hearing

Question 1

Sound helps to:

Answer 1

Find mates (sexual selection)

Avoid predators (humans too - car coming, explosion)

Communication (song/speech for humans and nonhumans)

Finding food (hunting/fishing, scavenging)

Please (music can change mood)

Question 2

Auditory system:

Answer 2

always on (can’t turn on/off, hard to block - can even hear yourself chew)

Distant sense (we don’t need physical contact, can hear from far)

Fastest sense (transmission/transduction of sound is fastest)

Question 3

Physics of Sound

Answer 3

sound is a vibrational disturbance of a medium (air), has a set of physical qualities that can be described (amplitude, frequency)

Question 4

Psychology of Sound

Answer 4

sound is a physical event that must be converted into a biological signal to produce the perceptual experience of hearing. Can be described by a set of qualities like loudness and pitch (determined perceptually).
Has to be detected by a biological system for us to hear

Question 5

inertia

Answer 5

vibrational property of sound

objects must have inertia (static, not moving)

Question 6

elasticity

Answer 6

vibrational property of sound

objects must have elasticity (if it’s moving, will keep moving) to vibrate

Question 7

Tuning Fork and it causes what kind of motion

Answer 7

tapping it causes prong to move in the direction of the force -> elasticity of the metal opposes this motion

Simple Harmonic Motion: The smoothly varying manner of the tuning fork from an outward maximum to the opposite direction (Displacement trace takes the form of a smoothly varying sinusoidal function)

Question 8

Impact of sound source on a medium (compression vs rarefaction)

Answer 8

Compression (blue): initial tuning fork displacement crowd neighbouring air molecules (peak)

Refraction (yellow): movement of fork in opposite direction causes air molecules to relax (trough)

Pattern of compression and refraction, wave travels
Nearby air particles act as vibrators that collide with neighbouring particles and cause them to vibrate

Question 9

Sound is:

Answer 9

a travelling wave of pressure
disturbance within a medium

The auditory system detects pressure changes in the air, allowing us to hear

Question 10

Pure tone (sound created):

Answer 10

characterised by single sinusoidal (sin) function
(simplest sound wave)

Question 11

Amplitude and its perceptual quality

Answer 11

pressure change from peak to peak

perceptual quality = loudness

Question 12

Frequency and its perceptual quality

Answer 12

number of cycles that occur per second (1 cycle/second = 1 hertz, Hz)

Wavelength: distance between peaks
Perceptual quality = pitch

Question 13

Tap tuning fork harder vs Tap a stiffer tuning fork

Answer 13

greater amplitude and sound waves (higher peaks)

shorter wavelength -> higher frequency sound (more waves)

Question 14

What range of frequencies can we hear?

Answer 14

Humans can only hear a limited audible range of frequencies present in environmental sounds

Young, healthy humans can hear sounds with frequencies ranging from 20-20,000Hz

Question 15

What range of amplitudes can we hear?

Answer 15

Humans can hear a wide range of intensities

The loudest sounds we can hear are >1,000,000x louder than the faintest sounds we can hear

To describe broad range of amplitudes, sound level are measured on a log scale using units called decibels (dB)

Decibels define the difference between two sounds in terms of the ratio between sound pressures

Question 16

equation defining decibels is

Answer 16

dB = 20log (p/p0)

20 = a constant, increases the dB range a bit
p = the pressure of the sound being described
p0 = the reference pressure (lowest audible sound = 0.000020 Pa)

Question 17

SPL (sound pressure level)

SPL of a movie theatre/concert

Answer 17

indicates the minimal audible sound was used as a reference (0 - minimal, not silence)

80-120 can be damaging

Question 18

Periodic Sound:

simple vs complex

Answer 18

occurs when the pattern of pressure change repeats itself over regular intervals over time

simple periodic waves = pure tones

complex periodic waves = repeating but not sinusoidal (combo of many sine waves)

Question 19

Complex periodic sounds include

Answer 19

human speech, notes on musical instrument, city traffic

Question 20

Harmonic series:

