october, 2004 - mitweb.mit.edu/hst.721/www/lectures/lecture_an.pdf · october, 2004. hair cell...
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HST 721 Auditory Nerve LectureOctober, 2004
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Hair Cell Afferent Innervation
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Type II: peripheral axon spirals basalwards among OHCseach contacts 10-100 OHCs5% of ANFsunmyelinated axons
Type II
Type I: peripheral axon takes radial course to IHCseach contacts only 1-2 IHC95% of ANFsmyelinated axons
Type I
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Hair Cell Afferent Innervation
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Unmyelinated type-II axons are too small to record fromAll ANF data are from Type-I / radial fibers innervating IHCs
Type I
Type I
Type IType I
Type I
Type I
Type I
Type IType I
Type II
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Each radial fiber (RF) contacts a single IHC by a single terminal swelling
IHC Afferent Innervation
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Each terminal swelling consists of a single synaptic plaque
Each synaptic plaque is continuous sheet of specialized (thickened) membrane pre- and post-synaptically, with a synaptic ribbon and associated halo of vesicles
IHC side
Nerve side
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IHC Afferent Innervation
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Each IHC is contacted by 10 to 30 radial fibers # Fibers/IHC varies from base to apex
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Rate Coding in the Auditory Nerve
•ANFs fire without acoustic stimulation: spontaneous rate (SR) varies from 0 -120 sp/sec•With tones at some frequency-intensity combos, ANFs increase average rate•Although there is post-onset adaptation, response continues as long as tone continues•Response is probabilistic: tone-off rate sometimes exceeds tone-on rate
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Tuning curves, CF and the cochlear frequency map
Frequency (kHz)
Th
resh
old
(d
B S
PL
)
Frequency (kHz)
Th
resh
old
(d
B S
PL
)
Intracellular labeling reveals cochlear frequency map: linear distance to log frequency
Characteristic Frequency (CF) defines where along cochlear spiral each radial fiber originates
Tuning curve defines iso-response contour: frequencies-intensities within curve evoke rate increase.
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Tuning curves, CF and the cochlear frequency map
Tuning characteristics arise largely in the cochlear mechanics stageBM iso-response contours show similar tuning to ANF isoresponse tuning curves.
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Click Response, Latency and BM traveling waves
Constructing a Post Stimulus Time Histogram
T=0
T=0
T=0
T=0
T=0
T=0
T=0
T=0
T=0
Spike Train
Clicks
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Increasing “latency” with decreasing CF reflects BM traveling wave delay
Interpeak time in PST for low-CF fibers = 1/CF
Increasing phase delays in mechanical measurements of basilar membrane motion showed traveling wave delays
Click Response, Latency and BM traveling waves
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Click Response, Latency and BM traveling waves
ANF PST Histograms
BM Motion
Click Stimuli
At low CFs (< 4 kHz), PST peaks mirror peaks in BM motionAt high CFs ( > 4 kHz), ANF cannot follow individual cycles of BM motion
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Click Response: the CAP
BASE
APEX
PST HISTOGRAMS
TimeV
olta
ge CM CAPCompound Action Potential,
recorded from round window
Click-evoked CAP dominated by basal ANFs
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Tuning and Non-linearities: two-tone suppression
1-tone sweep
CF tone+
1-tone sweep
Off-CF tone+
1-tone sweep
Off-CF tone+
1-tone sweep
2-tone suppression
Single tone is always excitatoryWith a fixed CF tone, 2nd tone above or below CF
will suppress
Two-tone suppression is seen in BM response
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Tuning and Non-linearities: two-tone suppression
A simple scheme to explain how transducer nonlinearity produces two-tone suppression
Same scheme produces distortion products (DPs), e.g. 2f1-f2, when driven with two tones f1 and f2DPs are seen in BM motion, ANF response and
ear canal sound pressureVisual Basic Distortion Simulator:http://oto.wustl.edu/cochlea/
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Low SR15%
Medium SR25%
High SR60%
Threshold difference is correlated with SRSR vs Threshold suggests three ANF subgroups
Tuning Curves, Threshold spread and SR groups
Threshold at CF for ANFs from one animal:At each CF region, >60dB threshold spread
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high SR fibers are large, mitochondrion-rich fibers on pillar sidelow/medium SR fibers are small, mitochondrion-poor fibers on modiolar side of IHC
Tuning Curves, Threshold spread and SR groups
Intracellular labeling experiments show:
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Tuning Curves, Threshold spread and SR groups
Central Projections:Many cochlear nucleus regions receive input from all 3 SR groupsOne region, the small cell cap, only receives input from low and medium SR fibers
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Rate vs. Synchrony Coding in ANFs
Information is carried in ANFs as action potentials or “spikes”Spikes are stereotyped voltage pulses of identical amplitudeInformation is coded in ANFs by changes in average rate or in fine timing of spikes
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Synchrony Coding in ANFs
Synchrony falls for f > 1.0 kHz irrespective of CF
Stim
ulus
Spik
e T
rain
s
0 dB SPL
50 dB SPL
ANF spikes can “phase lock” with a tone
PZC Histograms show synchrony: Synchronization index (SI) quantifies synchrony
PZ
C
PZ
C
PZ
C
PZ
C
Positive Zero CrossingHistogram
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Rate vs Synchrony Coding
Synchrony also seen in INT (interspike interval) histogramsPreferred intervals in INT histogram during tones = 1/CFINT histograms of SR: Poisson process with a dead timeInterspike intervals are random.
INT Histograms
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Adaptation in AN response
•Adaptation NOT in hair cell.•Adaptation arises in synaptic transmission:
vesicle depletion
Tone burst On
•AN discharge rate adapts: onset > steady state•Adaptation fit by two exponentials:
τ1=3 msec and τ2=60 msec
Tone burst On
Post Stimulus TimeHistogram
T=0
T=0
T=0
T=0
T=0
T=0
T=0
T=0
T=0
SR suppressed after tone burst offset
Max sustained rate: 200 - 300 sp/sec
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Adaptation and forward masking in AN response
Masker Probe
Masker Probe
•Adaptation decreases ANF response to “probe” tone for at least 100 msec after “masker” tone offset•This phenomenon is the basis for “forward masking” seen psychophysically
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Saturation and dynamic range
•Dynamic range is defined as the range of SPLs over which discharge rate increases
•Dynamic range is measured by increasing level of CF tone
•Dynamic range (DR) is < 30 dB in all ANFs:•High-SR fibers: hard saturation and small DRs (~20 dB);•Low-SR fibers: sloping saturation and larger DRs (~30dB)
Hi SR Fibers
Low-SR Fibers
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Saturation and dynamic range
Basilar membrane motion does not saturate IHC shows wide dynamic range (>50 dB)
Saturation arises at the IHC/ANF synapse
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Saturation of ANF response and the “Rate-Place” code
Cochlea is like a filter bank
Saturation non-linearity compromises fidelity
Brain might use Rate vs. Place to infer spectrum
Frequency (kHz)Frequency (kHz)Frequency (kHz)
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Saturation of ANF response and the “Rate-Place” code
At low SPLs, formants are visible in rate-vs-place profileAt high SPLs, the formant peaks are less obvious
Spectral information is also present in synchrony of response
ANF response rate vs CF for a vowel at two SPLs
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Flowchart View
Sharp Tuning, Sensitivity, Two-tone
Suppression
Synchrony Limits
Saturation