auditory perception

Auditory Perception Rob van der Willigenhttp://www.mbfys.ru.nl/~robvdw/DGCN22/Anatomy_Physiology/DGCN22_2011_Anatomy_Physiology_Part2.ppt

General Outline P4 P4: Auditory Perception

- Cochlear Mechanotransduction

- Physiology of the Auditory Nerve

P5: Auditory Perception

The Problem of Hearing

Tonotopie blijft in het auditief systeem tot en met de auditieve hersenschorsbehouden. De samenstelling van een geluid uit afzonderlijke tonen is te vergelijken met de manier waaropwit licht in afzonderlijke kleuren uiteenvalt wanneer het door een prisma gaat .John A.J. van Opstal (Al kijkend hoort men, 2006; p. 8)


Neurons within a brain area may be organized topographically (or in a map), meaning that neurons that are next to each other represent stimuli with similar properties.

Mapping can be an important clue to the function of an area. If neurons are arrayed according to the value of a particular parameter, then that property might be critical in the processing performed by that area.

Neurons do not need to be arranged topographically along the dimensions of the reference frame that they map, even if its neurons do not form a map of that space.


Problem I: Sound localization can only result from the neural processing of acoustic cues in the tonotopic input!

Problem II: How does the auditory system parse the superposition of distinct sounds into the original acoustic input?

*Joseph Dodds 2006*

Joseph Dodds 2006

Sensory Coding of Sound

Summary

Sensory Coding and TransductionCochlear MechanotransductionMammalian Auditory PathwayRecapitulation previous lectures

6 critical stepsSensory Coding and TransductionRecapitulation previous lectures

Peripheral Auditory SystemSensory Coding and TransductionThe Organ of Corti mediates mechanotransduction:

The cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, thousands of hair cells are set in motion, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells.

Cochlear nonlinearityActive processing of soundThe response of the BM at location most sensitive for ~ 9 KHz tone (CF).The level of the tone varied from 3 to 80 dB SPL (iso-intensity contours). Figure 3. Vertical lines mark the responses to a tone at either 4.5 or 9 kHz.BM input-output function for a tone at CF (~9 kHz, solid line) and a tone one octave below (~4.5 kHz) taken from the iso-intensity contour plot.INPUT level (dB SPL)OUTPUT Response in dBCF= 9 kHz~4.5kHzFrequency [kHz]

Cochlear nonlinearityActive processing of soundA: Tuning characteristic, from the basal turn of the cochlea. Data is shown by plotting the amplitudes of the variations evoked by various frequencies at intensities of between 10 and100 dB-SPL. X marks the best frequencies (BF) for each site, as defined by the stimuli evoking the largest vibration velocity (not displacement). (Data from Rhode and Recio, 2000).

B: Tuning characteristic, from the tectorial membrane in the apical turn of the cochlea.(Cooper and Rhode, 1997).

C: The degree of compression, measured at the basal turn of the cochlea. Strongcompression is indicated by low (i.e. closer to zero) growth rates, whereas linearity (i.e. thecomplete absence of compression) is indicated by growth rates of 1 dB/dB. (Rhode and Recio, 2000).

D: The degree of compression, from the tectorial membrane in the apical turn of the cochlea.(Cooper and Rhode, 1997).

Cochlear nonlinearityActive processing of soundDuring the 30s, Wever and Bray found that when playing music to a cat, an electrical voltage is produced near the cats middle ear round window. If that voltage waveform is amplified, the original music can be obtained.

That electrical occurrence is called the Cochlear microphonics (CM). The cochlear microphonics can accurately recreate the sound pressure wave presented to the ear.

Today, it is widely believed that the Inner and Outer hair cells are the source of cochlear microphonics. Therefore, Cochlear microphonics must predict compression.

Indeed, compression can be easily noticed. The figure plots an idealized input-output function for cochlear microphonics in response to pure tone stimuli presented at increasing levels.

The sine waves represent the cochlear microphonic response at various points of the function. (Notice the distortion at high levels).Cochlear MicrophonicsFigure shows an idealized input-output function for cochlear microphonics.An idealized input-output function for cochlear microphonics in response to pure tone stimulipresented at increasing levels. The sine waves represent the cochlear microphonic response atvarious points of the function. (Notice the distortion at high levels). Based on various dataand figures by Wever and Lawrence (1950, 1954), and Davis and Eldridge (1959).

