Theories of Hearing
Place
Volley
Theories of Hearing

Place - Helmholtz


Frequency - Rutherford

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Resonance
Auditory nerve firing codes frequency
Volley Principle - Wever

nerve fibers work as group
Resonance Theory (Helmholtz, 1857

Perception of pitch depends on selective vibration of a
specific place on the BM.
Telephone Theory (Rutherford, 1886)

Entire basilar membrane vibrates


Cochlea not frequency-specific along its length
Pitch is related to firing pattern of auditory nerve fibers

Frequency related to rate of firing
Problems with Telephone Theory

Presumes ability of neurons to fire synchronously with all
frequencies of stimulus



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Refractory = 1 ms
Maximum firing rate = 1000 x per sec
Does not account for high frequency hearing
Does not account for hearing loss due to BM damage at
specific places

i.e. damage to basal end = high frequency hearing loss
Volley Principle
In this
example:
Nerve fiber
responds
every 5 cycles
Theories of Hearing

Place - Volley Theory



Place = high frequency (~4k Hz & above)
Rate (volley) = low frequency (~400 Hz & below)
Combination = mid frequencies
Theories of Hearing

Traveling Wave - Bekesy
▪
BM Characteristics
▪
▪
▪
▪
Not under tension
Widest at apex
Stiffness gradient
BM acts as low pass filter
▪
TW travels from high to low impedance (base- to apex)
Movement of stapes in OW causes pressure
wave in cochlear fluids and displacement of
basilar membrane.
Traveling Wave
Auditory Nerve
Frequency & Intensity Coding
Reissner’s memb.
Ganglion Cells
*
Stria Vas.
Spiral Ligament
Osseous Spiral Lamina
*Habenula Perforata: between 2 plates of OSL: nerve fibers
Auditory Nerve Fibers


Spontaneous activity
Threshold



All or none
Refractory period


~1 ms
Maximum discharge rate


Minimum stimulus = increase in firing rate
~1000 spikes/sec
Compound AP

Sum of many fibers
Action Potential

Spontaneous discharge:


No stimulus
Increase firing rate:


Stimulus present
More intense stimulus
Frequency Coding
Tuning Curve
CF
Sensitivity of Auditory Nerve Fibers

Characteristic frequency


Smallest amount of intensity required to cause firing
Preservation of frequency selectivity in AN
Frequency Coding

Narrow tuning

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25 dB/octave below
CF
100 dB/octave above
CF
Low frequency tail
Reflects frequency
analysis of cochlea
Tuning curves for single units with
different CFs

Low frequency AN


broad tips
High frequency AN

sharp tips
PST Histogram
TW fast at base and slows toward apex
 Rapid, synchronous discharge from basal
fibers
 Reflects travel time of TW


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Shorter latency for high freq CF
Longer latency for low freq CF
PST Histograms
Responses of neurons with different CFs
Latency mirrors TW delay
Frequency Coding


Temporal theory (phase locking)
Low frequency tones
Phase Locking
Timing of response (spikes) synchronized with the
stimulus.
Spike occurs at the same point along stimulus waveform
Period Histogram
Single AN fiber to different frequency stimuli


Spikes at various time
intervals
Spikes time locked w/freq


Neural firing timed to the
period of the tone
Period of spikes decrease
with increase in frequency
Stim frequency < 1kHz
> 1k Hz
Intensity Coding
Intensity Coding
Action Potential

Spontaneous discharge:


Increase firing rate:


No stimulus
Stimulus present
Faster firing rate:

More intense stimulus
Intensity Coding
 Dynamic
Range
 Threshold to Saturation
 Dynamic Range of hearing =
 140 dB
 Dynamic Range of AN neurons
 20 to ~50 dB
 Single fiber cannot account for intensity
coding
Rate-Level


Effect of stimulus intensity on firing rate
Rate increases with stim intensity over a range of 20 – 50
dB above neuron’s threshold
Firing rate of AN fiber as function of increase in
intensity
Firing rate saturates ~ 45 – 50 dB
asymptotes
Firing rates of AN fibers with similar
CFs and different thresholds.
Intensity Coding
Across Fiber Tuning Curves
As stimulus intensity increases more and more fibers on both the low
frequency & high frequency sides of CF are activated.
Intensity Coding: Rate + # Neurons


Increase firing rate of more fibers over wider frequency
range with increase in stimulus intensity
Phase locking required to maintain frequency coding.
Intensity Coding: Rate + # Neurons


Increase firing rate of more fibers over wider frequency
range with increase in stimulus intensity
Phase locking required to maintain frequency coding.