Chapter 9: Hearing: Physiology and Psychoacoustics
I. What is Sound?
- Sounds are created when objects vibrate
- Sound waves travel faster through denser substances
- Sonic boom: when objects passes the sound waves that it is creating; huge pressure fluctuation
1. Basic Qualities of Sound Waves: Frequency and Amplitude
- The difference between the highest pressure and lowest pressure is called amplitude or intensity of a wave; loudness
- Frequency: how quickly the pressure fluctuates; hertz – one cycle per second; pitch – high freq = high pitch
- Speech, music, audible range, high risk threshold, pain threshold
- Sound levels are measured by decibels: difference between two sounds in terms of the ratio between sound pressures; small decibel changes can correspond to large physical changes
2. Sine Waves, Complex Tones, and Fourier Analysis
- Sine wave: pure tone; simplest kinds of sounds; air pressure changes continuously at same frequency; not common
- Complex tones: different frequencies, combination of different sine waves
- Fourier analysis: any sound can be divided into a set of sine waves
- Spectrum: energy at each frequency
- Timbre: quality of a sound that depends upon the relative energy levels of harmonic components.
II. Basic Structure of the Mammalian Auditory System
1. Outer Ear
- Pinna: the curly structure on the side of the heard that first collect sounds from the environment
- Sound waves funneled by the pinna into the ear canal; ear canal enhance sound frequencies; main purpose is to insulate the tympanic membrane (eardrum) from damage.
-Tympanic membrane: a thin sheet of skin that movies in and out in response to the pressure changes of sound waves
2. Middle Ear
- The tympanic membrane is the border between the outer ear and the middle ear
- Middle ear consists of three tiny bones called ossicles that amplify sound waves
a. First ossicle is malleus: connected to the tympanic membrane and second ossicle
b. Incus: second ossicle; connected to stapes
c. Stapes: third ossicle; transmits the vibrations of sound waves to the oval window
- Oval Window: another membrane that borders between the middle and inner ear
- Ossicles: smallest bones in the body; amplifies sound vibrations
a. joints between the bones are hinged to make them work like levers; small energy on one side becomes larger on the other; lever action increases the pressure change by 33%
b. increase energy transmitted to the inner ear by concentrating energy a larger to a smaller surface area; the tympanic membrane is 18 times as large as the oval window
c. the pressure on the oval window is magnified 18 times
d. Amplification allows us to hear faint sounds; inner ear is fluid-filled chambers; need more energy to move liquid than air.
e. Loud sounds: middle ear has two muscles – tenor tympanic (attached to malleus) and stapedius (attached to the stapes); smallest muscles; tense to loud sounds, restricting the movement of ossicles and muffling pressure changes that may cause damage à acoustic reflex
f. Acoustic reflex help for sustained loud environments but not abrupt loud sounds
g. Muscles tense when swallowing, talking, helping to keep auditory system from being overwhelmed by general body movement sounds.
3. Inner ear
- Sound pressure translated into neural signals; analogous to retina
- Cochlear canals and membranes:
a. Cochlear: tiny coiled structure in the temporal bone of the skull; size of pea; filled with watery fluids in three parallel canals: tympanic canal (scala tympani); vestibular canal (scala vestiguli) and the middle canal (scala media)
b. Tympanic and vestibular canals are connected by a small opening, helicotrema, and wrapped around the middle canal
c. T and V canals blow up and fold back on itself; middle canal is another balloon that is squeezed lengthwise
d. The three canals are separated by two membranes: Reissner’s membrane (between vestibular and middle canal) and Basilar membrane (between middle and tympani canal
e. Basilar membrane is not a membrane but a plate made up of fibers that have stiffness; forms the base of the cochlear partition – a complex structure through which sound waves are transduced into neural signals.
f. Vibrations through the tympanic membrane and middle ear bones cause stapes to push and pull the oval window in and out of the vestibular canal at the base of the cochlea
g. A “bulge” forms in the vestibular canal and travels from the base of the cochlear down to the apex; by the time the traveling wave reaches the apex, its displacement has mostly dissipated; if sounds are intense, pressure is transmitted back to the cochlear base through the tympanic canal, where it is absorbed by another membrane – round window
h. When vestibular canal bulges out, it pressures the middle canal; displaces cochlear partition
- The organ of Corti
a. Movements of cochlear partition are translated into neural signals by organ of Corti which goes from the top of the basilar membrane.