Cochlear nonlinearityHair cell functionIHC: Principal Sensor

Sends frequency-specific information to the brain based on the vibratory pattern of the basilar membraneOHC: Effector (Cochlear amplifier)

Provides frequency-specific energy to the basilar membrane.

Cochlear nonlinearityHair cell physiologyIHCs are responsible for turning the movement of the basilar membrane into changes in the firing rate of the auditory nerve.OHCs are anatomically and physiologically quite different from inner hair cells.

OHCs act as tiny motors that amplify the mechanical movement of the basilar membrane.

1. Nucleus 2. Stereocilia 3. Cuticular plate 4. Radial afferent ending (dendrite of type I neuron) 5. Lateral efferent ending 6. Medial efferent ending 7. Spiral afferent ending (dendrite of type II neuron)Cochlear nonlinearityHair cell anatomyIHCOHC

Cochlear nonlinearityIHC InnervationThe IHC is synaptically connected to all type I spiral ganglion neurons forming the radial afferent system (blue) going to the cochlear nuclei (CN).

The lateral efferent system (red) arising from small neurons in the ipsilateral lateral superior olivary complex (LSO) brings a feedback control to the IHC/type I afferent synapse.

Cochlear nonlinearityOHC InnervationOHC synapses with a few (at least in basal and mid-portions of the cochlea) small endings from type II spiral ganglion neurons, forming the spiral afferent system (green).

In turn, large neurons of the medial efferent system (red), from both sides of the medial superior olivary complex (MSO), form axo-somatic synapses with the OHC.

Cochlear nonlinearityCochlear InnervationInner hair cells: Main source of afferent signal in auditory nerve. (~ 10 afferents per hair cell)Outer hair cells: Primarily receiving efferent inputs.

Type I neurons (95% of all ganglion cells) have a single ending radially connected to IHCs.Type II small, unmyelinated neurons spiral basally after entering the organ of Corti and branch to connect about ten OHCs, in the same row.

Cochlear nonlinearityTotal Cochlear InnervationEach IHC is innervated by approximately 10 Type-I 8th nerve fibers.

Each Type II 8th nerve fiber synapses with about 10 OHCs but each outer hair cell synapses with several nerve fibers.

There are also approximately 900 efferent fibers (fibers that come into the cochlea from more central locations).

Innervation is both Ipsilateral and Contralateral.

Cochlear nonlinearityIn Vivo Cochlear InnervationCochlear Innervation by Temporally Regulated Neurotrophin Expression

The Journal of Neuroscience, 2001, 21(16):61706180

Cochlear nonlinearityHair cell numbers and life timeIn the human cochlea, there are 3,500 IHCs and about 12,000 OHCs.

This number is low, when compared to the millions of photo-receptors in the retina or chemo-receptors in the nose! In addition, hair cells share with neurons an inability to proliferate they are differentiated.

Thus, the final number of hair cells is reached very early in development (around 10 weeks of fetal gestation); from this stage on our cochlea can only lose hair cells.

Cochlear nonlinearityHair cell functionIHC: Principal Sensor

Sends frequency-specific Information to brain based on the vibratory pattern of the basilar membraneOHC: Effector (Cochlear amplifier)

Provides frequency-specific energy to the basilar membrane..

Cochlear nonlinearityFunctional relationship IHC and OHC

Cochlear nonlinearityThe response of the healthy mammalian basilar membrane (BM) to sound (1) is sharply tuned, (2) highly nonlinear, and (3) compressive.

Damage to the outer hair cells (OHCs) results in changes to all three attributes: in the case of total OHC loss, the response of the BM becomes broadly tuned and linear.

Many of the differences in auditory perception and performance between normal-hearing and hearing impaired listeners can be explained in terms of these changes in BM response.

Loss of OHCs affects nonlinearity

Cochlear nonlinearityCochlea is highly compressive:

In the mid-level region a change in input sound pressure of 40 dB (from 40 to 80 dB SPL) leads to a change of slightly less than 10 dB in the velocity of the BM.

