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Flashcards in Hearing Deck (22)
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The nature of sound

Sound is caused by changes in air pressure.
These pressure waves are characterised by amplitude, frequency, and phase.



dB= loudness



Hz= pitch



position within a cycle


Sine wave

A pure tone is the simplest sound wave


Complex sounds

Complex sounds can be built up from series of sine waves of varying amplitude, frequency, and phase.
We can decompose complex sounds into their sine wave components by a process called Fourier analysis.
As we will see, the auditory system essentially does the same thing!



The lowest frequency component of a complex sound is called the fundamental



Many complex sounds are made up of harmonics—integer multiples of the fundamental
If the fundamental frequency is 440 Hz, then the 2nd harmonic will be 880 Hz, the 3rd harmonic will be 1320 Hz, and so on


The outer ear: Pinna

increases the sound amplitude
helps determine the direction from which a sound is coming


The outer ear: External auditory

provides protection
increases the sound amplitude


The outer ear: Eardrum

(tympanic membrane)
vibrates in response to sound waves
moves bones in the middle ear


The middle ear: Ossicles

These are the smallest bones in the human body
The ossicles transmit the vibration of the eardrum (with some amplification) into the choclea through lever actions
They also provide protection against high amplitude sounds
Muscles attached to the ossicles restrict the bones’ movements


Inner ear: cochlea

The inner ear consists of:
Semicircular canals (important for our vestibular sense—i.e., sense of orientation)

The cochlea contains auditory sensory receptors
Oval window
Membrane covering an opening in the cochlea
The stapes is attached directly to the oval window (i.e., this is where vibrations get into the cochlea)
The oval window is much smaller than the eardrum—this size difference further helps amplify sound waves
The cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear


Central Auditory Pathways

Nerve fibres from each cochlea synapse in a number of sites on the way to the primary auditory cortex
The cochlear nucleus
The superior olivary nucleus
The inferior colliculus
The medial geniculate nucleus

The signal arriving at the cochlear nucleus splits and goes to each of the superior olivary nuclei
Beyond this point, input from both ears is present in both hemispheres


The auditory cortex

Granted that a number of structures exist before the primacy auditory cortex, what tasks are done there?
Animal studies have shown that many auditory tasks can be performed without the auditory cortex being present! These include responding to:
The onset of sound
Changes in sound intensity
Changes in sound frequency


What cannot be performed without the auditory cortex

Discriminating the pattern of several tones
Discriminating the duration of sounds
Localising sounds in space
Thus it seems that the cortex deals with more complex auditory tasks while the lower structures deal with simple aspects of sounds
Speech perception requires structures beyond the primacy auditory cortex


Frequency coding

The basilar membrane is about 30 mm long and varies in stiffness and width along its length.
Travelling waves move along the basilar membrane and peak at different point depending on the frequency of the sound
Thus, the location of a peak identifies the frequency of a sound

When people have damage to a specific part of the cochlea, they tend to suffer from frequency-specific hearing loss

Stimulating auditory nerves at different cochlear locations leads to perception of sounds in different pitch
Actually, this is one way in which a cochlear implant works


Hair cells

Hair cells are tuned to different ranges of frequency according to the location along the basilar membrane


Auditory neurons

The auditory neurons are arranged in an orderly manner

This organization is seen repeatedly in the auditory pathways
That is, tonotopic maps are present in the auditory system


Pitch perception

Perception of a missing fundamental
When higher-order harmonics are present in the absence of the fundamental (first harmonic), the missing fundamental is “filled in”
What if some harmonics are heard through only one ear?
A missing fundamental is perceived even when harmonics are presented to different ears


Binaural pitch encoding

Binaural pitch encoding
Structures beyond the cochlear nucleus should be contributing to pitch perception


Loudness perception

Human auditory range (in terms of loudness perception): approx. 0–120 dB
0 dB = absolute hearing threshold
120 dB = loud thunder

This range of intensity is roughly 1000000000000 to 1!
Generally, high-frequency sounds are perceived to be louder (up to about 5000 Hz)
Around 3000–5000 Hz, sounds are perceived to be loudest
As the amplitude goes up, the effect of frequency becomes smaller