Audition Flashcards
Congenital vs. Conduction Deafness
- Congenital Deafness – problem with inner ear (lack of hair cells in cochlea) at birth cochlear implant
- Conduction Deafness – in old people ossicles harden and unable to transfer/amplify sound as well
Inferior Colliculi
– main hearing parts of the brainstem; located just inferior to visual processing centers (superior colliculi)
External Ear
– spatial hearing
o Pinna (auricle) – allows vertical localization to take place as it collects sound
o External auditory canal – connects the auricle to the tympanic membrane (eardrum)
o Tympanic Membrane – vibrating air particles strike it and provide sound frequencies that the brain instantly analyzes
Middle Ear Structures
– air filled chamber internal to tympanic membrane & external to round window of cochlea
o Eustachian tube – connects middle ear to throat/nasopharynx
Equalizes the air pressure on both sides of eardrum
o Susceptible to ear infections (otis media)
o 3 bones (malleus, incus, anvil) convert sound waves striking eardrum into mechanical vibrations
Stapes fills the oval (vestibular) window – thin membrane leading to the inner ear
Middle Ear Function
Transforms incoming low-pressure high displacement vibrations into high-pressure low displacement vibrations in order to drive cochlear fluid
Impedance matching – transfer of acoustic energy from compression waves to fluid-membrane waves in cochlea
• Amplifies the small vibrations from low impedance large area of eardrum to smaller, high impedance oval window
• Bones move as the eardrum vibrates and the stapes hits the oval window of cochlea to amplify the sound and move the cochlear fluid
• Round window on other side of cochlea bulges in response to the added pressure
• Pressure is amplified in the cochlea because of the small area of oval window and force from the stapes
Sounds Entering Inner Ear
– temporal bone of the skull and contains the cochlea, semicircular canals, and vestibule
o Sounds entering cochlea are transferred to the basilar membrane
Different areas of the basilar membrane vibrate to different frequencies
Towards apex (semicircular canals) – responds to low frequencies
Towards the origin (windows) – responds to high frequencies
Fourier transform – how the cochlea separates complex waveforms into a single, simple frequencies
Inner Ear: Scala Media
– between the scala vestibule and scala tympani
Vestibular membrane divides it from scala vestibule
Basilar membrane divides it from scala tympani
Organ of Corti – in the scala media; comprised of hair cells which detect fluid movement
• Surrounded by endolymph – higher concentration of K+ than perilymph – essential for recycling of ions for the continuation of the propagation of action potential to the brain
Three rows of outer hair cells and one row of inner hair cells
• Hair cells are in gelatinous tectorial membrane of the Organ of Corti
o Outer hair cell cilia are in the membrane but cilia of inner hair cells are not
• Only inner hair cells are responsible for hearing
• Outer hair cells respond to efferent signals – that stiffen or weaken them
Inner Ear: Cochlea, Spiral Ganglia, other
o Cochlea has 2.5 windings & 3 compartments (scala vestibule, scala media, scala tympani)
Perilymph is called scala vestibule on the side of the semicircular canal
Perilymph is called scala tympani on the tympanic membrane
o Spiral ganglia – collection of nerves that form the auditory branch of CN 8
Located between the coils of the cochlea and run through basilar membrane
o Sound stimuli cause deflection which moves the basilar membrane up and down which pushes and pulls on the tectorial membrane causing the hairs to bend resulting in signal transduction
o Endolymph movement increases with the loudness of the sound stimuli
Transduction of Sound
o Vibrations go through middle ear and pass through the oval window and enter fluid in cochlea
o Small vibration travel from the base (oval window) to the apex & back to base (round window)
o Larger vibrations do not travel as far
o Lower frequency waves reach the apex of cochlea; higher frequency waves reach the base
Frequency is measured by where the wave stops in the inner ear
o Receptors on basement membrane react to different frequencies
o Inner ear breaks down complex sounds to individual frequencies & translate them into base code
Each nerve fiber of inner ear codes a particular frequency
Ascending Auditory Pathway
o Ascending Pathways: cochlear nerve synapse in (ventral/dorsal) cochlear nuclei superior olivary nuclei lateral lemniscus (external) inferior colliculus (in brainstem) (ventral/dorsal) MEDIAL geniculate nucleus of thalamus primary auditory cortex (temporal lobe)