A change in velocity by a factor of 10 corresponds to a 20-dB change in response.

This is equivalent to a compression ratio of approximately 5:1, compared to the essentially linear (1:1) relationship between sound pressure and BM velocity in the case of the post mortem cochlea.GAIN equals D Amplitude of motion divided by D Amplitude of stimulus pressureNo nonlinearity post mortemBasilar-membrane intensity-velocity coding functions for a chinchilla using a tone at the 10 kHzRugero et al. (1997)

Cochlear nonlinearityOHC motor driven by Tectorial membrane OHCs have a unique type of motility. They convert receptor potentials into cell length changes at acoustic frequencies.

The activation of the outer hair cell motor driven by the motion of the tectorial membrane into which the tips of the tallest stereocilia are inserted.

OHCs contract when depolarized (-60 mV)OHC lengthen when hyperpolarized (-70 mV)

A second class of sensory receptors, the outer hair cells couple visco-elastically the reticular lamina to the basilar membrane through their supporting Deiters' cells (yellow).

Cochlear nonlinearityOHC motor driven by the Tectorial membrane A virtuous loop. Sound evoked perturbation of the organ of Corti elicits a motile response from outer hair cells, which feeds back onto the organ of Corti amplifying the basilar membrane motion.

Cochlear nonlinearityOHC Boost BM vibrations at VmaxOHCs are proposed to generate positive (cell-body shortening)forces during maximum BM velocity toward scala media (a) and

negative (cell body lengthening) forces during maximum BM velocity toward scala tympani (b)

K. E. Nilsen and I. J. Russell. (2000)Scala MediaabVmaxVmaxScala MediaScala TympaniScala Tympani

Cochlear nonlinearityOHC ActivityOHC activity:Increases sensitivity (lowers thresholds)Increases selectivity (reduces bandwidth of auditory filter)Produces a non-linear amplitude response Produce Otoacoustic emissions

Cochlear nonlinearityNonlinearity is an active processFrom Pickles (1988)Cochlear Tuning is sharp and the responses are highly nonlinearBaseApex

Cochlear TransductionIHC mechanotransduction

Cochlear TransductionIHC mechanotransduction(1) Kinocilia / Stereocilia Linked

(2) Displacement Opens K+ Channels

(3) Depolarization (inward current) Less negative membrane potential release of glutamate

(4) K+ flows through cell

(5) Vesicle release in synaptic cleft Glutamate increase spike rate in auditory nervePositive displacementDepolarization

Cochlear TransductionIHC mechanotransduction(1) Kinocilia / Stereocilia Linked

(2) Displacement Closes K+ Channels

(3) Hyperpolarization (outward current) Less positive membrane potential inhibits release of glutamate

(4) K+ flows ceases

(5) Decreases spike rate in auditory nerveNegative displacementHyperpolarization

Cochlear nonlinearityHair cell anatomy

To enhance frequency tuning:

Mechanical resonance of hair bundles: Like a tuning fork, hair bundles near base of cochlea are short and stiff, vibrating at high frequencies; hair bundles near the tip of the cochlea are long and floppy, vibrating at low frequencies.

Electrical resonance of cell membrane potential.

Cochlear TransductionIHC mechanotransduction

Cochlear TransductionIHC mechanotransductionVery fast (responding from 10 Hz 100 kHz10 msec accuracy).

Cochlear TransductionIHC stimulus response relationshipAt low frequencies the membrane potential of the IHC follows every cycle of the stimulus (AC response, top).

At high frequencies the membrane potential is unable to follow individual cycles, but instead remains depolarized throughout the duration of the stimulus (DC response, bottom).

At intermediate frequencies the membrane potential exhibits a mixed AC + DC response.

Inner hair cells are thus responsible for turning mechanical movement of the basilar membrane into membrane potential changes NOT action potentials.

Cochlear TransductionIHC mechanotransductionReflect s Ca++ entry into the cell.

Motion of the Kinocilia modulates K+ influx, which causes Ca++ influx, but there is also background Ca++ leakage, so vesicles are released even without sound input.

The release rate varies among synaptic terminals, resulting in variation in sensitivity.

