effectiveness of intervention for children with primary - school of

Auditory system

Which properties characterize an auditory stimulus,and how is the sensitivity of the auditory system determined?

Q1

The 3 most relevant parameters of an auditory stimulus are:

- Intensity (dB) - Frequency (Hz) - Position of the auditory object (space coordinates, distance)

Auditory stimulation

Sound waves are pressure waves. The human auditory system perceives sounds at frequences between 20 und 20.000 Hz.

Frequency high

Maximum air compression

Minimum air compression

high

low

Pre

ssur

e

Sound spread velocity (v)

Wavelength, λ

Sound frequency, f (Hz, s-1)

In air: ca. 340 m/s

Frequency low

Sound velocity, v (m/s)

f=v/λ


Sound pressure (P): Pascal (1 Pa = 1 N/m2)

Sound pressure level (SPL)

SPL = 20 log Px/P0

dB Factor X0 1

Quiet countryside 20 10Low voice 40 100Street noise 80 10 000Aircraft 140 10 000 000

The sound intensity is measured in decibels. The decibel scale is a logarithmic scale and refers to the sound pressure level (SPL).

1W = 1Nms-1

1J = 1 Nm


Reference sound pressure (P0): 2x10-5 Pa

The sound threshold depends on the sound frequency. The audiogram displays possible deficits in the sound perception at a given frequency.

Frequency (kHz)

SPL(dB)

0 10

Absolute threshold(at about 4 kHz)

Main range of human language

1

1

100

1000


Presbyakusis is the age-dependent decrease of hearing capacity.The deficits are larger at higher frequencies.

The human sound threshold after a 5 minute presentation of a strong wide band noise at 115 dB (industrial noise, thunderstorm)

The human auditory system is very much prone to damage by noisy environment.

About 11% of the German population has hearing deficits.Tendency: rising. Cause: auditory trauma


Frequency (Hz)

Sou

nd p

ress

ure

leve

l (dB

SP

L)

How are auditory stimuli transduced in the inner ear?Q2

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Function of the inner ear

The sound waves are typically composed of several frequencies. The individual sound waves have their maximum at different sites, and the amplitude of the deflection of the basilar membrane in the inner ear represents the relative strength (pressure) of that frequency component.

Oval window Helicotrema


Travelling waves

Deflection

Therefore:at high frequencies - maximum close to helicotremaat low frequencies - maximum close to oval window

The mechanical properties of the basilar membrane change along its length

Tonotopy of the inner ear: Different frequencies produce their deflection maximumat different distance to the entry site of the inner ear.

Oval window

Round window


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The transduction of auditory signals is carried out in Corti‘s organ

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Tectorial membrane

Outer

Inner

Supporting cells

Basilar membrane Afferent nerve fibers

Hair cells

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22-6The stereovilli are connected by tip links.


Receptor activation Receptor deactivation


When the tip-links are under stress, ion channels open and depolarize the receptor cells.

Both the outer and the inner hair cells are depolarized under the influence of basilar membrane deflection. The outer hair cells also contain contractile elements that, by elongation or contraction, augment the travelling waves.This creates acoustic waves.

Tectorial membrane

Outer hair cells

Inner hair cells

Afferent fibers(VIII)Efferent fibers

95% of the fibers in the acoustic nerve are sensory afferents collectingsignals from the inner hair cells. Each inner hair cell is contacted by about 10 axons (high degree of divergence). The outer hair cells are not only contacted by afferent fibers (high degree of convergence), but are also controlled by efferent fibers.


Lesion of the outer hair cells results in a reduction of theoto-acoustic emissions


The movements of the outer hair cells generate microphone potentials which can be recorded for diagnostic purposes.

Click Tone

10 ms


How is the direction of sound analyzed?Q3

The distance of a sound-emitting object can be judged by the intensity or time difference of the acoustic waves at the left and the right ear.

At low frequencies:A sound wave arrives at different timeat the left and the right ear

At high frequencies:The sound has a shade, i.e. the stimulus intensityis different at the left and the right ear.

Positional information of acoustic objects

The signals from the left and right ear are compared in the superior olive.

Auditory cortex

Thalamus(CGM)

Colliculus inferior

Midline

Hair cell

VIIIth nerve

Nucl. cochl.

Nucl. lat. olivae superioris (LSO)

left right

MNTB

Positional information of acoustic objects

Q4 How do auditory neurons encode the intensity and the frequency of sound?