Cochlear Nuclei – where tonotopic and spatial information are divided
• Ventral cochlea – for frequency recognition; main one
• Dorsal cochlea – for spatial recognition
Tonotopic Information: organization of frequency is maintained the entire way
Superior olivary complex - sound localization; receives input from contralateral ear
Inferior colliculus – evolutionarily old integrative center for hearing
Descending Auditory Pathway
o Descending Pathways: auditory cortex medial geniculate nucleus inferior colliculus periolivary nuclei cochlear nuclei outer hair cells in cochlea
Decreases the sensitivity of the basilar membrane if sounds are too loud and may help distinguish sounds when background noise is present
Important with adaptation, learning, memory
Inner and Outer Hair Cells + Afferent vs. Efferent Nerve Fibers
o Sensitivity can be adjusted due to afferent and efferent nerve fibers
Auditory nerve fibers – transmit afferent signals from inner hair cells
Efferent nerve fibers to inner ear that regulate sensitivity by controlling the impedance of outer ear cells
o Outer hair cells – amplify quiet sound more than loud sound; may generate an echo (otoacoustic emission)
o Inner hair cells – transmit sound to nerve signal; does not fire an action potential
Receptor membrane potential is created from influx of positive ions (K+ or Ca+) from the endolymph of scala media opens voltage gated Ca+ channels Ca+ entering cell triggers the release of neurotransmitters that act on spiral ganglia cochlear nerv
Brainstem Nuclei
– lateral superior Olivary (LSO), medial superior olivary (MSO), and medial nucleus of trapezoid body (MNTB)
o Where SPATIAL hearing begins to occur with input from both ears
o Utilizes 3 things to identify where sound is coming from – time, intensity, & phase
Time – 7 microsecond delay
Intensity – sound is stronger in the ear on same side as sound
Phase – each ear receives sound at different pressure points because sound travels in phasic waves
Medial Superior Olivary Nucleus (MSO)
– used for interaural TIME differences (ITD)
Receives sound from both sides
Acts as co-incidence circuit so that you hear one sound at same time
• Inhibitory signals will be stronger on the shorter pathway (ear closest to sound) so that sound from contralateral ear will reach MSO at the same time
Left ear leading neuron is neuron closest to the right ear so that when a sound on left side is being received, it has a shorter path to the neuron on right side so that they reach the neuron at same time
Lateral Superior Olivary Nucleus
– used for interaural LEVEL differences (ILD)
Incoming sound excites LSO neurons on ipsilateral side and activates inhibitory neurons on contralateral LSO via Medial Nucleus of Trapezoid Body (MNTB) interneuron
• If excitation from ipsilateral side is greater than inhibition signal from contralateral side then the signal from ipsilateral side will continue
o Ratio of inhibition to excitation tells us where the sound is coming from
o Yelling in someones R ear results in R. LSO neurons excited and L. LSO inhibited; L. LSO also minimally excited and minimally inhibit R. LSO
Tonotopic Organization
o Variety of cell types in cochlea nucleus that react to different frequencies and intensities
Frequency dictates which neurons fire
o Frequency tuning – allows for fine-tuned hearing
o Basilar membrane in cochlea is tonotopicaly organized – different frequencies affect specific regions along the length of the membrane
Unique response of basilar membrane to unique frequencies is maintained throughout signaling pathway
Varying hair cilia length along the length of the basilar membrane make this possible
High frequency sounds are detected near the base (oval window) of the cochlea
• Recognized by posterior part of the cochlear nucleus before being sent to the ventral superior olivary nucleus
Low frequency sounds are detected near the apex (helicotrema) of the cochlea
• Recognized by the anterior part of the cochlear nucleus before being sent to the dorsal superior olivary nucleus
Tonal-Topical Organization: Medial Geniculus of Thalamus
o Secondary auditory complex composed of multiple areas around the primary complex and areas deep into lateral sulcus (Wernicke’s area classically considered responsible for decoding speech)
o 20-25 auditory areas of the brain, each with preserved tonal-topical organization
Primary auditory cortex is anterior to Wernicke’s area (secondary auditory cortex)
o Medial Geniculus of Thalamus – sends projections to auditory cortex
Ventral division projects to the main auditory areas
Dorsal division projects to caudal belt areas
