Audition

Question 1

Congenital vs. Conduction Deafness

Answer 1

• 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

Question 2

Inferior Colliculi

Answer 2

– main hearing parts of the brainstem; located just inferior to visual processing centers (superior colliculi)

Question 3

External Ear

Answer 3

– 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

Question 4

Middle Ear Structures

Answer 4

– 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

Question 5

Middle Ear Function

Answer 5

 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

Question 6

Sounds Entering Inner Ear

Answer 6

– 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

Question 7

Inner Ear: Scala Media

Answer 7

– 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

Question 8

Inner Ear: Cochlea, Spiral Ganglia, other

Answer 8

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

Question 9

Transduction of Sound

Answer 9

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

Question 10

Ascending Auditory Pathway

Answer 10

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

Question 11

Descending Auditory Pathway

Answer 11

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

Question 12

Inner and Outer Hair Cells + Afferent vs. Efferent Nerve Fibers

Answer 12

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

Question 13

Brainstem Nuclei

Answer 13

– 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

Question 14

Medial Superior Olivary Nucleus (MSO)

Answer 14

– 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

Question 15

Lateral Superior Olivary Nucleus

Answer 15

– 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

Question 16

Tonotopic Organization

Answer 16

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

Question 17

Tonal-Topical Organization: Medial Geniculus of Thalamus

Answer 17

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

Question 18

Lateral Belt Area and Lateral Sulcus

Answer 18

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

Question 19

Sound-Speech Extraction

Answer 19

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)

Question 20

Spectograms

Answer 20

– 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

Question 21

Sweeps and Speech Recognition

Answer 21

• 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

Question 22

Cochlear Implant

Answer 22

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

Question 23

Brainstem Implants

Answer 23

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

Question 24

Tinnitus and 3 Causes

Answer 24

– 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

Question 25

Peripheral Hearing Loss

Answer 25

 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

Question 26

Central Auditory Reorganization

Answer 26

– 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)

Question 27

Lack of Top-down Suppression

Answer 27

 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