Lecture 24 and 25 : Hearing

Question 1

what is frequency of sound measured in?

Answer 1

Hertz

Question 2

what stem frequency equation?

Answer 2

f=1/t

Question 3

what is amplitude measured in?

Answer 3

deciBel (dB)

Question 4

speed of a sound is ______ in a given _____

Answer 4

constant

medium

Question 5

what is the outer ear?

Answer 5

location-dependent sound collector.

Question 6

what is the middle ear?

Answer 6

• impedance-matching transformer
• a protector (from loud sound)
• a static pressure equaliser.

Question 7

auditory system processes sounds in how many streams?

Answer 7

in 2 parallel streams

Question 8

What is sound?

Answer 8

Sound is essentially a wave of molecules coming together and splitting apart

Question 9

What is condensation?

Answer 9

An increased pressure on a sound wave results in what is known as condensation (compression)

Question 10

Name the two parallel streams?

Answer 10

dorsal cochlear nucleus

lateral lemniscus

Question 11

What is the primary auditory cortex?

Answer 11

The structure of the auditory cortex is similar to that of the visual cortex
It consists of many layers, in which each to their own are responsible for detecting certain frequencies

Question 12

What do the pinna and the concha do?

Answer 12

funnel sounds into the external auditory meatus

Question 13

How does sound pass through the middle ear?

Answer 13

sound has to pass from air into the cochlear fluids

Water tends to reflect sound

Question 14

How do the ossicles concentrate the sound?

Answer 14

The ossicles act to concentrate the sound from a large area (the tympanic membrane) to a smaller area (oval window)

Allows better transmission of sound from air to cochlea

Question 15

what do the ossicles consist of?

Answer 15

scala vestibuli
scala media
scala tympani

Question 16

what do travelling sound waves result from?

Answer 16

Travelling waves result from interactions between fluid pressure and basilar membrane mechanics (mass, stiffness and damping).

Sound energy ‘travels’ (like energy along a flicked rope) from base towards apex… but the distance that each spectral component travels depends on its frequency.

Question 17

What is the mechanical excitation of hair cells? (1)

Answer 17

Upwards movement of BM leads to outwards movement of stereocilia

The tips of the outer hair cell stereocilia are firmly embedded in the TM (tectorial membrane), so the BM-OHC coupling is ‘displacement’ coupling.

Question 18

What is the mechanical excitation of hair cells? (2)

Answer 18

The tips of the inner hair cell stereocilia are not firmly embedded in the TM. They may simply be ‘dragged’ around by fluid flow in the sub-tectorial space (e.g viscous coupling)

Question 19

What is the Organ of Corti’s role in the mechanical excitation of hair cells?

Answer 19

• Converts transverse motion of basilar membrane into

* Shearing motion of hair cell stereocilia. (colours depict hair cell depolarisation and hyperpolarisation)

Question 20

how are sound localisation cues formed?

Answer 20

• Sound intensities decrease with increasing distance from the source
• Depending on direction of the sound source in relation to the inter-aural axis (line connecting both ears), these characteristics lead to two separate cues for sound localisation:
◦ Inter-aural time differences (ITDs)
◦ Inter-aural level differences (ILDs)

Question 21

What is Inter-aural time difference?

Answer 21

ITDs are maximal when sound source lies on interaural axis.

ITDs scale with head size, max for humans = 0.6 ms

Question 22

What is Inter-aural level difference?

Answer 22

ILDs depend on head size and frequency (sound shadows depend on size of shadowing object (e.g. head) relative to wavelength of sound).

ILDs negligible below 2 kHz – long wavelength sounds diffract (bend) too well

Question 23

Where does the vestibular nuclei lead to?

Answer 23

Ascending pathway from the vestibular nuclei leads to thalamus (VPN) and then to two cortical areas.

These are integrative centres (they receive additional sensory information) that are used to build up a picture of body position (vestibular cortex)

Question 24

How do the semi-circular canals respond to angular acceleration? (1)

Answer 24

1. The increase in firing is due to the endolymph lagging behind the head movement and thus displacing the cupula in the opposite direction (endolymph is fluid and therefore exhibits inertia)
   
   2. The decrease occurs as the endolymph catches up with the head movement.

Question 25

How do the semi-circular canals respond to angular acceleration? (2)

Answer 25

3. Now head decelerates but endolymph keeps moving, displacing the cupula back in the opposite direction*, eventually it too ceases moving (4).
   * with a resulting decrease in firing.

Question 26

What does a clockwise rotation lead to ? (1)

Answer 26

• Clockwise rotation leads to excitation of right horizontal SSC and inhibition of left horizontal SSC.
  
  • Via 2 groups of motor neurones controlling the eye muscles, this leads to a reflex anticlockwise movement of the eyes, ensuring that the image of a fixated object stays still on the retina.

Question 27

What does a clockwise rotation lead to ? (2)

Answer 27

• If rotation is continuous, the anti c/w (left) movements of the eyes are interrupted by fast eye movements c/w, producing a “saw-tooth” pattern called nystagmus. Anti c/w rotation would also produce nystagmus, but the movements would be opposite (a mirror image).