b. Organ of Corti made up of specialized neurons called hair cells, dendrites of auditory nerve fibers that terminate at the base of hair cells, and a scaffold of supporting cells
c. Hair cells: support the stereocilia that transducer mechanical movement into neural activity sent to the brain stem; also receive inputs from the brain
d. Auditory nerve fivers: collection of neurons that convey information from hair cells to (afferent) and from (efferent) the brain stem; includes neurons for the vestibular system
e. Hair cells are arranged in four rows that run down the length of the basilar membrane
f. Inner and outer hair cells provide the foundation for minuscule hair-like bristles called stereocilia which are hair-like extensions on tips of hair cells that initiate the release of neurotransmitters when they are flexed
g. Tectorial membrane: not really a membrane either; attached on one end and floats above the outer hair cells on the other end; taller stereocilia of outer hair cells are in the tectorial membrane, and the cilia of inner hair cells are nestled against it; shears across the width of the cochlear partition whenever partition moves à causes the stereocilia of both inner and outer hair cells to bend back and forth.
h. Deflection of hair cells’ stereocilia causes a voltage change that releases NT à firing by auditory nerve fibers with dendritic synapses on hair cells.
i. Summary 216
- Coding of amplitude and frequency in the cochlea
a. As amplitude increases, tympanic membrane and oval window move more à larger bulge in the vestibular canal à cochlear partition to move farther up and down à tectorial membrane to shear across the organ of Corti more forcefully à hair cells to bend back and forth more à more NT release à faster firing of auditory nerve fiber action potential
b. Different parts of the cochlear partition are displaced to different degrees by different sound wave frequencies
c. High freq cause displacements closer to the oval window, near the base of the cochlea, and lower freq cause displacements nearer to the apex à different parts of cochlea are tuned to different freq à place coding for sound freq
d. Cochlear freq tuning is caused by the way the structure of the basilar membrane changes along the length of the cochlea
e. Cochlea becomes narrower from base to apex but the basilar membrane becomes wider toward the apex; the basilar begins thicker at the base and thinner as it gets wider
f. Higher freq bend the narrower, stiffer regions of the basilar membrane near the base more, the lower freq cause greater displacements in the wider, more flexible regions near the apex
- Inner and Outer Hair cells
a. Afferent fibers: auditory nerve fibers that give information to the brain; synapse on the inner hair cells
b. Efferent fibers: take information from the brain; synapse to outer hair cells; the longer they synapse, the stiffer the cochlear partition à less sensitive to pressure changes
c. Outer hair cells: feedback system
4. The Auditory Nerve
- Response of individual AN fibers to different freq are related to their place along the cochlear partition à freq selectivity
- Characteristic frequency: the freq that increases the neuron’s firing rate at the lowest intensity; the lowest point on the threshold tuning curve
- Transduction of acoustic energy at different freq to neural responses: low-intensity sine wave tone with a certain freq will cause certain AN fivers to increase their firing rates; as long as the brain knows which AN fibers have which characteristic freq, the brain can interpret the pattern of firing rates across all the AN fibers to determine the freq of any tone
-Two-Tone Suppression
a. A decrease in the firing rate of one auditory nerve fiber due to one tone, when a second tone is presented at the same time; caused by mechanical changes to the basilar membrane
-Rate saturation
a. For relatively quiet sounds, the neuron is still selectively tuned, but at louder sounds, the neuron fire at about the same rate for any freq; freq such as 1000 Hz to which AN fiber had no response at low intensity, had substantial response when intensity is increased.
b. Rate saturation: broadening of freq selectivity; the point at which a nerve fiber is firing as rapidly as possible and further stimulation is incapable of increasing the firing rate.