Answer 20

many common sounds have this series

Caused by a vibrating vibrating source (eg. string of guitar)
Each frequency component is called a harmonic

Question 21

Fundamental frequency

Answer 21

lowest frequency component of a sound. All other harmonics are integer multiples of the fundamental (ex: 1st harmonic is a guitar string, 2nd harmonic is 2 times the first guitar string. 100Hz to 200Hz and so on)

Question 22

Complex Aperiodic Sounds

Answer 22

sound like noise, caused by random vibrations

White noise: noise containing all of the frequencies within a particular range (eg :traffic, whirring fan approximate white noise, static from radio)

Question 23

Fourier Analysis and Spectrum

Answer 23

any complex waveform can be decomposed into a series of sine-wave patterns without prior knowledge of what those constituent patterns are

Represented by Fourier Spectrum: displays how much energy, or amplitude, is present at multiple frequencies (a way to decompose complex sounds)

Question 24

A moving wave can be thought of as

Answer 24

a progressively growing sphere

SA of a sphere = 4πr2

Sound energy must be spread over a larger area

Inverse square law: intensity (I) = 1/r2
Eg. at twice the distance (2r), I = 1/22 = ¼

Question 25

interaural time differences

Answer 25

brain uses differences in intensity values to figure out sound source

Question 26

Sound interacts with objects in the environment but...

Answer 26

inverse square law is only true if sound travels through a medium free of other matter. Otherwise waves are: reflected, absorbed or diffracted 1. Reflected -> sound bounces back after hitting a boundary. (When sounds persist within an enclosed space due to multiple reflections, the sounds are called echoes or reverberations) ex: music hall 2. Absorbed -> energy is transferred from one medium to another. (Amount depends on absorption coefficient of the 2nd medium (% of original sound energy that is absorbed)) 3. Diffracted -> wave bends around an object, reforms and continues. (Easier for low frequency sounds) long wavelength characteristics are important to consider when designing halls and auditoriums, to maximize sound quality perceived by the audience

Question 27

auditory system can detect

Answer 27

long-range signals Sound intensity diminishes with distance

Question 28

ear must encode 2 features of sound stimulus:

Answer 28

amplitude and frequency

Question 29

The Outer Ear

Answer 29

1. Pinna: sound funnel, cartilage + skin, bumps and grooves (structure helps enhance specific sound frequencies) , good for prey animals to locate predators 2. External auditory (ear) canal (EAC): channels sound to eardrum, lined by wax-secreting glands, main job is to protect eardrum. ½ cm - 1cm in diameter (lined with ear wax) 3. Tympanic Membrane: aka eardrum, elastic membrane (made of skin) that seals off the EAC (with skin), sound waves cause it to vibrate

Question 30

The Middle Ear

Answer 30

auditory ossicles: conduct vibration of the eardrum to the inner ear 1. Malleus (Mallet): long body protrusion (handle) attached to eardrum, head connected to incus 2. Incus (anvil): body is connected to head to malleus, shaft is connected to stapes 3. Stapes (stirrup): oval-shaped footplate makes tight connection with oval window Eustachian tube: equalizes pressure across the eardrum (inner ear + external auditory canal)

Question 31

The Inner Ear

Answer 31

1. Vestibular Organs (part of vestibular system): semicircular canals (fluid-filled bony structures), balance and body position in space 2. Cochlea (part of auditory system): fluid-filled bony structure embedded in temporal bone (2.5 turns/folds), filled with watery fluids in 3 parallel canals separated by 2 membranes. Cochlea contains transduction apparatus beg: tip end: apex

Question 32

3 membrane in cochlea

Answer 32

Reissner's (top) Tectorial (middle) Basilar (bottom)

Question 33

2 canals in cochlea

Answer 33

Vestibular (top) Tympanic (bottom)

