SLHS 302 Exam 2: Auditory Nerve (14 &15)

Auditory Nerve

Robust signal processing

Robustness

______ has proven difficult to restore after sensorineural hearing loss and replicate with machines for automatic speech recognition

Fundamental limitation to success of auditory prostheses and speech recognitions systems

lack of understanding of the neural code underlying basic auditory percepts

Cranial Nerve VIII

vestibulocochlear nerve

Cranial Nerve VIII

All sensory information from cochlea and vestibular organs

Cranial Nerve VIII

~30,000 auditory neurons

Cranial Nerve VIII

Projects cochlea to the brainstem through the internal auditory meatus

Cranial Nerve VIII

Comprised of all Type I and Type II afferent auditory nerve neurons (spiral ganglion neurons)

Auditory nerve fibers

peripheral axons of spiral ganglion neurons (connect IHCs and OHCs)

spiral ganglion neurons

House in modiolus

spiral ganglion neurons

Wrap together to make big bundle (auditory nerve)

Apical

low frequency in center

basal

high frequency on outside

Hair cell receptor potentials

Graded electrical potentials from graded influx of potassium (K+) ions through stereocilia

IHCs

store chemical neurotransmitter vesicles on ribbon proteins near the bottom of the cell

ANF action potentials

All or none. Spikes (rapid change in potential) of individual myelinated neurons

ANF action potentials

send spikes to cochlear nucleus

Single AN fibers

• Rate (# of spikes)

• Timing (temporal = neural phase-locking)

population responses

• Which fibers are active

• Summed activity

Action Potentials

rapid sequence of voltage changes across the cell membrane of a neuron

Absolute refractory period

after a spike is generated the neuron most recover; no additional spikes can be generated for a short period of time

Relative refractory period

additional spikes are possible but more difficult to generate

Types of firing patterns of auditory nerve

low, medium, high SRs

low SR

few if any (<0.5 sp/sec)

medium SR

>0.5 and <18 sp/sec

high SR

many per sec (up to 100)

Neural codes

Single AN fibers and population responses

primary-like auditory neuron responses

Prolonged stimulus (non-transient) → the number of spikes/sec decreases over time (depletion of neurotransmitter)

Post-stimulus time histogram (PSTH)

plot total number of spikes at each point in time for all presentations of stimulus; record number of action

Post-stimulus time histogram (PSTH)

Steady-state response: continues until stimulus changes

Post-stimulus time histogram (PSTH)

Highest number of spikes near onset

Post-stimulus time histogram (PSTH)

Adaptation: neurotransmitters begin running out

Post-stimulus time histogram (PSTH)

Rate-level function

plot of a neuron’s firing rate in response to a sound of constant frequency at increasing sound levels

rate-level function

Firing rate tells about intensity of sound

rate-level function

Rate saturation

point at which a neuron fires as rapidly as possible; rate will not change with further increase in sound level

Neural threshold

single neuron threshold to a particular stimulus

Neural threshold

Varies with/depends on spontaneous rate

High SR

low threshold

Low SR

high threshold

increase; increasing

Neurons _____their firing rate with _____ sound intensity above threshold

dynamic range

Each neuron has a limited

20-50 dB

limited dynamic range

100 dB

perceptual loudness range

dynamic range

Limits the ability of individual neurons to code sound level

Population of neurons

important for encoding intensity changes

population of neurons

Low, medium, and high SR neurons together encode a much wider range of intensities

Very intense sounds

• Wider range of basilar membrane to vibrate

• More neurons to fire

• More neurons to be saturated

highly sensitive (sound detection) and sharply tuned (discriminated)

OHCs enable cochlear to be

Threshold tuning curve

graph plotting thresholds of a neuron or fiber in response to pure tones with varying frequencies

y-axis

lowest intensity that will give rise to a response for a given frequency

best/characteristic frequency

frequency for which a neuron’s threshold is lowest

neural threshold

single neuron threshold to be a particular stimulus

tonotopic

neuronal representation of sound is

threshold tuning curve

labeled line coding scheme

“If a 2-kHz neuron is firing, there must be a 2-kHz sound”

Neural tuning curve bandwidths

bandwidth increase with BF

Neural tuning curve bandwidths

Determined by the logarithmic stiffness gradient along the basilar membrane

center frequency

Neuronal frequency selectivity varies as a function of

increase; increasing

Absolute bandwidths ____  with  ______ characteristic frequency

constant

Relative bandwidths stay roughly

absolute bandwidth

relative bandwidth

Isointensity Curves

response areas

isointensity curve

AN fiber firing rates to a wide range of frequencies all presented at the same intensity (dB SPL)

rate saturation

limits place code for frequency at high sound levels

hard to determine BF

when neuron fires a lot at high levels, it is

isointensity function

Phase locking

firing of a single neuron at one distinct point in the period (cycle) of a sound wave at a given frequency

phase locking

Ability of neuron to synchronize firing to a particular phase of the stimulus

phase locking

Up to 4-5 kHz; degrades above 5 Hz

peaks

Neuron will most likely fire at _____ of stimulus

phase locking

Can be seen in PSTHs

temporal coding of frequency

phase locking & volley principle

frequency

At a place of excitation, the frequency of BM vibrations are determined by the ______ of the stimulus

Volley Principle

combing spikes across multiple fibers fills in the temporal code

volley theory

Time between spikes tells the brain about sound frequency

place theory

the frequency of the sound can be determined by noting which nerve fiber (place) fires with the greatest relative discharge rate

Outer edge of the AN bundle

basal ANFs, high frequency

Inner portion of the AN bundle

apical ANFs, low frequency

temporal theory

the periodicity (timing) of AN discharges are used to determine the frequency of an input stimulus

temporal theory

For a 500 Hz tone, neurons tuned to 500 Hz will fire every 2 ms (period of a 500 Hz sinusoid)

proportional

AN fibers discharge at rates ______ to the period of the input stimulus

rate-place code

place of maximal discharge

temporal-place code

periodicity

Individual neuron

Encodes intensity changes by increasing firing rate as sound grows louder

individual neuron

Encodes frequency by place code and phase locking to the frequency of stimulus

Groups of neurons

Encodes much wider range of intensities with low, medium, and high SR neurons

group of neurons

Encodes frequency by pooling output from neurons in region of basilar membrane