FROM EAR TO THALAMUS

Question 1

Audible Sound

Answer 1

Audible sounds vary from 0-120 dbSPL;
>120 dBSPL causes permanent hearing damage

Question 2

Sound is rapid pressure fluctuations in a medium such as air

Answer 2

– Sound pressure level (SPL) measures the magnitude of pressure fluctuations (loudness)
– 0 dbSPL is a pressure fluctuation close to threshold of hearing (20 db ↑ is 10× ↑ in pressure fluctuation)
– Frequency is how rapidly the pressure fluctuates (1 Hz is 1 up and down cycle per second) (pitch)
– Human hearing ranges from 20Hz-20,000Hz (speech ranges from about 200Hz-2,000Hz or more)
– Speed sound travels from source is about 340 meters/second (760 miles/hour) in dry air, at sea level

Question 3

ITD and ILD are binaural cues:

Answer 3

Based on comparison of sounds reaching the left and right ears

mainly useful as horizontal location cues (as our
ears are separated horizontally)

Question 4

HRTF is a spectral cue:

Answer 4

Body scatters (reflects and diffracts) sound
This influences sound frequencies

Question 5

Interaural time difference (ITD)

Answer 5

Difference in time taken for a sound to reach each ear
– Times will differ when the sound source is on one side
– No time difference when the source is directly in front or behind

Used at low frequencies

Question 6

Interaural level difference (ILD)

Answer 6

Difference between the sound pressure level at each ear
– Head casts a “sound shadow”
– Reduction in sound level at the ear further away from the sound
– ILD more useful at higher frequencies
(head does not attenuate lower frequencies as much)

Used at high frequencies

Question 7

Head-related transfer function (HRTF)

Answer 7

Pinna, head and torso influence the sound before it reaches the inner ear
– Sound pressure level at different frequencies affected by location of sound source
– HRTF specifies how the body influences the sound
– HRTF provides a vertical location cue based on changes in frequency spectrum

Question 8

Middle Ear

Answer 8

Sound moves tympanic membrane:
– Ossicles (malleus, incus, stapes)
move with tympanic membrane
Ossicles move oval window:
– Ossicles act as amplifier (converting air
movement into cochlear fluid movement)

Question 9

Inner Ear

Answer 9

Cochlear fluids:
– Perilymph (lo K+) in scala
tympani and scala vestibuli
– Endolymph (hi K+) in scala media
Basilar membrane:
– Moves up and down with sound
Inner hair cells:
– Transmit information to the brain
Outer hair cells:
– Amplify movement of basilar membrane

Question 10

Cochlea acts as a frequency analyzer

Answer 10

Sound causes travelling
wave in cochlea
– Resulting from pressure differences
between fluid-filled compartments
Basilar membrane moves
at frequency of stimulation
Size of travelling wave
varies along basilar membrane
– Due to varying membrane stiffness
Where membrane moves most
depends on sound frequency
– Maximal movement at base
for high-frequency sounds
– Maximal movement at apex
for low-frequency sounds

Question 11

Hair cells

Answer 11

Human cochlea contains 1 row of inner hair cells and 3 rows of outer hair cells
– Hairs, called “stereocilia”, form a “hair bundle” on the surface of hair cells
– Stereocilia vary systematically in height across hair cell
– Kinocilium is the tallest hair
– Protein filaments called “tip links” interconnect successive stereocilia

Question 12

Sound leads to deflections of stereocilia

Answer 12

Hair cells depolarize when stereocilia deflect towards kinocilium
Hair cells hyperpolarize when stereocilia deflect away from kinocilium
– Tectorial membrane is attached to tips of tallest stereocilia of outer hair cells
– Movement of the tectorial membrane deflects stereocilia of outer hair cells
– Stereocilia of inner hair cells are deflected by motion of fluid beneath tectorial membrane
– Tip links are associated with ion channels, which open or close depending on stereocilia deflection

Question 13

Hair cell membrane potential

Answer 13

• Endolymph has high K+ concentration
• K+ influx depolarizes hair cell
• Depolarization opens calcium channels
• Calcium influx triggers glutamate release
• Glutamate activates spiral ganglion cells which form auditory nerve

Question 14

Auditory nerve

Answer 14

Cochlea sends information via auditory nerve
– Hair cells connected to spiral ganglion cells
– Axons of spiral ganglion cells form auditory nerve
– Spiral ganglion contains cell bodies of spiral ganglion cells
Each spiral ganglion cell has best frequency
– Also called “characteristic frequency”
Auditory nerve tonotopically organized
– Best frequency changes systematically across nerve

Question 15

At low frequencies (<4 kHz):

Answer 15

– Neurons fire action potentials at a
particular phase of the sound wave
– This is called “phase-locking”
– Provides frequency information

Question 16

At high frequencies (>4 kHz):

Answer 16

– Phase-locking does not occur
– Frequency information must be
derived from tonotopic arrangement
of auditory nerve fibers

Question 17

Medial geniculate nucleus (MGN):

Answer 17

• ventral division is first-order thalamus
• dorsal division is higher-order thalamus

Question 18

Inferior colliculus

Answer 18

• integrates spatial and spectral analysis
• codes both the complexity of sounds and their direction in space?

Question 19

Superior olive

Answer 19

• 1st site receiving input from both ears
• sound direction analysis

Question 20

Ventral cochlear nucleus

Answer 20

• relays information tonotopically to olive
• preserves response timing of auditory nerve

Question 21

Dorsal cochlear nucleus:

Answer 21

• complex spectral analysis (includes HRTF)
• projects directly to inferior colliculus

Question 22

Binaural neurons in the superior olive sensitive to sound location

Answer 22

Superior olive earliest area sensitive to binaural cues
– Interaural time differences (ITDs encoded in medial superior olive)
– Interaural level differences (ILDs encoded in lateral superior olive)
Best ITD differs between neurons
– Different best ITDs derived from different
path lengths from each ear
Best ILD differs between neurons