Question 34

Sound Transmission through the ear

Answer 34

Inward displacement of stapes on oval window -> increases pressure in vestibular canal Pressure gradient is transmitted across Reissner’s membrane into middle canal Causes movement of basilar membrane, upon which the transduction apparatus sits Increased pressure in tympanic canal is relieved by outward movement of the round window

Question 35

to equalize pressure

Answer 35

round window: bulges out to equalize pressure, oval window bulges in

Question 36

organ of Corti

Answer 36

conversion of vibrational to neural signals occurs in the organ of Corti structure that extends along the top of the basilar membrane (vibrations from basilar membrane are transferred to it) Made up of a scaffold of cells that support specialised neurons called hair cells Organ of corti is located entirely within the cochlear duct (middle canal) -> thus, it is immersed in fluid

Question 37

Structural features of the organ of Corti

Answer 37

Stereocilia of inner (30-100 per hair cell, bilateral symmetry by size) and outer hair cells( V shaped pattern): Fine filaments that protrude from the upper surface of hair cells Outer Hair Cells: 3 rows, Outer side of the arch of Corti Inner Hair Cells: Single row, Inner side of the arch of Corti Auditory Nerve: Collection of afferent and efferent neurons conveying information from and to hair cells, respectively Tectorial Membrane: Soft gelatinous structure, attached on one end, that extends into the middle canal, Floats above inner hair cells and attached to outer hair cell stereocilia

Question 38

How does vibration of the basilar membrane contribute to auditory transduction?

Answer 38

A travelling wave causes basilar membrane to move up and down As a result, the tectorial membrane and hair cells move in opposite directions (shear) Upward phase (up, out) and downward phase (down, in) - stereocilia bend in direction of the force

Question 39

Inner Hair Cell Auditory Transduction

Answer 39

Sensory receptors that transform vibrational energy (ex: shearing force) into an electrical signal are epithelial cells Hair-like stereocilia protrude into the middle canal -Pivot about a hinge (attached to hair cell at a “hinge” so they can swing back and forth) -Arranged in straight rows, graded in height -Are connected to each other by tip links Innervated by (afferent) auditory nerve fibers

Question 40

Tip Links

Answer 40

Fine filamentous structures that run parallel to the plane of bilateral symmetry (mechanically gated ion channels that open gates, allowing ions to flow)

Question 41

Auditory transduction

Answer 41

1. Displacement of hair bundle toward tallest stereocilia stretches tip link 2. Tip links directly open cation selective channels 3. Entry of cations (including K+) depolarizes the hair cell 4. Voltage-gated Ca2+ channels open and allow Ca2+ entry 5. Neurotransmitter (glutamate) is released onto cochlear nerve fibres that innervate the hair cell 6. Action potentials in cochlear nerve carry info to higher auditory centres

Question 42

Transduction mediated by hair cells

Answer 42

Allows hair cell to generate a sinusoidal receptor potential in response to a sinusoidal stimulus, thus preserving the temporal information present in the original signal (up to 3 kHz)

Question 43

Mechanoelectrical transduction in hair cells: but..

Answer 43

is fast: Can convert sound to electrical signal in 10 μs Uses a directly-gated channel, NOT second messengers! is sensitive (risk of damage): Hair bundle movements are on the atomic (nm) scale But, exquisite mechanical sensitivity of the stereocilia presents risks → high intensity sound can shear off the hair bundle → hearing loss!

Question 44

How do the ossicles dampen loud sounds? problem with this...

Answer 44

Two tiny muscles in the middle ear: tensor tympani and stapedius ( Smallest muscles in the body) Produce auditory reflexes in response to loud sounds Motor neurons in the brainstem stimulate their contraction in response to sudden sound bursts Descending pathway (info to brain -> brain send back to ear, talking muscles to contract and dampen sound) Muscles tense → reduce movement of ossicles → dampen pressure changes that could damage inner ear structure Reflexes have ~200 ms delay after sound (this is too slow to protect hair cells from sudden, high intensity sounds - gunshot or explosion)

