutamate onto…

spiral ganglion cells

→ SGC are bipolar neurons that get input from hair cells and extend to CNS, somas located in SG in modiolus 

→ SGC send axons to auditory nerve → cochlear nuclei in medulla Paradoxical Connections 

One IHC connects to…

15-20 SGCs

→ auditory transduction

Many OHC connect to …

one SGC

→ cochlear amplification 

IHC Stereocilia are unique:

• Actin filaments make it so they only bend at the base (near the top of the hair cell)

• Tip-link K+ channels at the tip of each

IHC Tip-link K+ channels are mechanically gated:

• Tips point straight up → channels half open, half closed 

• Tips bend toward kinocilium → channels open more → K+ enters → depolarization 

• Tips bend away from kinocilium → more channels close → no depolarization

What causes K+ entry and depolarization in IHC

No [K+] gradient (high in endolymph and in cell), BUT… large voltage difference will drive K+ into cell

Endocochlear potential = +80 mV, which is 140 mV higher than inside cell

in age-related degeneration of stria terminalis - the K+ concentration of endolymph is insufficient → deafness results w/o endocochlear potential

Effect of depolarization in IHC?

Depolarization → VG Ca++ channels open → Ca++ enters → IHCs release Glu onto spiral ganglion cells

Graded synapse: amt. of depolarization determines how much Glu is released

Sound waves → displacing BM → stereocilia bend back and forth → alternating depolarization and hyperpolarization

What are OHCs doing?

OHCs amplify movement of BM, particularly so we can hear low intensity stimuli

OHC length changes → BM movement amplified → V m changes amplified (pos feedback)

Prestin

motor protein in OHC membrane; can change length in response to changes in Vm

Full Summary of Hair Cell path

1. Sound waves cause tympanic membrane to vibrate

2. Sound amplified from tympanic membrane via ossicles to oval window

3. Displacing oval window membrane displaces perilymph in scala vestibuli

4. Displacing perilymph displaces Reissner’s membrane → displaces endolymph

5. Displacing endolymph displaces BM → makes stereocilia bend towards kinocilium

6. BM displacement amplified by OHCs 

7. Tip link K+ channels open and let K+ into IHC due to endocochlear potential difference

8. In IHCs, Depolarization by K+ opens VG Ca2+ channels

9. Ca2+ triggers Glu release from IHCs a. Graded synapse - voltage level determines amount of Glu released 

10. Glu affects postsynaptic spiral ganglion cells → AP!

auditory transduction is faster than would be possible with ion diffusion and faster than visual transduction that uses G-proteins

What non-bony structure does the stapes directly push on?

Oval window

How is a receptor potential generated in a cochlear hair cell when sound makes stereocilia bend?

The fluid surrounding the cilia is more positive than the inside of the hair cell and this potential difference allows K+ ions to flow inward

Auditory Pathway

1) Cochlea

2) Spiral ganglion neurons

Auditory nerve 

3) Ventral and dorsal cochlear nuclei (Medulla)

4) Ventral cochlear nuclei (Decussation)

5) both superior olive

lateral lemniscus 

6) Inferior colliculus

7) MGN (in thalamus)

Acoustic radiation

8) Auditory cortex

Tonotopy: characteristic frequency

The frequency of sound at which a given neuron is most responsive

→ neurons near each other have similar characteristic frequencies

*tonotopy does not extend to frequencies <200Hz bc neurons do not have this low of characteristics 

Tonotopy on Basilar Membrane

A specific point on the BM is displaced maximally by a specific frequency; frequencies are represented by their location on the BM

Tonotopy on Auditory Nerve

Each auditory nerve fiber is sensitive to a range of frequencies around a characteristic frequency; which neurons are active indicates frequency

Tonotopy on Cochlear Nuclei 

Auditory axons synapse in cochlear nuclei in an organized pattern based on characteristic frequency

Primary Auditory Cortex (A1)

Tonotopic organization on basilar membrane is preserved in A1!

Isofrequency bands: neurons organized in columns according to frequency

Similar to orientation columns in V1

Both A1 cortices get input from both ears → lesioning one side impairs sound identification and localization BUT doesn’t cause unilateral deafness

Dorsal Stream

pure tones and sound localization (“where?”) → areas above A1 toward parietal and frontal lobes 

Ventral stream

complex sound identity (“what?”) → areas below A1 toward temporal and frontal lobes (ie monkey vocalizations)

A lesion of which of the following would produce deafness in the left ear?

