Lecture 19 Auditory Coding and Processing Flashcards
how we’re going to signal frequency information
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
place theory
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
temporal coding
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
Two ways nerve fibers signal frequency:
- Which/where nerve fibers are responding
- Specific groups of hair cells on basilar membrane respond to particular frequencies.
- Place Theory
- How/when fibers are firing
- Rate or pattern of firing of nerve impulses in response to specific frequencies.
- Temporal coding
Envelopes at different frequencies show different maximum vibration points. This indicates…
….the place
where most hair cell activity is predicted.
this is for single frequencies!
Low frequencies activate toward
the
apex
high frequencies
activate hair cells near the
base
complex frequency
built from several frequencies that are multiples of that first harmonic
If I play a complex sound into the ear, the traveling wave
vibrating the basilar membrane will have:
A single peak at the highest amplitude frequency
The basilar membrane can be described as an
acoustic prism
the basilar membrane as an acoustic prism
–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.
what the basilar membrane does when it gets a single complex tone
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
auditory transduction
problem with Békésy’s model ?
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
Better Békésy
New research with live membranes shows
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.
why outer hair cells are called the cochlear amplifier
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.
By selectively destroying
outer hair cells (chemical
ablation)
we can see a decrease in the firing rate of the inner hair cell at its characteristic frequency.
Temporal coding
Phase Locking
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.
Phase locking across fibers
– 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.
Place coding is effective for
the entire frequency range of hearing.
from the lowest (20Hz) to the highest (20,000 Hz)
Temporal coding with phase locking seems effective up to
4 - 5 kHz.
this is the range where the most human vocal information exists = functionally important = capture vocal inflection (peaks in speech)
A 20 Hz tone comes in, what happens?
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)
both place coding and temporal coding work for frequencies…
below 4-5kHz
breaking up a signal across many neurons
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
both place and temporal coding work for frequencies
below 4-5 kHz
Conductive hearing loss
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
Sensorineural hearing loss
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
Presbycusis
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
How can you use age-related differences in hearing?
- 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.”
Noise-induced hearing loss
– 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)
Tinnitus
- Perception of a ‘ringing’ sound in the ears.
- Objective tinnitus
- Subjective tinnitus
- Associated with both conductive and sensorineural hearing loss.
Objective tinnitus
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
Subjective tinnitus
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
If someone ruptures their tympanic membrane, they are most likely to experience:
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
Where do the signals go from Cochlea?
Sending signals to the brain
- 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)
Primary auditory cortex (A1)
– 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.
temporal lobe has a
- 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)
Tonotopic maps are found in A1 (core area)
– 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.
Cases of humans with brain damage to auditory cortex
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
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.