c. For moderately intense tones, the brain cannot rely on a single AN fiber to determine freq
d. Solution: use AN fibers with different spontaneous firing rates
e. Low-spontaneous fibers: auditory nerve fibers with low rates of spontaneous firing; require intense sound before firing at higher rates; cones; require higher intensity but retain freq selectivity over a broad range of intensity
f. High-spontaneous fibers: auditory nerve fibers with high rates of spontaneous firing; increase firing to low levels of sound; reaches saturation quickly à poor freq selectivity when intensity is high
g. Mid-spontaneous fibers: medium rates
- The Temporal Code for Sound Freq
a. Another way to encode freq; phase locking: AN fibers fire action potentials at one particular point in the phase of a sound wave
b. AN fibers fire when stereocilia of hair cells move in one direction but not in the other direction
c. Phase locking à firing patter of an AN fiber carries a temporal code for sound freq
d. Volley principle: multiple neurons can provide a temporal code for freq if each neuron fires at a distinct point in the period of a sound wave but does not fire on every period; “took turns”
5. Auditory Brain Structures
- Cranial Nerve 8; from cochlea to the brain stem; AN gibers synapse in the cochlear nucleus; neurons fire when multiple freq are heard but stop firing if the sound continues playing
- Cochlea à superior olive à inferior colliculus à medial geniculate nucleus (thalamus) à cerebral cortex
- Tonotopic organization: an arrangement in which neurons that respond to different freq are organized anatomically in order of freq
- TO is maintained in the primary auditory cortex (A1): neurons from A1 project to the belt area and neurons from this belt synapse with neurons in the parabelt area
III. Basic Operating Characteristics of the Auditory System
-Psychoacoustics: the physical characteristics of sounds and the impressions of these sounds for listeners.
-Inequality of sound pressure and loudness: equal amplitude sounds can be perceived as softer or louder depending on the freq
-The loudness of sound depends on how long the sound is: longer sounds are louder
-The difference between the intensity at which a neuron just starts firing and the intensity at which te neuron's firing rate saturates is less than the window that humans can detect loudness differences
-AN fibers have different intensity thresholds; a population can encode a broader range of intensities
1. Frequency and Pitch
- For any freq increase, listeners perceive a greater rise in pitch for lower freq than they do for higher freq.
- Critical bandwidth: adding more energy to the noise stops affecting the detectability
IV. Hearing Loss
1. Damage to any structures along the chain of auditory processing
2. Conductive hearing loss: middle-ear bones lose their ability to conduct vibrations from tympanic membrane to oval window; otitis media: middle ear fills with mucus during ear infections
3. Otosclerosis: abnormal growth of the middle ear bones
4. Sensorineural hearing loss: inside the cochlea; damage of hair cells
Summary:
1. Sounds are fluctuations of pressure. Sound waves are defined by the frequency, intensity (amplitude), and phase of fluctuations. Sound freq and intensity correspond to our perception of pitch and loudness, respectively.
2. Sound is funneled into the ear by the outer ear, made more intense by the middle ear, and gtransformed into neural signals by the inner ear.
3. In the inner ear, cilia on the tops of inner hair cells are flexed by pressure fluctuations in ways that provide information about freq and intensity to the auditory nerve and the brain. Auditory nerve fibers convey information through both the rate and the timing patterns with which they fire.
4. There are multiple places in the brain stem where different characteristics of sounds are processed before information reaches the cortex. Information from both ears is brought together very early in the chain of processing. At each stage of auditory processing, including primary auditory cortex, neurons are organized in relation to the freq of sounds (tonotopically).
5. Humans and other mammals can hear sounds across an enormous range of intensities. Not all sound freq are heard as being equally loud. Hearing acorss such a wide range of intensities is accomplished by the use of many auditory neurons. Some neurons respond across certain levels of intensity; others span different levels of intensity. In addition, more neurons overall respond when sounds are more intense.
6. A series of channels (or filters) processes sounds within bands of feq. Depending on freq, these channels vary in how wide (many freq) or narrow they are. Consequently, it is easier to detect differences between some freq than between others. When energy from multiple freq is present, lower-freq energy makes it relatively more difficult to hear higher freq.
7. Hearing loss is caused by damage to the bones of the middle ear, to the hair cells in the cochlea, or to the neurons in the auditory nerve. Although hearing aids are helpful to listeners with hearing impairment, they cannot restore hearing as well as glasses can improve vision.
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