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
Auditory stimulation
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
Auditory stimulation
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
Auditory stimulation
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
Auditory stimulation
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
Function of the inner ear
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
Function of the inner ear
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Function of the inner ear
The transduction of auditory signals is carried out in Corti‘s organ
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Function of the inner ear
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.
Function of the inner ear
Receptor activation Receptor deactivation
Function of the inner ear
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.
Function of the inner ear
Lesion of the outer hair cells results in a reduction of theoto-acoustic emissions
Function of the inner ear
The movements of the outer hair cells generate microphone potentials which can be recorded for diagnostic purposes.
Click Tone
10 ms
Function of the inner ear
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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Auditory signalling in the CNS
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.
Auditory signalling in the CNS
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Auditory signalling in the CNS
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
Auditory signalling in the CNS
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.
Auditory signalling in the CNS
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
Auditory signalling in the CNS
The auditory maps in the superior colliculus and the primary auditory cortex are multidimensional.
Auditory signalling in the CNS
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.
Auditory signalling in the CNS
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