Lecture 19 Auditory Coding and Processing

Question 1

how we’re going to signal frequency information

Answer 1

we need to get that physical signal (sound stimulus out in the environment) and transduce it into neural energy so the brain can process and use it

Question 2

place theory

Answer 2

basic idea: the frequency of the stimuli was going to vibrate certain PLACES in the cochlea on the basilar membrane

based on that frequency we get a nice organization in the cochlea that would always faithfully reproduce the frequency information in the stimulus and reproduce that neurally for the brain

Question 3

temporal coding

Answer 3

what info is in the frequency that we can get out?

not necessarily just based on the place or location in the cochlea that’s being stimulated, but also by taking repeated pattern information that’s in the frequency, that firing

Question 4

Two ways nerve fibers signal frequency:

Answer 4

1. Which/where nerve fibers are responding

• Specific groups of hair cells on basilar membrane respond to particular frequencies.
• Place Theory

2. How/when fibers are firing

• Rate or pattern of firing of nerve impulses in response to specific frequencies.
• Temporal coding

Question 5

Envelopes at different frequencies show different maximum vibration points. This indicates…

Answer 5

….the place
where most hair cell activity is predicted.

this is for single frequencies!

Question 6

Low frequencies activate toward

the

Answer 6

apex

Question 7

high frequencies

activate hair cells near the

Answer 7

base

Question 8

complex frequency

Answer 8

built from several frequencies that are multiples of that first harmonic

Question 9

If I play a complex sound into the ear, the traveling wave

vibrating the basilar membrane will have:

Answer 9

A single peak at the highest amplitude frequency

Question 10

The basilar membrane can be described as an

Answer 10

acoustic prism

Question 11

the basilar membrane as an acoustic prism

Answer 11

–There are peaks in the membrane’s vibration that correspond to each harmonic in a complex tone.

– Each peak is associated with the frequency of a harmonic.

Question 12

what the basilar membrane does when it gets a single complex tone

Answer 12

those characteristic areas on the membrane will vibrate if those harmonics are present

has a peak for each of the harmonics present in the complex tone

each of the peaks will be associated with each of the frequencies there

Question 13

auditory transduction

problem with Békésy’s model ?

Answer 13

place theory dude

• Békésy used basilar membranes isolated from cadavers and his results showed no vibrational difference in response for close frequencies that people can distinguish (didn’t match psychophyiscal data = we are pretty good at telling close frequencies apart).

• why that’s the case: it’s dead tissue (DUH): can’t respond: drys out fast

Question 14

Better Békésy

New research with live membranes shows

Answer 14

not only was it the vibrational properties of the basilar membrane,
- the outer hair cells respond even more greatly when there’s vibration at a particular location

• their cilia are already embedded in the tectorial membrane:
• their cells change shape in response to vibration:
• this amplifies the signal at that location

the entire outer hair cells
respond to sound by slight tilting and a change in length.
– For this reason these cells are called the cochlear amplifier.

Question 15

why outer hair cells are called the cochlear amplifier

Answer 15

amplify the signal at that location: targeted amplification: so the threshold for this neuron at this particular frequency is much lower

Traveling wave lifts basilar 
membrane (1) .After the cilia 
are deflected (2), the outer 
hair cells contract (4), 
lowering the reticular lamina 
(basically pulling down the tectorial membrane) and pivoting the inner 
hair cells (IHC) upwards.

Question 16

By selectively destroying
outer hair cells (chemical
ablation)

Answer 16

we can see a decrease in the firing rate of the inner hair cell at its characteristic frequency.

Question 17

Temporal coding

Phase Locking

Answer 17

responding in time (phase) to a particular frequency

– Inner hair cells (nerve fibers) fire in bursts when being maximally activated.

– Bursts happen at or near the
peak of the traveling wave (at the peak of vibration on the basilar membrane).

– As the basilar membrane is going up and down: as it bounces up there’s a burst of activity and as it bounces down the cell recovers (no activity) Thus, they are “locked in phase” with the wave (will fire “in phase” with that frequency).

– A single neuron must rest
after firing (the refractory period). This establishes a
limit for its individual signaling
capacity.

Question 18

Phase locking across fibers

Answer 18

– Groups of fibers may fire at
different peaks.

– Cells don’t need to activate to each peak. Different cells can respond to different peaks. Combining the activity of these cells can recreate a
frequency beyond the
response capacity of a single
neuron.

Question 19

Place coding is effective for

Answer 19

the entire frequency range of hearing.

from the lowest (20Hz) to the highest (20,000 Hz)

Question 20

Temporal coding with phase locking seems effective up to

Answer 20

4 - 5 kHz.

this is the range where the most human vocal information exists = functionally important = capture vocal inflection (peaks in speech)

Question 21

A 20 Hz tone comes in, what happens?

