Audition 2 Flashcards
How is sound frequency represented?
- Tonotopy
- Spike timing
Tonotopy
Each auditory nerve fiber is sensitive to a range of frequencies around a characteristic frequency
- which neurons are active indicates frequency
Spike timing
Auditory nerve fibers fire action
potentials in synchrony with sound waves
- The firing rate indicates frequency
(this only works at lower frequencies)
Tonotopy in the cochlea
The cochlea is a spectral frequency analyzer, it
separates frequency into a place code
Tonotopy in auditory nerve fibers
Central auditory connections
are also tonotopically arranged
- In each of these structures, and later tonotopic areas, the frequency could be inferred from the region most activated.
- Note that tonotopy does not extend to the lowest frequencies (below about 200 Hz) because neurons do not have characteristic frequencies this low
Tonotopy in auditory cortex
Spike timing - a second neural code for frequency
Phase locking
* At low frequencies, spike timing is locked to the
phase of the sound wave
* If there are spikes on each cycle of the sound
wave, sound frequency = spike rate
* Some neurons phase lock but spikes do not
fire on every cycle
* At higher frequencies (e.g. 3kHz), spikes cannot
fire on every cycle. Why?
* Above about 5kHz, phase locking does not occur
Phase locking and the Volley Principle
Phase locking can still indicate sound frequency even if it does not occur on every cycle of the sound wave:
- Here the activity summed over 3
neurons indicates sound frequency.
- This is called the “Volley Principle”.
Above 5kHz, the volley
representation can’t be used because
neurons don’t phase lock.
Frequency coding summary
Describe the organization of auditory cortex
- There are multiple auditory areas below the Sylvian fissure on the superior temporal gyrus
- They form an oval with a center of “core” areas
(includes A1), and surrounding “belt” areas
(You don’t need to know the names of these
surrounding areas) - Lesions to these areas impair both sound identification and localization
Auditory parallel pathways
Analogous to the visual system, dorsal “where” and ventral “what” streams have been proposed in auditory processing
Sound information in the auditory cortex
Different areas respond strongest to different stimuli
* Areas above A1 toward parietal and frontal lobes –
pure tones and sound location
* Areas below A1 toward temporal and frontal lobes
– complex sounds such as monkey vocalizations
What vs where in human auditory cortex
- Pictures and sounds simultaneously presented
“Where” cues
- Pictures appeared in left or right hemifield
- Sounds appeared in left or right headphone
“What” cues
- Sound and picture semantically consistent (dog photo with barking sound)
- Sound and picture inconsistent (horse photo and sound of violin)
Subjects were asked to report either:
1. Location task: did visual and auditory stimuli
appear on the same side?
2. Recognition task: were visual and auditory stimuli
semantically consistent
Brain activation in what vs where tasks is
consistent with parallel pathways
- Higher parietal lobe activity in location task than recognition task
- Higher temporal lobe activity in recognition task than location task
There is higher ___ lobe activity in the location task than in the recognition task
Parietal
There is higher ___ lobe activity in the recognition task than in the location task
Temporal
Wernicke’s area - a higher
auditory area unique to humans
- Wernicke’s area appears to store memories of
the sounds that make up words - A high-order area for sound recognition
analogous to the inferior temporal cortex thought to be a high-order area for visual recognition
Wernicke’s Aphasia
- Aphasia - loss of language abilities without the loss of cognitive faculties. e.g. inability to speak
- One type is Wernicke’s aphasia – lesion to superior temporal lobe (Wernicke’s area) adjacent to auditory cortex (stroke, tumor)
- Poor comprehension of language (spoken,
written) - Fluent speech but much of it gibberish
- Largely unable to understand speech, even simple commands: “Wave goodbye,” “Pretend to brush your teeth”
- Curiously, they appear undisturbed despite not
understating the speech of others or even
themselves!