The auditory neurons that synapse on the IHC use AMPA receptors and have a very short time constant (~200 sec). Vesicle release

Cochlear TransductionIHC mechanotransductionDiagram of bending and flow of potassium

Conversion from mechanical (motion of basilar membrane) energy into neural (electrical) responses in hair cells is called transduction. As the stereocilia are bent, (positively charged) potassium ions flow into the neuron. This is the electrical signal that initiates neural conduction. This voltage is graded, an analog signal. Hair cells do not fire action potentials. The hair cells themselves are attached to a bundle of nerve fibers. If the voltage signal in a hair cell is large enough, then it will cause an auditory nerve fiber to fire an action potential.

Cochlear TransductionIHC activity and Action Potentials Action Potentials are generated in the auditory nerve cells

NOT in the IHCs

and mostly when the basilar membrane moves upward.

Cochlear TransductionIHC activity and ElongationThe outer hair cells (a) elongate when cilia bend in one direction; (b) contract when the cilia bend in the other direction. This results in an amplifying effect on the motion of the basilar membrane.

Cochlear nonlinearitynonlinearity is an active processDepends on the endolymphatic cochlear batteryFurosemide decreases [K+], stops process.

Not present in post-mortem cochlear preparations (in vitro). Requires metabolic energy.

Stria-Vascularis (dark red area) battery maintains the potential difference and powers the active process in a living animal.SVSTSMDisruption of electrical equilibrium via:drugsElectrical stimulationblood supply effects hearingCochlear nonlinearityEndolymphatic cochlear batterySMScala mediaSTScala tempanySVScala vestibuliDrawing from www.the-cochlea.info0 mV0 mVK+ lowK+ low

Cochlear TransductionIHC versus OHC mechanotransductionFluid movement bends the hairs of the IHCs.Tectorial membrane shearing (b) moves the tallest stereocilia that are inserted in the tectorial membrane.

Cochlear TransductionIHC versus OHC mechanotransductionThe OHCs are depolarized in the same way as the IHCs

When an OHC depolarizes, the entire cell contracts and shortens, thereby literally pulling the basilar membrane towards the cell, because the OHCs are affixed to thebasilar membrane through the supporting Deiter cells

This phenomenon, which is known as electromotility, causes the OHCs to actively feed mechanical energy back into the system!

Electromotility it is powered by a specialized protein (Prestin), lodged in the OHCsmembrane

Cochlear TransductionIHC versus OHC mechanotransduction

The Auditory NerveAnatomyNeural information from inner hair cells: carried by cochlear division of the VIII Cranial Nerve, 30,000 myelinated fibers in Humans (cats have 50,000).The auditory nerve is formed by the axons of spiral ganglion cells, of which there are two types .

Type I neurons have myelinated cell bodies and innervate IHCs. In humans, each IHC forms synaptic terminals with about 10 Type I fibers.

Type II neurons are unmyelinated and innervate many OHCs longitudinally distributed along the cochlea. Both types of neurons project to the cochlear nucleus, albeit to different types of cells. Type I neurons form the vast majority of the AN population (95% in cats). All existing physiological data are from Type I neurons.

The Auditory NerveFunctionThe auditory nerve conveys information about sound from the ear to the brain, which decodes this information to control behavior. Data on responses of the auditory nerve to sound are useful both to infer the processing performed by the ear, and to assess the brains performance in various perceptual tasks against that of an ideal observer operating on auditory-nerve information.

The Auditory NerveSpontaneous rate / All-or-none potentialsDepolarizing currents exceeding a threshold produce a large, all-or-none action potential which travels along the axon without attenuation.

The Auditory NerveStatistical analysis of neural dischargesHistograms:

Because neural discharges (action potential or "spikes") occur at discrete, punctuate instants in time, histograms are used to analyze and display the temporal discharge patterns. Histograms estimate the distribution of spike times along a temporal dimension which is divided into small time intervals or "bins". http://web.mit.edu/hst.723/www/Labs/LabANF.htm

The Auditory NerveRaster plot.

A single occurrence of an action potential (spike) is represented by a single dot as a function of time after stimulus onset. Representing firing ratesPost-stimulus histogram (PSTH).

A count of the average number of spikes in each bin (10 ms in this case) following stimulus presentation, and normalization to the number of presentations and the bin size, produces the normalized PSTH.

