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
