Hearing, physiology and psychoacoustics

Basic qualities of sound waves: frequency and amplitude

Amplitude/intensity: the magnitude of displacement (the difference between the highest pressure area and the lowest pressure area) of a sound pressure wave; amplitude is perceived as loudness o dB: define the difference between two sounds in terms of the ratio between sound pressures; each 10:1 sound pressure ratio is equal to 20dB
dB = 20 log(p/p₀) → p:the pressure of the sound being described; p₀:

reference pressure (0.0002 dyne/cm²)

o The more intense a sound wave is, the louder it will sound

Frequency: the rate of fluctuation of a sound wave; frequency is perceived as pitch o Hz: 1 Hz is one cycle per second

o Low-frequency sounds correspond to low pitches. High-frequency sounds correspond to high pitches

Sine waves and complex sounds

Spectrum: a representation of the relative energy present at each frequency
Harmonic spectrum: the spectrum of a complex sound in which energy is at integer multiples of the fundamental frequency
Fundamental frequency: the lowest-frequency component of the sound
Timbre: the psychological sensation by which a listener can judge that two sounds with the same loudness and pitch are dissimilar. Timbre quality is conveyed by harmonics and other high frequencies

Basic structure of the mammalian auditory system

Outer ear

Outer ear: the external sound-gathering portion of the ear, consisting of the pinna and the ear canal
Pinna: the outer, funnel-like part of the ear
The length and shape of the ear canal enhance sound frequencies between about 2000 and

6000 Hz, but its main purpose is to protect the structure at its end, the tympanic membrane(eardrum)

Tympanic membrane: a thin sheet of skin that moves in and out in response to the pressure changes of sound waves

Middle ear

Middle ear: consists of three tiny bones, the ossicles, that amplify sound waves (fig 9.8 p248) o Malleus: the first ossicle, receives vibration from the tympanic membrane and is attached to the incus
- Incus: the middle ossicle, connects the malleus and the stapes
- Stapes: the last ossicle, which transmits the vibrations of sound waves to the oval window
Oval window: the flexible opening to the cochlea through which the stapes transmit vibration to the fluid inside; forms the border between the middle ear and the inner ear -The amplification of sound waves by the ossicles:
- The joints between the bones are hinged in a way that makes them work like levers:

the modest amount of energy on one site of the fulcrum (joint) becomes larger on the other → increases the amount of pressure change by a third

The ossicles concentrate the energy from a larger to a smaller surface area: the tympanic membrane, which moves the malleus, is about 18 times as large as the oval window, which is moved by the stapes → pressure on the oval window is magnified

18 times relative to the pressure on the tympanic membrane

Amplification is essential for our ability to hear faint sounds, because the inner ear is made up of a collection of fluid-filled chambers, and it costs more energy to move liquid than to move air
The middle ear has two muscles:
- Tensor tympani: the muscle attached to the malleus; tensing this muscle decreases vibration
- Stapedius: the muscle attached to the stapes; tensing this muscle decreases vibration
Acoustic reflex: tensing of the tensor tympani and the stapedius in response to a very loud sound, which restricts the movement of the ossicles and thus muffles pressure changes that might be large enough to damage the inner ear o Cannot protect the inner ear against abrupt loud sounds, because it follows the onset of loud sounds by about one-fifth of a second. So it only helps in environments that are loud for sustained periods

Inner ear

The inner ear translates the sound waves into neural signals

Cochlear canals and membranes

Cochlea: a spiral structure of the inner ear; contains watery fluids in three parallel canals (figure 9.9 page 251): o Tympanic canal o Vestibular canal o Middle canal
The tympanic and vestibular canals are connected by a small opening, the helicotrema, and are effectively wrapped around the middle canal
Reissner’s membrane: separates the vestibular canal and the middle canal

Basilar membrane: separates the tympanic canal and the middle canal; forms the base of the cochlear partition

Cochlear partition: combined basilar membrane, tectorial membrane and organ of Corti, which transduces sound waves into neural signals
Vibrations transmitted through the tympanic membrane and middle ear bones cause the stapes to push and pull the flexible oval window in and out of the vestibular canal at the base of the cochlea. This movement of the oval window causes waves of pressure changes, travelling waves, to flow through the fluid in the vestibular canal. A displacement forms in the vestibular canal and travels from the base of the cochlea down to the apex. Because the vestibular and tympanic canals are wrapped tightly around the middle canal, when the vestibular canal bulges out it puts pressure on the middle canal. This
Round window: a soft area of tissue at the base of the tympanic canal that releases excess pressure remaining from extremely loud sounds

