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Auditory system Q1 Which properties characterize an auditory stimulus, and how is the sensitivity of the auditory system determined? Auditory stimulation 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. Maximum air compression Pressure high low Frequency low Frequency high Minimum air compression Wavelength, λ Sound spread velocity (v) Sound frequency, f (Hz, s-1) Sound velocity, v (m/s) f=v/λ In air: ca. 340 m/s Auditory stimulation The sound intensity is measured in decibels. The decibel scale is a logarithmic scale and refers to the sound pressure level (SPL). Sound pressure (P): Pascal (1 Pa = 1 N/m2) Sound pressure level (SPL) 1W = 1Nms-1 1J = 1 Nm SPL = 20 log Px/P0 Reference sound pressure (P0): 2x10-5 Pa Quiet countryside Low voice Street noise Aircraft dB 0 20 40 80 140 Factor X 1 10 100 10 000 10 000 000 Auditory stimulation The sound threshold depends on the sound frequency. The audiogram displays possible deficits in the sound perception at a given frequency. SPL (dB) 1000 Main range of human language 100 Absolute threshold (at about 4 kHz) 1 0 1 10 Frequency (kHz) Presbyakusis is the age-dependent decrease of hearing capacity. The deficits are larger at higher frequencies. Auditory stimulation The human auditory system is very much prone to damage by noisy environment. The human sound threshold after a 5 minute presentation of a strong wide band noise at 115 dB (industrial noise, thunderstorm) About 11% of the German population has hearing deficits. Tendency: rising. Cause: auditory trauma Frequency (Hz) Sound pressure level (dB SPL) Q2 How are auditory stimuli transduced in the inner ear? KS-97-22-3 Function of the inner ear 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 Tonotopy of the inner ear: Different frequencies produce their deflection maximum at different distance to the entry site of the inner ear. Travelling waves Oval window Round window Deflection The mechanical properties of the basilar membrane change along its length Therefore: at high frequencies - maximum close to helicotrema at low frequencies - maximum close to oval window KS-97-22-7 Function of the inner ear The transduction of auditory signals is carried out in Corti‘s organ Tectorial membrane Hair cells Outer Inner Supporting cells Basilar membrane Afferent nerve fibers KS-97-22-5 Function of the inner ear The stereovilli are connected by tip links. KS-97-22-6 Function of the inner ear Function of the inner ear When the tip-links are under stress, ion channels open and depolarize the receptor cells. Receptor activation Receptor deactivation Function of the inner ear 95% of the fibers in the acoustic nerve are sensory afferents collecting signals 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. Tectorial membrane Outer hair cells Inner hair cells Efferent fibers Afferent fibers(VIII) 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. Function of the inner ear Lesion of the outer hair cells results in a reduction of the oto-acoustic emissions Function of the inner ear The movements of the outer hair cells generate microphone potentials which can be recorded for diagnostic purposes. 10 ms Click Tone Q3 How is the direction of sound analyzed? Positional information of acoustic objects 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 high frequencies: The sound has a shade, i.e. the stimulus intensity is different at the left and the right ear. At low frequencies: A sound wave arrives at different time 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 Midline Thalamus (CGM) Colliculus inferior Hair cell Nucl. cochl. MNTB Nucl. lat. olivae superioris (LSO) left VIIIth nerve right Q4 How do auditory neurons encode the intensity and the frequency of sound? Auditory signalling in the CNS 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 KS-2001 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. Auditory signalling in the CNS But the frequency is faithfully reproduced by the neuron ensemble. Stimulus Single neuron Neuron ensemble Up to 4 KHz - good reproduction of the frequencies by this mechanism. Auditory signalling in the CNS KS-2001 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' Auditory signalling in the CNS With each new level of information processing, the tuning curves become sharper and sharper. dB Tuning curve of a neuron in the cochlear nucleus 80 60 Inhibitory surround 40 20 Excitatory response 2 7 4 KHz 10 15 Characteristic frequency 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 discharge pattern of the individual auditory neurons or neuron ensembles By the position of the active elements with regard to the entire tonotopic map 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. Q5 How is language represented in the cerebral cortex? Language processing The basic acoustic elements of the language are the phonems. Spoken sentences can be presented in a frequency spectrogram. Frequency (Hz) To cat----ch pink s----alm----on Language processing To perform quantitative tests of language perception and execution, it is convenient to work with reduced sound patterns („artificial language“) Language processing Language processing can also be tested in non-human primates Keyboard with lexical symbols for communication with signs Language processing 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 Cortical areas involved in language processing Parietal associative cortex Face representation in the motor cortex (MS1 ) A.39 atus u c r sa u l u ic Fasc Associative visual cortex A. 18,19 Primaryr visual cortex Broca‘s field Primary auditory cortex (A1) Associative auditory cortex (A2) A.41,42 Wernickefield A.17 Language processing The Wernicke-Geschwind model of language representation in the cerebral cortex Broca MS cortex Wernicke LGN Par. ass. cortex Ass. vis. cortex Vis cortex Task: 'Point to the butterfly and tell me its color' Language processing Humans communicating by the sign language display activity in the cortical language fields. „We arrived in Jerusalem and stayed there“ 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 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 and l remains separate Representation of r und l fuses 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