Question 45

Amplitude coding in the cochlea

Answer 45

Tympanic membrane and oval window move farther in and out Bulge in vestibular canal becomes bigger, causing greater deflection of basilar & tectorial membranes Greater shearing forces on stereocilia Greater depolarization of hair cells and more neurotransmitter release

Question 46

Frequency encoding in cochlea (Place Code)

Answer 46

Vibrational disturbances in the cochlear fluids set up a travelling wave within the basilar membrane, whereby a pure tone of a particular frequency produces maximum displacement only at the point where it coincides with the resonant frequency of the basilar membrane I.e., sounds of different frequencies produce a vibrational pattern whose maximum amplitude occurs at different places along the basilar membrane Differences in width and tension of the basilar membrane produce a systematic change in resonant frequency along its length Note: Higher frequency regions are actually stimulated earlier than lower frequency regions. base (shorter/stiffer/more tension) = high frequency apex (longer/less stiff/less tension) = low frequency

Question 47

Tonotopic map

Answer 47

sound frequency is mapped onto the basilar membrane Important consequence of this map is that neural coding of frequency can be accomplished quite effectively if it is linked to the tonotopic organization of the basilar membrane.. Low frequency = further from basilar membrane

Question 48

Basilar membrane acts as:

Answer 48

Fourier analyzer to encode complex tones Maximum vibrational disturbance is produced by each of the frequency components only at the corresponding tonotopic locations! (broken down into pure tone components)

Question 49

how many auditory nerve (AN) fibres innervate each hair cell!

Answer 49

10-30

Question 50

Cell bodies of afferent nerves lie in the

Answer 50

cochlear (spiral) ganglion (one in each ear) Just outside the body of the cochlea ~95% terminate on inner hair cells (the rest innervate outer hair cells)

Question 51

Transmission of signals to auditory nerve fibres occurs through the

Answer 51

cochlear branch of the via the vestibulocochlear nerve (cranial nerve VIII)

Question 52

1st way of Neural coding of sound frequency

Answer 52

Hair cells are innervated by auditory nerve (AN) fibres in a systematic and topographic manner Responses of individual AN fibres to different frequencies is related to the position along the basilar membrane (Tones near the characteristic frequency will increase the firing rate of an AN neuron) Cochlear nerve simply preserves place coding of frequency sensitivity

Question 53

Tuning curve and Characteristic frequency (CF)

Answer 53

A graph plotting the thresholds of a neuron in response to sine waves with varying frequencies at the lowest intensity that will give rise to a response, threshold (y) and frequency (x) CF: * Lowest point on tuning curve * Frequency at which afferent fibre is most sensitive Obtain this curve by: extracellular neuron recording, play a sound (10Hz), see if neuron fires. Keep increasing till neuron changes firing rate. Change frequency (ex: 9kHz) and repeat

Question 54

What do outer hair cells do?

Answer 54

Fibres on outer hair cells are almost all from efferent axons of cells in the superior olivary complex Efferent input causes the outer hair cells to become physically longer → extend further into tectorial membrane (efferent fibers: convey info from the brain; terminate on IHCs. each efferent fiber terminates on a single inner hair cell) Influences movement of the basilar membrane Makes inner hair cells more sensitive/ more sharply tuned to particular frequencies

Question 55

Dancing Outer Hair Cell

Answer 55

Isolated guinea pig outer hair cell, Current passed through a Micropipette, dances/moves in response to sound - in sync with incoming sound to help enhance their responses, microelectrode passes current to electrically excite hair cell

Question 56

2nd way of encode sound frequency

Answer 56

Directly related to the pressure changes of the sound wave → phase-locked response (According to maximum neural discharge rates (because of AP refractory period), true phase-locking can only occur up to ~400-500 Hz, 2 milliseconds is max firing rate of neuron). neurons themselves fire at the frequency related to incoming sound waves. Volley principle: multiple neurons, population, encode higher frequencies as a group (By fibres firing once every few cycles of the wave, they retain a form of “dispersed” phase locking)

Question 57

Neural coding of sound intensity

Answer 57

At intensities above threshold, AN fibres become less selective about the frequencies to which they respond Recall: the farther stereocilia on inner hair cells pivot, the faster AN fibres linked to that hair cell fire, more firing AN fibres become less selective at higher intensities → rate saturation Thus, the brain cannot rely on a single AN fibre to determine the frequency of a tone!