The organ of Corti

Organ of Corti: extends along the top of the basilar membrane; translates movements of the cochlear partition into neural signals; made up of:
- Hair cells: a cell that had stereocilia for transducing mechanical movement in the inner ear into neural activity sent to the brain; some also receive input from the brain; arranged in four rows:
  - One row of inner hair cells: receptor cells, pick up the sounds and send responses → afferent fibres
  - Three rows of outer hair cells: efferent fibres: when they become active the outer hair cells become longer, making the nearby cochlear partition stiffer. By making some parts stiffer than other parts the outer hair cells make the cochlea more sensitive and more sharply tuned to particular frequencies

o Dendrites of auditory nerve fibres: a collection of neurons that convey info from the hair cells in the cochlea to (afferent) and from (efferent) the brain stem

Supporting cells

Stereocilium: a hair-like extension on the tips of hair cells in the cochlea, that, when flexed, initiate the release of neurotransmitters

Tectorial membrane: gelatinous flap that is attached on one end and floats above the outer hair cells on the other end. Taller stereocilia of outer hair cells are embedded in the tectorial membrane, and the cilia of inner hair cells are nestled against it. Because the tectorial membrane is attached on only one end, it shears across the width of the cochlear partition whenever the partition moves up and down, this causes the stereocilia of both inner and outer hair cells to bend back and forth

Inner and outer hair cells

Deflection of a hair cell’s stereocilia causes a change in voltage potential that initiates the release of neurotransmitters, which in turn encourages firing by auditory nerve fibres that have dendritic synapses on hair cells
Tip link: a tiny filament that stretches from the tip of a stereocilium to the side of its neighbour → mechano-electrical transduction (MET) is responsible for extreme speed and sensitivity (ion pores open when the deflection is as little as 1nm) of the hair cells.
When a stereocilium deflects, the tip link pulls on the taller stereocilium in a way that opens an ion pore. This action permits potassium (K⁺) ions to flow rapidly into the hair cell, causing rapid depolarization, which in turn leads to a rapid influx of calcium (Ca²⁺) ions and initiation of the release of neurotransmitters from the base of the hair cell to stimulate dendrites of the auditory nerve fibres. The firing of the auditory nerve fibres finally completes the process of translating sound waves into patterns of neural activity

An air pressure wave is funnelled by the pinna through the auditory canal to the tympanic membrane, which vibrates back and forth in time with the sound wave. The tympanic membrane moves the malleus, which moves the incus, which moves the stapes, which pushes and pulls on the oval window. The movement of the oval window causes pressure bulges to move down the length of the vestibular canal, and these bulges in the vestibular canal displace the middle canal up and down. This up-and-down-motion forces the tectorial membrane to shear across the organ of Corti, moving the stereocilia atop hair cells back and forth. The flexing of the stereocilia initiates rapid depolarization that results in the release of NTs into synapses between the hair cells and dendrites of the auditory nerve fibres. These NTs initiate action potentials in the auditory nerve fibres that are carried to the brain.

Coding of amplitude and frequency in the cochlea

Coding amplitude: as the amplitude of a sound wave increases, the tympanic membrane and the oval window move farther in and out with each pressure fluctuation. The result is that the bulge in the vestibular canal becomes bigger, which causes the cochlear partition to move farther up and down, which causes the tectorial membrane to shear across the organ of Corti more forcefully, which causes the hair cells to bend farther back and forth, which causes more NTs to be released, which causes the auditory nerve fibres to fire action potentials more quickly → the larger the amplitude, the higher the firing rate of the neurons that communicate with the brain.
Coding frequency: different parts of the cochlear partition are displaced to different degrees by different sound wave frequencies. High frequencies cause displacements closer to the oval window, near the base of the cochlea; lower frequencies cause displacements nearer the apex → different places on the cochlea are tuned to different frequencies: place code for sound frequency (figure 9.12 page 255)

The auditory nerve

The responses of individual auditory nerve fibres to different frequencies are related to their place along the cochlear partition → different fibres selectively respond to different sound frequencies o Characteristic frequency: the frequency that increases the neuron’s firing rate at the lowest intensity

Two-tone suppression

Two-tone suppression: a decrease in the firing rate of one auditory nerve fibre due to one tone, when a second tone is presented at the same time
The threshold tuning curve (dark red) plots the response of one auditory nerve fibre with a characteristic frequency of 8kHz. Whenever a second tone is played at the frequencies and levels within the light red area, the response of this fibre is reduced

(suppressed)

