Chap 10. Pt. III

Question 1

Hair Cells

Answer 1

2 types of hair cells:
Inner hair cells (primary receptors)
-3500
-single row
-flask shaped
-95% of auditory nerve fibers
Outer hair cells (amplifying role)
-12000
-3-5 rows
-cylindrical
-5% of auditory nerve fibers
-cilia of the outer hair cells touch the tectorial membrane

Question 2

Sound coding in the inner ear:

Answer 2

Motion of the stapes against the oval window causes the basilar membrane to vibrate
Hair cells project from Organ of Corti onto tectorial membrane
As basilar membrane moves, hair cells also move


Question 3

The Neural Encoding of Pitch: Place Theory

Answer 3

-Which fibers are responding
-Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers
-Suggests that different frequencies will disturb different regions of the basilar membrane (i.e. different cells)
-Helmholtz first proposed a “resonance” theory
basilar membrane wider at apex
suggested that fibers in basilar membrane resonated at different frequencies at different locations along its length (long fibers at apex, short at base)
-Problems for resonance theory
no fibers
not under tension
TRAVELLING WAVES: low freq/long at apex, high freq/short at base

Question 4

The Neural Encoding of Pitch: Frequency Theory or Temporal Code

Answer 4

-Rate or pattern of firing of nerve impulses
-Proposes the basilar membrane vibrates in synchrony with the pressure changes (at the same frequency as the sound)
-Frequency representation based on a match between the frequencies in incoming sound waves and the firing rates of auditory nerve fibers.
-similar to a diaphragm in a speaker
vibrates as a unit
results in impulses in the auditory nerve at same frequency as sound itself
-Two problems:
basilar membrane not like a diaphragm
Nerve fibres can’t fire above about 1000 Hz

Question 5

Travelling Waves

Answer 5

Bekesy observed the movement of the basilar membrane directly – drilled hole in cochlea of cadaver and viewed movement
sound produced a travelling wave along the basilar membrane
waves affected all of the membrane, but the location of maximum displacement varied with frequency
Peak of envelope moves towards basal end of basilar membrane with increasing frequency

Question 6

Stimulation Deafness Experiments

Answer 6

Experimental procedure
First, a low level test tone is presented
Then, masking tones are presented with frequencies above and below the test tone
Measures are taken to determine the level of each masking tone needed to eliminate the perception of the test tone
Assumption is that the masking tones must be causing activity at same location as test tone for masking to occur – common neural mechanism
Resulting tuning curves show that the test tone is affected by a narrow range of masking tones
Psychophysical tuning curves show the same pattern as neural tuning curves which reveals a close connection between perception and the firing of auditory fibers

Question 7

Volley Theory & Phase Locking

Answer 7

frequency could be coded by pattern of activity in a number of cells

Question 8

Problems for theories of the encoding of pitch 3

Answer 8

• Loudness
• changes in pitch perception in noise
• missing fundamental

Question 9

dynamic range

Answer 9

The range of amplitudes that can be heard and discriminated; when applied to an individual auditory nerve fiber, the range of amplitudes over which the firing rate of the fiber changes.

Question 10

Tinnitus

Answer 10

Persistent noise in the absence of any auditory stimulation
Chronic
Debilitating
May be related to “Phantom Limbs”

Question 11

Conductive loss:

Answer 11

~mechanical impediment to sound transmission~
Common causes
-blockage in external ear
-Otitis media (infection)
-Otosclerosis
Diagnosis
-Relatively “flat” loss or low -frequencies more affected
-Difference between bone and air thresholds
-maximum loss no more than about 6o dB
Treatment
-Medical/surgical
-Hearing aid

Question 12

Sensori-neural loss:

Answer 12


~damage to neural transduction (i.e. hair cells)~
Common causes
-Age: gradual deterioration (presbycusis)
-Noise: progressive cumulative effects
-Ototoxic drugs: nicotine, aspirin, streptmycin
-Infections: mumps, maternal rubella, syphilis
Diagnosis
-Similar bone and air thresholds
-Typical high frequency loss (“ski-slope”)
-Loss may be profound
Treatment
-Hearing aids very limited
-In severe cases, cochlear implants

Question 13

Presbycusis: 


Answer 13

Hearing loss with age
Progressive sensori-neural loss
More severe in males than females

Question 14

Problems for theories of the encoding of pitch: Change in pitch perception in noise

Answer 14

• tones heard in background noise - pitch appears to be shifted in direction opposite to the pitch of the noise
• tone in lower frequency noise appears higher in pitch
• tone in noise higher in frequency appears lower in pitch

Question 15

Problems for theories of the encoding of pitch: Missing Fundamental

Answer 15

• Most natural sounds have a fundamental frequency and harmonics, which are always some multiple of the fundamental
• The defining characteristic of the sound is the fundamental
• The missing fundamental phenomenon occurs when only the higher harmonics of a tone are presented
• In this situation, the fundamental is still heard
• Poses problems for a simple place theory

Question 16

Problems for theories of the encoding of pitch: Loudness

Answer 16

• as the intensity of tone changes, so does the pitch
• low frequency tones - high intensity sounds appear lower in pitch
• high frequency tones - high intensity sounds appear higher in pitch