Causes of congenital deafness
- Genetic causes (some associated with Down or
Usher syndrome) - Infection in mother or fetus (e.g. rubella,
cytomegalovirus, meningitis) - Conduction deafness – developmental defects,
occlusion of ear canal or middle ear - Ototoxic medications (generally hair cell loss) in
utero (e.g. certain antibiotics)
Deafness genes and locations of their expression in the cochlea
- Hereditary deafness (257 genes at last
count) - One of the most common is GJB2 that
codes for a connexin protein (recall electrical synapses) - While not found in hair cells, connexins
and gap junctions are in the stria terminalis and the supporting cells around the hair cells
Acquired hearing loss causes
Many things can reduce hearing:
* loud sounds – loud music, fireworks, sporting events,
motorcycles, blow dryers lawn mowers, kitchen blenders,
shouting (many soldiers and musicians experience
significant hearing loss)
* infection, autoimmune disease
* aging
* ototoxins – aspirin, certain antibiotics, diuretics (e.g.
furosemide), chemotherapy drugs
How loud is safe?
Centers for Disease Control and Prevention says:
* Environmental noise below 60 dB is safe
* Avoid prolonged exposure above 70 dB
* Club concerts are often 120 dB
Maximum earbud/headphone output is typically
about 110 dB
* Volume loud enough to block out ambient
sound is typically 80 dB or higher
* Limit exposure to 90 min at 80% max volume
* Noise isolating or canceling headphones help
What are most cases of acquired hearing loss caused by?
Problems with the inner ear and hair cells
Acquired hearing loss pathology
Most cases are caused by problems with the inner ear and hair cells
- Tip link breakage, hair cell distortion/swelling, loss of stereocilia, thinning of auditory nerve myelin, hair cell death, burst eardrum, membranes…Hair cells do not regenerate
- Usually loss is gradual and it goes undetected until serious (can lose about 50% before detected)
- Outer hair cells are more vulnerable than inner hair cells
- Hair cells responding to high frequencies are more vulnerable than cells responding to low frequencies
Can hair cells regenerate? (acquired hearing loss)
No
Tinnitus
- Sound perception in the absence of a
stimulus (ringing, whooshing, clicking, buzzing)
Sound perception in the absence of a
stimulus (ringing, whooshing, clicking, buzzing) - Affects about 1 in 6 people
- Risk factors – hearing loss, anxiety and stress-related disorders, exposure to loud
sound, ototoxic drugs - Caused by damage or dysfunction in the auditory system. Linked to hyperexcitable and
elevated spontaneous activity in the cochlear nucleus, inferior colliculus, MGN, auditory
cortex - No cure. Treatment with white noise or sound masking machines, possible changes to
medications, hearing aidsAffects about 1 in 6 people - Risk factors – hearing loss, anxiety and stress-related disorders, exposure to loud
sound, ototoxic drugs - Caused by damage or dysfunction in the auditory system. Linked to hyperexcitable and
elevated spontaneous activity in the cochlear nucleus, inferior colliculus, MGN, auditory
cortex - No cure. Treatment with white noise or sound masking machines, possible changes to
medications, hearing aids
What causes tinnitus?
- Damage or dysfunction in the auditory system.
- Linked to hyperexcitable and elevated spontaneous activity in the cochlear nucleus, inferior colliculus, MGN, auditory cortex
Amusia
Problems perceiving, memorizing, and producing music, pitch
discrimination, discriminating instruments, rhythm, timing
* The several components of music (pitch, timbre, rhythm) may each be
differentially located.
Identifying music-specific brain circuits is difficult because amusia is usually
accompanied by other deficits – cognitive, aphasia. But it has been seen in
isolation suggesting some specialized circuitry. The right hemisphere is
generally considered dominant (opposite of language)
Hearing aids
- Sound from microphone is amplified and delivered to speaker in the ear
- The frequencies amplified can be tailored to the hearing loss
- Different modes for quiet conversation vs noisy
background
Cochlear implants
- Unlike a hearing aid, a cochlear implant bypasses damaged hair cells and electrically stimulates the auditory nerve
- It takes advantage of the tonotopy of the cochlea by stimulating different places on the basilar membrane (base…apex) to evoke different sensations of pitch
Cochlear implant electrodes
A cochlear prosthesis has about 12-24
electrodes stretching along the length of the
scala tympani.