Answer 21

20 times per second of the hair cells bursting

a single neuron or group of neurons can keep up with a 20Hz tone but there has to be a rest period for each neuron: basilar membrane gets to rest as it comes down

works maximally of about 400-500 Hz (basically right after a neuron fires it’s going to need about a millisecond to recover)

Question 22

both place coding and temporal coding work for frequencies…

Answer 22

below 4-5kHz

Question 23

breaking up a signal across many neurons

Answer 23

so rather than one neuron always responding to the peak, it can SKIP peaks (it’s neighbor fires)

neurons mass their activity to make a pattern that captures all of the activity in the original time wave

group of neurons converge or fire in an associated fashion then they can reproduce the frequency that was there

Question 24

both place and temporal coding work for frequencies

Answer 24

below 4-5 kHz

Question 25

Conductive hearing loss

Answer 25

Blockage of sound (physical stimulus) from the receptor cells (some kind of functional, anatomical breakdown that keeps the signal from getting to the point of being transduced)
– e.g. damage to tympanic membrane or ossicles

Question 26

Sensorineural hearing loss

Answer 26

some damage to the cells that do the transduction or the cells that process the signals once they’ve been transduced: before transduction and at transduction and beyond

• Damage to hair cells
• Damage to the auditory nerve or brain
• Most common type is presbycusis

Question 27

Presbycusis

Answer 27

most common!

(“old hearing”)
• Accompanies aging

• Greatest loss is at high frequencies
• Affects males more severely than females (historically men have been involved in more histories that involve loud noises)

• Can be caused by accumulated exposure
to damaging noises or drugs over a lifetime = adds up over time

Question 28

How can you use age-related differences in hearing?

Answer 28

• product development :
• mosquito tone: high frequency tone that drives young people crazy: “If you have problems with teenagers
  loitering near your property, causing criminal damage, putting off customers or abusing your customers and staff, the Mosquito MK4 is the most effective method of putting a stop to it.”

Mosquito Ringtone: “A tone outside the audible range of hearing for most people over the age of 30. This means that you can get phone calls and 
receive text messages in class or 
school without teachers hearing it.”

Question 29

Noise-induced hearing loss

Answer 29

– Loud noise can severely damage the Organ of Corti (loss of inner and/or outer hair cells).

– Damage associated with certain types of work (e.g. factory, heavy machinery, etc.)

– Leisure noise can also cause hearing loss (e.g. your iPod - very close to the membrane, can’t turn your head away, can’t cover ears, etc..)

• anything that’s approaching the threshold of pain (constant exposure to anything over 85dB)

Question 30

Tinnitus

Answer 30

• Perception of a ‘ringing’ sound in the ears.
• Objective tinnitus
• Subjective tinnitus
• Associated with both conductive and sensorineural hearing loss.

Question 31

Objective tinnitus

Answer 31

An actual sound is produced in the ears.

This may be due to middle ear muscle spasms, otoacoustic emissions (vibration of the basilar membrane becoming audible), or vascular problems (blood rushing in the ear).

it’s objectively present: you could record the frequency

Question 32

Subjective tinnitus

Answer 32

Experienced by the individual, usually in association with another disorder: you THINK you hear something

Most common with noise-induced 
hearing loss (nerve damage in the cochlea or spiral ganglion cells coming out of the cochlea).

damage to inner or outer hair cells so they start firing sporadically

can be either conductive or sensorineural hearing loss

Question 33

If someone ruptures their tympanic membrane, they are most likely to experience:

Answer 33

Conductive hearing loss

because if you damage that membrane then the signal coming in will be interfered with in someway: you can’t have that mechanical translation of the sound pressure as functionally pure as you had it before: has to be conductive hearing loss

Question 34

Where do the signals go from Cochlea?

Sending signals to the brain

Answer 34

• cochlea sends its signals out via auditory nerve fibers (called spiral ganglion cells)
• auditory nerve goes to the cochlear nucleus (on both sides: bilateral)
• each ear sends it nerve signals via the auditory nerve to the cochlear nucleus on either side
• then signal sent to two structures: the Superior olivary nuclei (in the brain stem); binaural information mixed (mixing info really quickly! - in comparison to eye stuff).

– Inferior colliculus (in the midbrain)

– Medial geniculate nucleus (in
the thalamus - where almost all sensory info goes)

[CoNuc SON-IC MG]

– Auditory receiving area (A1 in
the temporal lobe)

Question 35

Primary auditory cortex (A1)

Answer 35

– The first cortical region to receive auditory information is in the temporal lobe.

– It has a tonotopic organization, matching the structure
established in the cochlea.

Question 36

temporal lobe has a

Answer 36

• tonotopic organization that is a reproduction of the tonotopic organization of the basilar membrane in the cochlea!!!!!!!!!!
• there are cells that respond to lower (apex) and higher frequencies (base)

Question 37

Tonotopic maps are found in A1 (core area)

Answer 37

– within the temporal cortex in the auditory areas, there’s a hierarchical organization

– in the “core” (A1) they found a nice frequency organization:

– Neurons that respond better to low frequencies are on the left and those that respond best to high frequencies are on the right.

– Below, single cell recordings in macaque monkeys shows tonotopy in A1.

Question 38

Cases of humans with brain damage to auditory cortex

Answer 38

show perception difficulties with pitch.

they can tell what the duration of the tone was, but if you change the tone (1000 Hz 0 1500Hz) they have difficulty in determining that there was a difference and CANNOT say the direction of the difference (higher or lower)

This provides support for the
link between perception and physiological response in A1
- link between pitch representation and how WE PERCEIVE IT, how we experience that pitch

Question 39

There are peaks in the membrane’s vibration

Answer 39

that correspond to each harmonic in a complex tone.

– Each peak is associated with the frequency of a harmonic.