Normalization gives the firing rate (or probability per unit time of firing ) as a function of time. http://mulab.physiol.upenn.edu/analysis.html#Introduction

The Auditory NerveRaster plot.

A single occurrence of an action potential (spike) is represented by a single dot as a function of time after stimulus onset. Representing firing ratesPost-stimulus histogram (PSTH).

A count of the average number of spikes in each bin (10 ms in this case) following stimulus presentation, and normalization to the number of presentations and the bin size, produces the normalized PSTH.

Normalization gives the firing rate (or probability per unit time of firing ) as a function of time.

The Auditory NerveSpontaneous firing(Kiang et al., 1965)

The Auditory NerveTime CodingSpike train of spontaneous activity of a single nerve fiber and 5 tonal responses

(Kiang et al., 1965)

PSTH of the spike train. Notice the adaptation after ~20 msec.

Adaptation improves sensitivity to transients, can adjust sensitivity to preserve dynamic range of stimulus-response.Post-stimulus time histogram

The Auditory NerveTime Coding: Phase lockingIn response to low-frequency (< 5 kHz) pure tones, spike discharges tend to occur at a particular phase within the stimulus cycle.

However, spikes do not always occur on every cycle, i.e. there can be 2, 3, or more cycles between consecutive spikes.

Phase locking can be quantified using period histograms (PSTH), which display the distribution of spikes within a stimulus cycle.

With perfect phase locking, the period histogram would be an impulse.

Period histograms of AN fibers for low-frequency pure tones are nearly sinusoidal at near-threshold level, and become more peaky at moderate and high levels.See Evans, E. (1975). The cochlear nerve and cochlear nucleus. In D. N. WD Keidel (Ed.), Handbook of Sensory Physiology (pp. 1-109). Heidelberg: Springer.

The Auditory NerveTime Coding: Phase lockingAlternatively, phase locking can be visualized from interspike interval histograms (ISIH).

This analysis is appealing from the viewpoint of central auditoryprocessing because, unlike period histograms, it does not require an absolute time reference locked to each stimulus cycle.

The Auditory NerveTime Coding: Phase lockingAlternatively, phase locking can be visualized from interspike interval histograms (ISIH).

Here, phase locking shows up as modes at integer multiples of the stimulus period, i.e. at 1/f, 2/f, 3/f, etc for a pure tone of frequency f.

This analysis is appealing from the viewpoint of central auditoryprocessing because, unlike period histograms, it does not require an absolute time reference locked to each stimulus cycle.See Evans, E. (1975). The cochlear nerve and cochlear nucleus. In D. N. WD Keidel (Ed.), Handbook of Sensory Physiology (pp. 1-109). Heidelberg: Springer.

The Auditory NerveTime Coding: Phase lockingSynchrony coding:

For very low-frequency pure tones, period histograms can show severe deviations from a sinusoidal waveform, with sometimes two peaks per cycle (peak splitting).

Period histograms for higher frequency tones show less distortion. The synchronization index (vector strength) is a measure of the degree of phase locking varying for 0 for a flat period histogram (no phase locking) to 1 for a pulsatile histogram (perfect phase locking).

Synchronization index falls rapidly with frequency for pure tones above 1 kHz. Above 5-6 kHz, the synchronization index reaches the noise floor of the measurements.

There is no absolute upper frequency limit to phase locking. See Johnson, D. H. (1980). The relationship between spike rate and synchrony in responses of auditory-nerve fibers to single tones. J. Acoust. Soc. Am., 68(4), 1115-1122.

The Auditory NerveTime Coding: CFSee Evans, E. (1975). The cochlear nerve and cochlear nucleus. Handbook of Sensory Physiology (pp. 1-109). Heidelberg: Springer. Each vertical bar represents one spike (action potential) recorded from an AN fiber in response to a pure tone swept in frequency at different intensities.

Spike discharges occur in all conditions: There is spontaneous activity.

For low intensities, discharge rate increases above spontaneous only for a narrow range of frequencies.

The Auditory NerveTime Coding: CFAs intensity increases, so does the range of frequencies to which the fiber responds.

The outline of the response area (red) represents the pure tone tuning curve or frequency threshold curve.