Question 58

Isointensity Curves

Answer 58

plot the firing rate of a single AN fibre at different frequencies and intensities

Question 59

pattern of firing across all AN fibres 2 types of fibers

Answer 59

Auditory system determines the frequency of incoming sound waves by using the pattern of firing across all AN fibres * Allows human hearing to span range of sound pressure levels to >100 dBSP High-Spontaneous Fibers: High AP freq, Low activation threshold, Saturates at low intensities Low-Spontaneous Fibers: Low AP freq., High activation threshold, Saturates at higher intensities Inner hair cell can release different amounts of neurotransmitter at different synapses. One inner hair cell can trigger different levels of neural activity among parallel fibers (ex: amount of Ca+ released, neurotransmitters.. etc)

Question 60

Primary auditory cortex (area A1 - medial geniculate nucleus)

Answer 60

assembles incoming neural signals into meaningful acoustic events * Located on the superior temporal gyrus (mound) in the temporal lobe (temporal lobe, most of superior gyrus, tucked within sylvian fissure groove) * Receives point to point input from MGN; thus contains tonotopic map

Question 61

Secondary auditory cortex (area A2/ belt area)

Answer 61

immediately surrounds and receives input from A1 * Less sensitive to simple sounds (e.g., constant pure tone) * Processes more complex auditory signals

Question 61

Isofrequency sheets

Answer 61

populations of neurons that all respond to same frequency, tonotopically organized

Question 61

Auditory Dysfunction and Hearing Loss

Answer 61

Variability in hearing loss * Moderate impairment to total deafness (higher hearing threshold) * Brief (auditory fatigue) to permanent Hearing loss: reduction in auditory sensitivity and perception due to deficiency in sound processing by the ear

Question 61

Hearing loss can be described in 2 ways:

Answer 61

1. Described by site of damage * Obstructed ear canal: sound waves cannot exert pressure on tympanic membrane * Conductive (mechanical) loss: middle-ear bones lose ability to conduct vibrations from tympanic membrane to oval window Reduced transmission of sound to the cochlea (don't reach cochlea, inner ear) * Sensorineural loss: damage to the cochlea or to the nerves of the inner ear (reach cochlea but hair cells/auditory nerves damaged) 2. Described by age of onset * Congenital hearing loss: occurs due to either a genetic cause or problem associated with the birth process Hearing loss in children before language development hinders speech development Total loss of hearing in early childhood would result in deaf-mutism (can't create sounds, don't hear them), the absence of language vocalization ability * Acquired hearing loss: occurs later in life

Question 61

Conductive hearing loss and its causes and an example

Answer 61

Affects the mechanical conduction of sound through the ear Causes: perforation of the eardrum (poke - no vibration/changes in air pressure), middle ear infection, otosclerosis

Question 61

Conductive Hearing Loss examples

Answer 61

Middle ear infection, otitis media * Inflammation of eustachian tube (leads to breakage/closure, pressure build up in ear) * Changes in vibrational properties (air pressure) of the eardrum reduce sound transmission * Fluid buildup (in middle ear) can interfere with conductive operation of the ossicles (dampens vibrations) * Prevalence of otitis media is greater in children (easier for fluid to build up, eustachian tube is less developed/smaller) → tube is smaller and more horizontally inclined ** tube is inserted into kids ears to drain fluid, equalize pressure Otosclerosis: inherited bone disease that produces abnormal development and function of the ossicles * Increased accumulation of calcium in ossicles * Reduces amplification and transfer of sound energy * Advances in microsurgery make it possible to repair the ossicles or replace them with artificial implants!