The encoding of sound intensity is based on 2 mechanisms: - increase in AP frequency - increase in the number of active neurons

Stimulus

Neuron A

Neuron B

Auditory signalling in the CNS

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In a single fiber of the N. acusticus, each AP is strictly in phase with the sound wave, but a single fiber does not discharge with each sound wave.

Stimulus

Single neuron

Neuron ensemble

Up to 4 KHz - good reproduction of the frequencies by this mechanism.

But the frequency is faithfully reproduced by the neuron ensemble.


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Each neuron has a tuning curve. At the preferred frequency ('characteristic frequency'), the intensity threshold is the lowest.

Recording from individual axons of the acoustic nerve. After exposure to a cytoxic drug - loss of 'best frequency'

With each new level of information processing, the tuning curves become sharper and sharper.

Characteristicfrequency

Tuning curve of a neuronin the cochlear nucleus

Excitatory response

60

80

40

20

dB

2 4 7 10 15KHz

Inhibitorysurround


The receptive fields in the inferior colliculus and auditory cortex have an inhibitory surround. A decrease of inhibition (deprivation from a GABA-potentiating drug) can cause disturbance of sound perception. Also impaired is the localization of an auditory object in space.This may cause panic.


The frequency of an auditory stimulus is, in addition, encoded by the position of active neurons within the tonotopic map.

Thus, sound frequency is determined by two mechanisms:

By the position of the active elementswith regard to the entire tonotopic map

By the discharge pattern of the individual auditory neurons or neuron ensembles


The auditory maps in the superior colliculus and the primary auditory cortex are multidimensional.


The coordinate system of the auditory space is head-related.

In order to attribute auditory and visual signals to the same object at a given position in space, the auditory map is coupled to the retina-related visual map.

The superposition of the auditory, head-related map onto the visual, retina-related map is carried out in the superior colliculus and develops postnatally.


How is language represented in the cerebral cortex?Q5

The basic acoustic elements of the language are the phonems. Spoken sentences can be presented in a frequency spectrogram.

To cat----ch pink s----alm----on

Frequency(Hz)

Language processing

To perform quantitative tests of language perception and execution,it is convenient to work with reduced sound patterns („artificial language“)

Language processing

Keyboard with lexical symbols for communication with signs

Language processing

Language processing can also be tested in non-human primates

Are non-human primates able to use language for communication?

In higher primate species, stimulation with language symbols can elicit neuronal activity in areas equivalent to the human language areas (Wernicke‘s and Broca‘s fields), but organs for speech (articulation) are missing.

Language processing

PET analysis of cortical activity in connection with different language tasks

Language processing

Primaryauditorycortex (A1)

Associativeauditorycortex (A2)

Primaryr visual cortex

Associative visual cortex

Wernicke-field

Broca‘sfield

Face representation in themotor cortex (MS1 )

A. 18,19

A.17

A.41,42

A.39

Parietal associative cortex

Fasciculus arcuatus

Cortical areas involved in language processing

Language processing

The Wernicke-Geschwind model of language representation in the cerebral cortex

Task: 'Point to the butterfly and tell me its color'

LGN

Vis cortex

Par. ass. cortex

Ass. vis. cortex

Wernicke

Broca

MS cortex

Language processing

Humans communicating by the sign language display activity in the cortical language fields.

The American Sign Language (ASL) of the deaf is a language that largely follows the linguistic rules of spoken language. A lesions in the left temporal lobe produces sign language aphasia

„We arrived in Jerusalem and stayed there“

Language processing

A common basis of language in all humans?

Basic observation:Newborns recognize the phonems in the same way, regardless of where they are born

Topography of the phonems in the cortex

Newborns(up to 6 Mo)

Initially: separate representation of the phonems r and l Later on: grouping of phonem representation according to similarity

Grown up with German or English

Grown up with Japanese Representation of r und l fuses

Representation of r and lremains separate

Language processing

Different forms of aphasia:

Wernicke‘s aphasia: patient does not understand the task, says nothing but nonsense.

Broca‘s aphasia: patient understands the task (points to the right figure,understands that this is an insect, etc.), but cannot say „it‘s yellow“; difficulties in word finding; incorrect grammar; articulation preserved

Language processing

effectiveness of intervention for children with primary - school of

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