Left cochlear nucleus

Auditory Coding Pathway

Sound Vibrations → Hair Cells → Auditory Nerve Fibers (ANFs) → Auditory Centers

What does the Auditory Pathway encode?

1) Intensity (loudness)

2) Frequency (pitch)

○ At high, intermediate, and low frequencies

3) Location

○ At high and low frequencies ○ Vertical and horizontal localization

How does the Auditory System encode intensity of sound?

At higher sound intensities, the BM vibrates with a greater amplitude, which causes…

● hair cells to depolarize and hyperpolarize at a higher frequency, which causes auditory neurons to fire APs at a higher frequency

● more hair cells to be depolarized and hyperpolarized, which then causes more auditory neurons to fire APs

What is Tonotopy?

A specific point on the BM is displaced maximally by a specific frequency

Range of 200-20,000 Hz (high & intermediate frequencies)→ because neurons do not have characteristic frequencies < 200 Hz

What is Phase locking (spike timing)?

• Neuron fires consistently at same phase of a sound wave

• A frequency might be represented by one cell alone or by multiple cells working together (volley principle)

• Range of 20-5,000 Hz (intermediate & low frequencies)→ because neurons can only fire so often 

What is Volley principle?

take a group of phase-locked neurons, we can make up for the fact that they might not fire every cycle

• Some neurons will fire when others don’t, so with a good enough group, we will have at least 1 neuron firing every cycle

• Very helpful for intermediate frequencies that might be a little too fast for neurons to fire every cycle

Horizontal Location

AKA azimuth

Starting at the superior olive, ascending auditory structures get input from both ears (i.e. binaural input) → inputs from ears combined for sound localization 

Interaural Intensity Difference

higher frequency sounds (>1.5 KHz), the head casts a “sound shadow” that dampens intensity of sound at the other ear

moving head around detects interaural intensity difference → infer sound direction 

Interaural Time Delay

Lower freq. sounds do not produce a sound shadow due to diffraction (sound waves bend around head)

Sound wave peak reaches each ear at different times (right ear hears sound from right side first)

What parts of the superior olive are used for Azimuth Sound Localization?

MSO: medial superior olive nucleus

• Neurons in the MSO are tuned for interaural timing difference (delay) at lower frequencies (20 - 2000 Hz)

LSO: lateral superior olive nucleus

• Neurons in the LSO are tuned for interaural intensity difference at higher frequencies (2,000 - 20,000 Hz)

What are some MSO neurons sensitive to?

delays in time → arranged in delay lines and tuned for specific time delays

(Negative delay = sound reached left ear first ;  Positive delay = sound reached right ear first)

Interaural Time Delay example pathway (MSO)

sound comes from left side → reaches left cochlea first → creates AP in left auditory nerve → cont down left auditory nerve → X ms later, sound reaches right cochlea → right cochlea generates AP down right auditory nerve ——→ both AP converge at neuron 3 (sensitive to amt of Xms it took for sound to reach other ear)

• AP sum at synapse to generate an EPSP high enough to generate an AP in neuron 3

Interaural Intensity Difference example pathway (LSO)

Excitatory input from the ear ipsilateral to LSO

Inhibitory input from the ear contralateral to LSO

— Spike rate varies continuously as sound direction moves:

• More intense sound to ipsi ear → gradual decrease in spiking

• More intense sound to contra ear → steeper decrease in spiking

Vertical Auditory Coding

Pinna directs sound to auditory canal

→ better at higher frequency sounds (esp from high elevations) into canal 

→ can enter directly or indirectly (sound reflected towards canal, cause intensities of diff freq to inc or dec based on origin)

direct & indirect paths interfere → spectral shaping → brain can tell original elevation of sound

monaural - each pinna get own unique input

Timing in Speech Interpretation (graphs)

Axes: frequency vs time (darker color = more intense sound)

VOT: voice onset time = Time btwn consonant release and vocal cord vibration (difference in VOT can make the difference in the sound we hear)

Otoacoustic Emissions

Ear is both a detector and emitter

Otoacoustic emission: A sound generated by the ear == Caused by OHC amplification of BM

What are the 2 types of Otoacoustic Emissions?