Voice onset time (VOT)
- Timing is a critical feature for speech interpretation
- Voice onset time (VOT) – the time
between consonant release and vocal
cord vibration
Auditory-Visual Interactions
The McGurk Effect
A lesion of which of the following would produce deafness in the left ear
a) left cochlear nucleus
b) left inferior colliculus
c( left medial geniculate nucleus
d) left superior olive
a) left cochlear nucleus
Explanation:
The auditory pathway is highly bilateral and redundant, meaning that most of the structures beyond the cochlear nuclei receive input from both ears. A lesion in structures such as the inferior colliculus, medial geniculate nucleus, or superior olive on one side would generally not cause complete deafness in one ear because auditory information from both ears is processed by both sides of the brain.
However, a lesion in the left cochlear nucleus would disrupt the primary auditory input from the left ear before it has a chance to cross over or be processed bilaterally, resulting in deafness in the left ear.
Primary auditory cortex is located in which lobe of the brain?
Temporal
Which of the following structures on one side receive input from both ears?
a) Spiral ganglia
b) Cochlear nuclei
c) Medial geniculate nucleus
d) More than one of the above
e) All of the above
c) Medial geniculate nucleus
Low-frequency sounds (e.g. around 500Hz) are localized primarily through ___
Interaural time differences (the brain detects the slight delay between th sound reaching each ear)
High-frequency sounds (e.g. around 3000 Hz) are localized through ___
Interaural level differences (the head casts a ‘sound shadow’)
What is voice onset time?
The delay between the release of a consonant sound and the movement of the vocal cords
Restoration of hearing with a cochlear implant works by artificially
a) Electrically stimulating auditory axons
b) Moving the basilar membrane
c) Causing the hair cell cilia to move
d) Activating the cochlear amplifier
a) Electrically stimulating auditory axons
The most common genetic cause of deafness is a mutation in the GJB2 gene that codes for
which protein
a) Hair cell stereocilia
b) Connexin
c) Prestin
d) Actin
b) Connexin
One of the biggest differences between the auditory system when compared to both the
somatosensory and visual systems is the fact that
a) the auditory system projection does not have a synapse in the thalamus on its way to cortex
b) spatial maps of the environment must be computed in the brain because there is no two- dimensional sensory surface that keeps track of the location of stimuli.
c) it is the only sensory system that is capable of sensing stimuli that are far away from the body.
d) neurons in the auditory system are not able to fire action potentials at a high frequency
b) spatial maps of the environment must be computed in the brain because there is no two- dimensional sensory surface that keeps track of the location of stimuli.
The presence of a 5,000 hertz sound wave would result in all of the following EXCEPT
a) a portion of the basilar membrane vibrating at 5,000 hertz
b) movement of the tympanic membrane back and forth 5,000 times a second
c) firing of 5,000 action potentials per second in an auditory nerve axon
d) lengthening and shortening of outer hair cell bodies 5,000 times per second
c) firing of 5,000 action potentials per second in an auditory nerve axon
As auditory information is relayed from the cochlea to auditory cortex (not considering
feedback), which is the correct order of structures ascending to the brain
a) Cochlear nucleus to spiral ganglion to superior olive to inferior colliculus
b) Cochlear nucleus to spiral ganglion to inferior colliculus to superior olive
c) Spiral ganglion to cochlear nucleus to inferior colliculus to superior olive
d) Spiral ganglion to cochlear nucleus to superior olive to inferior colliculus
d) Spiral ganglion to cochlear nucleus to superior olive to inferior colliculus
