Hearing

Question 1

The nature of sound

Answer 1

Sound is caused by changes in air pressure.

These pressure waves are characterised by amplitude, frequency, and phase.

Question 2

Amplitude

Answer 2

dB= loudness

Question 3

Frequency

Answer 3

Hz= pitch

Question 4

Phase

Answer 4

position within a cycle

Question 5

Sine wave

Answer 5

A pure tone is the simplest sound wave

Question 6

Complex sounds

Answer 6

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!

Question 7

Fundamental

Answer 7

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

Question 8

Harmonics

Answer 8

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

Question 9

The outer ear: Pinna

Answer 9

increases the sound amplitude

helps determine the direction from which a sound is coming

Question 10

The outer ear: External auditory

Answer 10

provides protection

increases the sound amplitude

Question 11

The outer ear: Eardrum

Answer 11

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

Question 12

The middle ear: Ossicles

Answer 12

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

Question 13

Inner ear: cochlea

Answer 13

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

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

Question 14

Central Auditory Pathways

Answer 14

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

Question 15

The auditory cortex

Answer 15

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

Question 16

What cannot be performed without the auditory cortex

Answer 16

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

Question 17

Frequency coding

Answer 17

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

Question 18

Hair cells

Answer 18

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

Question 19

Auditory neurons

Answer 19

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

Question 20

Pitch perception

Answer 20

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

Question 21

Binaural pitch encoding

Answer 21

Binaural pitch encoding

Structures beyond the cochlear nucleus should be contributing to pitch perception

Question 22

Loudness perception

Answer 22

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