Lecture 8 - Auditory Sensation and Perception 1

Question 1

How does the human auditory system work?

Answer 1

Outer, middle and inner work
Transduction

Question 2

What is sound caused by?

Answer 2

By the movement/vibration of an object.
The movement alternately squeezes air molecules together and pulls them apart.
Creates a longitudinal pressure wave in the air.

Question 3

What is the decibel scale?

Answer 3

A logarithmic scale of relative intensities.
Reduces a wide range of values to a smaller scale.
Calculated with reference to our hearing threshold.

Question 4

What is a period?

Answer 4

Time taken to complete one wavelength.

Question 5

What is frequency?

Answer 5

Number of periods per second.

Question 6

What is timbre?

Answer 6

Refers to that quality which can make two sounds with the same pitch and loudness seem dissimilar.
Related to complexity

Question 7

What is the difference between pure and complex tones?

Answer 7

Pure tone - single frequency
Complex tones - more than one frequency

Question 8

What are harmonics?

Answer 8

For a sound to have a clear pitch, the partials must be integer multiples of the fundamental frequency.
If a sound has inharmonic partials, it will be unpitched.

Question 9

What fundamental frequency does a harmonic complex tone have?

Answer 9

150Hz

Question 10

Explain the properties of the outer ear

Answer 10

Visible part of the ear (auricle/pinna)
Not vital for perception, but does have a perceptual effect.

Question 11

Explain the properties of the ear canal

Answer 11

3cm long
Extends down to the eardrum
Resonant frequency of 1-5kHz

Question 12

Explain the properties of the middle ear

Answer 12

Eardrum –> ossicles –> oval window.
Two membranes joined by bones.

Question 13

Why do we need the middle ear?

Answer 13

Vibrations must travel from air to fluid
The eardrum is much bigger than the oval window.

Question 14

What is the main structure of the inner ear?

Answer 14

The cochlea

Question 15

How does the inner ear work?

Answer 15

It works as a frequency analyser by breaking incoming complex sounds into pure tone components.
It also works as a transducer by converting mechanical energy at these different frequencies into electrical activity to travel to the brain.

Question 16

What are the elements of the cochlea?

Answer 16

• Two chambers separated by the cochlear partition.
  Filled with perilymphic fluid
  Vibrations flow from oval –> round window and back.

Question 17

What does the cochlear partition contain?

Answer 17

The basilar membrane

Question 18

What sits on top of the basilar membrane? (BM)

Answer 18

The Organ of Corti

Question 19

What does the organ of corti contain?

Answer 19

Tectorial membrane
Hair cells (inner and outer)

Question 20

Explain the transduction process (4)

Answer 20

1. BM moves in response to vibrations in the perilymph
2. BM vibrates at the same frequencies as the incoming sound
3. These vibrations bend the stereocilia of the inner hair cells against the TM.
4. Triggers the release of glutamate (neurotransmitter) and an electrical signal is sent to the brain.

Question 21

How do we know what pitch we’re hearing?

Answer 21

Place coding - WHERE on the BM the firing is coming from.
Temporal (rate) coding - WHEN the firing is occuring.

Question 22

What is place coding?

Answer 22

At the base, the BM is anchored, narrow and stiff.
At the apex, it is free, wide and loose.
Different resonant frequencies at different points!

Question 23

Explain temporal coding (4)

Answer 23

1. BM moves in response to vibrations in the perilymph.
2. BM vibrates at the same frequencies as the incoming sound.
3. Stereocilia stimulated by peaks in BM vibration.
4. Firing occurs at the period of the incoming waveform.

Question 24

What is the problem with place coding?

Answer 24

Place coding is less reliable at low frequencies.
Broader regions of activity on the BM at lower frequencies.
Less specific encoding of pitch

Question 25

What is the problem with temporal coding?

Answer 25

Temporal coding breaks down at high frequencies.
Not enough time for cells to “recharge”.
This leads to temporal “smearing”.

Question 26

What do inner hair cells detect?

Answer 26

Motion on the BM

Question 27

What is cochlear turning?

Answer 27

Outer hair cells amplify the motion of the BM

Question 28

What does intensity coding rely on?

Answer 28

The fact that there are low-and-high threshold auditory nerve fibres.
Low-threshold fibres discriminate quiet and moderate (low-vs. medium-amplitude) sounds.
High-threshold fibres kick in to discriminate moderate and loud (high-amplitude) sounds.

Question 29

What is the cochlear nucleus?

Answer 29

A relay station.
Sends neural activity to other nuclei in the brainstem.

Question 30

What is the primary auditory area?

Answer 30

Analyses higher-order features.
Less reliant on precise timing.

Question 30

What is the acronym to remember the elements of the auditory pathway?

Answer 30

• Carrying: Cochlear nucleus
• Sounds: Superior olive
• Into: Inferior colliculus
• My: Medial geniculate
• Primary auditory cortex!

Question 31

What is the superior olive?

Answer 31

It analyses location.
Relies on precise timing so happens early.