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
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)
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
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