1. Spontaneous Otoacoustic Emissions: No stimulus needed -occurs in about 50% of people - sometimes associated with damage to ear

2. Evoked Otoacoustic Emissions: CLICK played into ear, after short delay, sound will be played back. Used for testing hearing noninvasively in babies

Causes of Congenital Deafness

• Genetics (some associated w down or usher)

• Prenatal infection (in mother or fetus - rubella, cytomegalovirus, meningitis)

• Ototoxins in utero (hair cell loss - certain antibiotics)

Causes of Acquired Hearing Loss

• Excessive exposure to loud noise

• Infection

• Autoimmune disease 

• Aging

• Ototoxins: aspirin, certain antibiotics, diuretics (e.g. furosemide), chemotherapy drugs

What is the pathology cause of most hearing issues?

Hair cell damage or inner ear damage.

• very fragile, and over time they can be damaged by loud noises, infection etc. → goes undetected till serious

• OHCs are more vulnerable to damage

• Hair cells responding to high frequencies are more vulnerable to damage

• Hair cells DO NOT regenerate

Causes of conductive hearing loss (outer or middle ear damage)

• Bacteria or virus causes inflammation in middle ear

• Abnormal bone growth in the middle ear

• Occlusion of ear canal

• Developmental defects

Tinnitus

Sound perception in absence of stimulus

• Linked to hyperexcitable and elevated spontaneous activity in auditory system

• Associated with: hearing loss, anxiety, exposure to loud noise, ototoxic drugs

• no cure, treat w white noise or sound masking machines 

Amusia

Tone deafness

• Issues with perceiving, memorizing, and producing music, pitch
  discrimination, discriminating instruments, rhythm, timing

• Usually accompanied by other deficits (cognitive, aphasia)

• right hemi considered dominant (opp of language)

Wernicke’s Aphasia

poor language comprehension

Wernicke’s area: higher order auditory area, involved in word interpretation (damage of this area), may be specialized for storing memories of sounds that make up words

Hearing aids

• Amplify sound from microphone and deliver to speaker in the ear

• Frequencies amplified can be tailored

Cochlear Implants

• Takes sound wave frequencies and stimulates the respective spiral ganglion cells directly

• Bypasses hair cells and exploits the tonotopy of the cochlea → stimulating different
  places on the basilar membrane (base...apex) to evoke different sensations of pitch

has about 12-24 electrodes stretching along length of scala tympani 

The following statements about sound localization are correct EXCEPT:

High frequency sounds do not generate sound shadows

What is meant by the “cochlear amplifier”?

Outer hair cells amplify movements of the basilar membrane

Which nucleus in the auditory system receives input only from the ipsilateral ear?

Cochlear nucleus

Which is the most important factor used to represent the frequency of a 50 Hz sound?

Phase locking

The auditory system determines the horizontal location of a 10 kHz sound (azimuth) based on

Interaural intensity difference

what is the speed of sound?

340 m/sec (770 mph)

what is hearing damage related to?

sound intensity (not loudness)

why do we use the ossicles?

Because the cochlea is filled with fluid. Without the
ossicles, 99.9% of sound energy would be reflected off the
eardrum and back into the environment.

• Large, low-pressure eardrum vibrations become small, high-pressure movements at the oval window

• The stapes footplate is about 30× smaller than the tympanic membrane

• Ossicles amplify force through lever action

• Movements are extremely tiny—only a few nanometers at hearing threshold

What neurons are sensitive to sound elevation?

Neurons in the MGN and A1 are sensitive to sound elevation
This is probably based on sensitivity to spectral shape
(i.e. the ratio of different sound
frequencies)

how do owls hunt in the darkness?

with precise azimuth and elevation information (their ears are at diff heights)

cocktail party effect

ability to focus attention on one stimulus and filter out others

→ based on sound frequencies & location 

What vs where in human auditory cortex
Pictures and sounds simultaneously presented

Where= pictures appear on L or R side, sound came from L or R side of headphones

What= sound and picture semantically consistent (dog barking) or inconsistent (horse picture with dog barking)

subjects asked to report:

1. Location task: did visual and auditory stimuli
appear on the same side?
2. Recognition task: were visual and auditory stimuli
semantically consistent?

Brain activation in what vs where tasks

Higher parietal lobe activity in location task than recognition task

Higher temporal lobe activity in recognition task than location task

How loud is safe?

• below 60 dB is safe
  • Avoid prolonged exposure above 70 dB
  • Club concerts are often 120 dB
  

• Maximum earbud/airpod output is typically ≥ 100 dB
  • Volume loud enough to block out ambient
  sound is typically 80 dB or higher
  • Limit exposure to 90 min at 80% max volume
  • Noise isolating or canceling headphones help