Lateral Belt Area and Lateral Sulcus
Lateral Belt Area
Antero-lateral (AL) belt projects to ventral locations in the frontal cortex
• Responds to communication calls
Caudal-lateral (CL) belt projects to dorsal locations in the frontal cortex
• Responds to sound localization, spatial positioning
Lateral Sulcus – separated frontal/parietal lobe from temporal lobe
Parietal Lobe (dorsal to auditory area) – involved in spatial sound localization (“where”) with projections to the dorsal lateral prefrontal cortex (site of working memory for location )
Temporal Lobe (ventral to auditory area) – involved in objected recognition (“what”) with projections to ventral lateral prefrontal cortex
o Projections in dorsal and ventral streams merge within frontal cortex and send signals back to auditory cortex
Sound-Speech Extraction
o Auditory cortex responsible for sound decoding but another area responsible for producing sound
o Sound decoding in primary and secondary auditory cortexes (Wernicke’s area )
Arcuate fasciculus sends projections to Broca’s area
o Sound production occurs in Broca’s area (more anterior; frontal lobe)
Sends projections to motor cortex for vocal apparatus movements (vocal cords, tongue)
Spectograms
– descriptions of complex sounds that represent vocal cord vibrations as sound frequency variations over time, with strong harmonic tones occurring at vowels
o Formants – characteristic frequency changes
2 formants form resonances in the vocal apparatus
o Mechanical properties of sound production are encoded in spectrograms
Sweeps and Speech Recognition
• Sweeps – exemplify frequency changes with variation in speed, direction, etc.
• Speech recognition is fairly selective with different areas of the brain activated with different syllable stimuli
o Syllables stimulate anterior regions of the superior temporal cortex (ventral “what” pathway)
o Vowel stimuli shows distinct areas of activation compared to noise, with activity in the anterior superior temporal cortex into the inferior frontal cortex
No observed activation in response to language in Wernicke’s area
New word recognition site exists anterior to the classical location of Wernicke’s area
Cochlear Implant
o Ccochlea/basilar membrane undergo frequency decoding & encoding via fourier analysis
o Deafness – result of non-functioning hair cells; nerve fibers remain functional
Implant that allows the comprehension of speech requires 20-30 electrode contacts along a single integrated wire that is placed inside the scala tympani of cochlea
o Old Patients – regain hearing easily via cochlear implant that bypasses non-functional hair cells with direct stimulation of nerve fibers
o Children – must have cochlear implant placed as an infant (max age ~2) to correct deafness
Place too late – language system develops without auditory input and patient loses ability to respond to auditory stimuli; unused auditory cortex is taken over by other senses
o External microphone connected to a sound processor that processes the signal
Greater hearing deficit the more space between microphone and processor
Brainstem Implants
o Loss of hearing due to loss of auditory nerve fibers cochlear implant can’t be used
o Place penetrating or surface stimulatory electrode into the cochlear nuclei that stimulate with various frequencies
Tinnitus and 3 Causes
– ringing sensation of various presentations that effects millions of people
o Associated with depression and suicide in severe cases
3 Causes (all MUST be present)
o Peripheral hearing loss
o CENTRAL auditory reorganization
o Lack of top-down (limbic) suppression
Peripheral Hearing Loss
Loss of hair cells due to loud noise exposure (120+ db)
30% of hearing loss patients experience tinnitus
• Advanced high frequency hearing loss starting at lower frequencies or notch-like hearing loss
Cutting of auditory nerve
• Cochlear lesion resulting in no input of auditory cortex allows neighboring frequencies to expand into the vacant cortical region, resulting in overrepresentation of the border edge frequency
Central Auditory Reorganization
– tinnitus continues even after cutting auditory nerve
Shows auditory complex hyperactivity
Originates in the brain, resulting from reorganization of the auditory cortex (lesion induced plasticity)
Lack of Top-down Suppression
Frontal cortex reduces noise in a top-down manner which is damaged
• Associated with dissonant/unpleasant sounds with projections into limbic system that controls emotional reactions/processing
• This non-auditory system receives auditory signals and is able to compensate for any tinnitus present
• People with persistent tinnitus are unable to suppress the signal