Rate saturation

Isointensity curve: a map plotting the firing rate of an auditory nerve fibre against varying frequencies at a steady intensity (figure 9.15 page 258)
Rate saturation: the point at which a nerve fibre is firing as rapidly as possible and further stimulation is incapable of increasing firing rate
The brain cannot rely on the response of a single auditory nerve fibre for determining the frequency of an incoming sound, thus:
- It uses auditory nerve fibres with different spontaneous firing rates:
  - Low-spontaneous fibre: an auditory nerve fibre that has a low rate of spontaneous firing; requires relatively intense sound before it fires at high rates, but retains its frequency selectivity over a broader range of intensity
  - High-spontaneous fibre: an auditory nerve fibre that has a high rate of spontaneous firing; increases its firing rate in response to relatively low levels of sound, but quickly reaches saturation → sensitive to low levels of sound, but poor selectivity at high intensities
- It accurately determines the frequency of incoming sound waves by integrating the info across a broad range of auditory nerve fibres, and uses a pattern of firing rates across all these fibres

The temporal code for sound frequency

Phase locking: auditory nerve fibres tend to fire action potentials at one particular point in the phase of a sound wave o The firing pattern of an auditory nerve fibre carries a temporal code: tuning of different parts of the cochlea to different frequencies, in which info about the particular frequency of an incoming sound wave is coded by the timing of neural firing as it relates to the period of the sound → if the auditory nerve fibre fires an action potential 100 times per second, the downstream neurons listening to the auditory nerve fibre can infer that the sound wave includes a frequency component of 100 Hz
- Temporal coding becomes inconsistent for frequencies above 1000Hz, and is absent above 4000/5000Hz, because the fibres simply cannot produce action potentials quickly enough to fire on every cycle of the sound
Volley principle: the idea that multiple neurons can provide a temporal code for frequency if each neuron fires at a distinct point in the period of a sound wave but does not fire on every period

Auditory brain structures

Cochlear nucleus:
- Contains many different types of specialized neurons:
  - Sensitive to just the onsets of sound at particular frequencies
  - Sensitive to the coincidence of onsets across many frequencies: fire when multiple frequencies are initially heard, but stop firing if the sound continues playing

Use lateral inhibition to sharpen the tuning to one frequency by suppressing nearby frequencies
Respond in exactly the same way as the auditory nerve fibres that feed them
Serve as quick relays from the cochlea to the superior olive, where input from both ears converges

Consists of three sub-nuclei

Cochlear nuclei – superior olive – inferior colliculus – medial geniculate nucleus – primary auditory cortex(A1) – belt (secondary) area of the cortex – parabelt (associational) area (figure 9.19 page 262 & figure 9.20 page 263)
Inferior colliculus: mostly contralateral input
Medial geniculate nucleus: more efferent than afferent connections
Tonotopic organization: an arrangement in which neurons that respond to different frequencies are organized anatomically in order of frequency: reflects both the early mechanical properties of transduction and the importance of frequency composition of sounds for auditory perception

Basic operating characteristics of the auditory system

Psychoacoustics: the response of a human being to a sound is a combination of the physical characteristics of the sound and the impressions of this sound for the listener o Frequency – pitch o Intensity – loudness

Intensity and loudness

Audibility threshold: the lowest sound pressure level that can be reliably detected at a given frequency o Equal loudness curve: pressure level and loudness are unequal

Temporal integration: a sound at a constant level is perceived as being louder when it is of greater duration (presented <100ms will be perceived softer; presented =>300ms will be perceived louder)

Frequency and pitch

For any given frequency increase, listeners will perceive a greater rise in pitch for lower frequencies than they do for higher frequencies
Masking experiment: a second sound is used to make the detection of the first sound more difficult: how effectively can one sound hide another sound. A sine wave test tone is presented with a narrow band of noise. The intensity of the test tone is then adjusted, until the listeners can just hear the tone over the noise. Next the bandwidth of the noise is increased, therefor the listener would need to increase the intensity of the test tone to be able to hear it over this broader range of noise frequencies. If the bandwidth keeps widening, we eventually reach a point at which adding more energy to the noise stops affecting the detectability of the test tone → the critical bandwidth
The width of the critical bandwidth changes depending on the frequency of the test tone, and these widths correspond to the physical spacing of frequencies along the basilar membrane. Critical bandwidths for low frequencies are smaller than the critical bandwidths for high frequencies, because the spacing between low frequencies is larger on the basilar membrane.
Masking effects are asymmetrical o The upward spread of masking: a mask whose bandwidth is below the critical bandwidth of a test tone, the mask is more effective if it is centred on a frequency below the test tone’s frequency

Hearing loss

Obstruction of the ear canal inhibits the ability of sound waves to exert pressure on the tympanic membrane
Conductive hearing loss: occurs when the middle-ear bones lose their ability to freely convey vibrations from the tympanic membrane to the oval window o Otitis media: inflammation of the middle ear o Ostosclerosis: abnormal growth of the middle ear bones
Sensorineural hearing loss: hearing loss due to defects in the cochlea or auditory nerve o Injured hair cells
Age related hearing loss first affects the perception of high frequency sounds