The frequency for which threshold in dB is minimum is the characteristic frequency (CF). RECEPTIVE FIELDSee Evans, E. (1975). The cochlear nerve and cochlear nucleus. Handbook of Sensory Physiology (pp. 1-109). Heidelberg: Springer.

The Auditory NerveCF curve versus iso-intensity contours

The Auditory NerveTime Coding: CF, IHC are criticalOuter hair cell destruction decreases the sensitivity and broadens the tuningof auditory nerve fibers.

Tuning curves show the threshold for response in an auditory nerve fiber as a function of frequency

Kanamycin is a drug which can produce a specific lesion of outer hair cells, leaving inner hair cells normal.

Place Theory:Place of maximum vibration along basilar membrane.Tuning curves (or FTC=Frequency Threshold Curve)Tuning curves measured by finding the pure tone amplitude that produces a criterion response in an 8th nerve fiber.

Tuning curves for four different fibers (A-D) are shown.The Auditory NerveTime Coding

The Auditory NerveTime Coding: Summary Responses of individual AN fibers to differentfrequencies are related to their place along the cochlear partition (Basilar menbrane).

Frequency selectivity: Clearest when sounds are very faint.

Threshold Tuning Curve: Map plotting thresholds of a neuron or fiber in response to sine waves with varying frequencies at lowest intensity that will give rise to a response.

The Auditory NerveTime Coding versus Place CodingThe Place Theory stipulates that frequencies are encoded by activity across the tonotopic array of fibers in the AN, as well as in tonotopic nuclei along the auditory pathway within the brain.

The Timing Theory posits that temporal information conveyed through phase locking provides the dominant cue to frequency information. Upper limit is 5-6 kHz.

Neither the Place nor the Timing Theory can account for all psychophysical data. For example the human hearing range is from 200 up to 20,000 Hz.

The Auditory NerveTime Coding versus Place CodingPlace Code Theory:

The place code theory is given that name because it identifies each pitch with a particular place along the basilar membrane. It assumes that any excitation of that particular place gives rise to a specific pitch.

Shown is an illustration of how place code theory relates to what we have learned about the frequency tuning in the cochlea. For a low frequency tone (top row), the largest motion is at position 1 along the basilar membrane. Hence, there are action potentials in auditory nerve fibers connected to position 1. For a high frequency tone, the largest motion is at position 2 so there are action potentials in auditory nerve fibers connected to position 2.

Temporal (Time) Code Theory:

According to temporal code theory, the location of activity along the basilar membrane is irrelevant. Rather, pitch is coded by the firing rates of nerve cells in the auditory nerve. In principle, this makes a lot of sense. A low frequency tone causes slow waves of motion in the basilar membrane and that might give rise to low firing rates in the auditory nerve. A high frequency tone causes fast waves of motion in the basilar membrane and that might give rise to high firing rates.

Shown is an illustration of how temporal code theory relates to the cochlea. Both the low and high frequencies evoke responses at both positions, but there are more action potentials in response to the high frequency.

The Auditory NerveRate code: The Volley PrincipleBecause of refractory intervals, no individual fiber can fire with a period equal to that of the input signal. Individual fibers catch a cycle, miss one or more, catch another one, miss a few, etc. The period of the input signal is not preserved on any individual fiber, but it is reflected in the most common interspike interval of a population of fibers.

The Auditory NerveRate code: The Volley PrincipleVolley Principle: The volley principle reconciles the fact that the cochlear microphonic mimics the sound pressure waves with the implausibility of the temporal code. Wever suggested that while one neuron alone could not carry the temporal code for a 20,000 Hz tone, 20 neurons with staggered firing rates could. Each neuron would respond on average to every 20th cycle of the pure tone, and the pooled neural responses would jointly contain the information that a 20,000 Hz tone was being presented.

The Auditory NerveRate code: The Volley PrincipleThe volley theory is oversimplified since it requires neurons to always firing at the time when the signal amplitude reaches a peak. This is not the case since the entire hair cell-nerve fiber relationship is probabilistic rather than deterministic. The point of maximum amplitude is the time when the probability of a pulse is greatest (though not guaranteed). However, if the fiber is most likely to fire at the amplitude peak, the most common interspike interval (of a population of fibers) will equal the period of the input signal.

The Auditory NerveTime Coding: Phase lockingPhase Locking is an empirical observation that supports the volley principle.

When auditory nerve neurons fire action potentials, they tend to respond at times corresponding to a peak in the sound pressure waveform, i.e., when the basilar membrane moves up.

The result of this is that there are a bunch of neurons firing near the peak of each and every cycle of a pure tone. No individual neuron can respond to every cycle of a sound signal, so different neurons fire on successive cycles. Nonetheless, when they do respond they tend to fire together.

The Auditory NerveTime Coding versus AuditionSo, we have a Place or Tonotopic Code: Frequency is coded by the place along the BM where 8th N electrical activity is greatest (base=high freq; apex=low freq, etc.)We also have a Synchrony Code (with the Volley Principle tacked on to make it work even with the limits imposed by refractory intervals) based on the timing of 8th N pulses: Frequency is coded by the interspike interval of a population of fibers (short interspike interval=high freq; long interspike interval=low freq).

Is one of these theories right and the other one wrong? Probably not.

Commonly Held View

~15 to ~400 Hz:Mainly synchrony~400 to ~5000:Combination of place and synchrony above ~5000:Only place

The thresholds of auditory nerve fibers vary regularly with low threshold fibers on the pillar side of the inner hair cell and high threshold fibers on the modiolar side.

The low threshold fibers have high spontaneous activity rates, while the high threshold fibers have low spontaneous activity rates.Spontaneous Activity and ThresholdThe Auditory Nerve High spontaneous + low threshold

Low Spontaneous + high threshold

PillarModiolar

The distribution of spontaneous discharge rates (SR) is bimodal, separating AN fibers into two groups.

The high-SR group (SR > 18 spikes/s) forms 60% of the fiber population.

The remaining fibers are further subdivided into a low-SR group (SR < 0.5 sp/s, ~15%) and a medium-SR group (0.5 < SR < 18 sp/s, ~25%).

High-SR fibers form a large synaptic terminal on the pillar side of inner hair cells, while low- and medium-SR fibers form smaller terminals on the modiolar side.

Spontaneous discharge rate is inversely related to threshold at the CF, high-SR fibers being the most sensitive.

The auditory nerve can be thought of as a two-dimensional array of fibers organized by CF (cochlear place) and sensitivity or threshold (spontaneous rate).

Low-SR fibers are more sharply tuned than high-SR fibers, even though the same inner hair cell is innervated by fibers from both groups. This is consistent with the compressive nonlinearity in basilar-membrane motion.

Spontaneous Activity and ThresholdThe Auditory Nerve

The Auditory NerveIntensity InformationRate versus Level Functions

Example of a Low-SR fibers with a high threshold.Data is represented by curve (c) in the right plot.

Curves (a), (b) and (c) are typical of what is observed for neurons with High, Medium and Low SRs, respectively.

The Auditory NerveIntensity Information

The Auditory NerveIntensity InformationHigh intensityAction potentialsLow intensityLow thresholdneuronsHigh thresholdneurons

SummaryThe Auditory Nerve

A

September 6, 2007***Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen******Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen*Cochlear, as well as vestibular, sensory cells are called hair cells because they are characterised by having a cuticular plate with a tuft of stereocilia bathing in the surrounding endolymph. The cell body itself is localised in the perilymph compartment (see transverse section of the organ of Corti). Schematically, both types of cells, inner hair cells (IHCs) and outer hair cells (OHCs), differ by their shape and the pattern of their stereocilia.

****Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen***Lecturer: Rob van der Willigen**Lecturer: Rob van der Willigen************************************However, there's a problem with temporal code. The ear is sensitive to frequencies from about 20 Hz up to 20,000 Hz. But a single nerve cell can not signal at a rate of 20,000 Hz. Therefore, the possibility of a temporal code accounting for the detection of the pitch of a 20,000 Hz tone seems impossible because no nerve cells can conduct that many impulses per second. And, in fact, Hallowell Davis, in the 1930s, showed that the maximum response rate of auditory neurons in the cat is about 1000 action potentials per second.

Lecturer: Rob van der Willigen******

auditory perception

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