Ear to Thalamus + Auditory cortex and Vestibular System

Sound

rapid pressure fluctuations in medium such as air

sound pressure level measures...

magnitude/amplitude of pressure fluctuations (loudness)

pressure fluctuation close to threshold of hearing

0 dB SPL

human hearing range

20-20,000 Hz

human speech range

200-2,000 Hz

speed of sound

340 m/s (760mph, in dry air at sea level)

three main cues to localize sound

interaural time difference (ITD), interaural level difference (ILD), and head-related transfer function (HRTF)

biaural cues

ITD and ILD, based on comparisons of sounds reaching left v.s. right ear, horizontal direction

spectral cue

HRTF, body scatters sound which influences sound frequency

Interaural Time Difference (ITD)

difference in time taken for a sound to each each ear, quicker for ear closer to source, no difference when source is directly in front or behind of person

Interaural Level Difference (ILD)

difference in sound pressure level at each ear, head can absorb and reduce energy of incoming sound, reduction in sound level at ear further away, more useful at higher frequencies

Head-Related Transfer Function (HRTF)

the pinna, head, and torso influence the sound before it reaches the inner ear, HRTF specifies how body influences sound and provides vertical location cue based in changes in frequency spectrum

ITD is used for ____ and ILD is used for ____

ITD mainly used for low frequencies and ILD mainly used for higher frequencies

middle ear

sound moves tympanic membrane, ossicles move with tympanic membrane which ossicles then moves oval window and also act as amplifier (converting air movement into cochlear fluid movement)

inner ear

cochlear fluids (perilympth in scala tympani and scala vestibuli, endolymph in scala media), basilar membrane (moves up and down with sound), inner hair cells (transmit information to brain), outer hair cells (amplify movement of basilar membrane)

cochlea

acts as frequency analyzer: sound causes traveling wave in cochlea, resulting from pressure differences between fluid-filled compartments, basilar membrane moves at frequency of stimulation, size of traveling wave varies due to membrane stiffness changes, where membrane moves most depends on sound frequency)

basilar membrane

different parts of basilar membrane respond maximally to different frequencies

hair cells

human cochlea contains one row of inner hair cells and three rows of outer hair cells, hairs bundle on surface of hair cells to form stereocillia, height of these systematically vary)

tallest hair

kinocilium

what interconnects successive sterocilia

protein filaments called "tip links"

Sound and stereocilia

sound leads to deflections of stereocilia, hair cells depolarize when deflection is towards kinocilia and hyperpolarize when away

K+ influx causes...

hair cells to depolarize, happens when stereocilia deflect towards kinocilium, which leads to opening of voltage-gated Ca2+ channels, Ca2+ influx, and transmitter release

tectorial membrane

attached to tips of tallest stereocilia of outer hair cells, movement deflects stereocilia of outer hair cells

deflection of inner hair cells

stereocilia of inner hair cells are deflected by motion of fluid beneath tectorial membrane

tip links are associated with...

ion channels which open and close depend on stereocilia deflection

auditory nerve

cochlea sends info via auditory nerve (hair cells connected to spiral ganglion cells, axons form auditory nerve)

spiral ganglion

contains cell bodies of sensory neurons, each spiral ganglion cell has best frequency also called "characteristic frequency", best frequency changes systematically across nerve

auditory nerve is _____ organized

auditory nerve is tonotopically organized, which carries information to cochlear nucleus

auditory nerve responses at low frequencies

neurons fire action potentials at a particular phase of the sound wave, called "phase locking" which provides frequency information

auditory nerve responses at high frequencies

phase-locking doesn't occur, frequency information must be derived from tonotopic arrangement of auditory nerve fibers

auditory pathway to cerebral cortex

spiral ganglion --> ventral cochlear nucleus --> superior olive - lateral lemniscus --> inferior colliculus --> medial geniculate nucleus --> auditory cortex

Binaural neurons in superior olive are...

sensitive to sound location (via binaural cues, ITD encoded in medial superior olive and ILD encoded in lateral superior olive) different neurons respond best to different ITDs and ILDs

Auditory Cortex Anatomy

auditory cortex occupies dorsal and lateral superior temporal gyrus, can be divided into core, belt, and parabelt regions

Core auditory cortex region

Brodmann area 41 amd primary auditory cortex

belt auditory cortex region

surrounds core region, mostly anterolaterally

parabelt auditory cortex region

posterolateral superior temporal gyrus

commonly thought that core, belt, and parabelt are....

hierarchically organized, core projects to belt which projects to parabelt

auditory cortex is...

tonotopically organized, core contains three tonotopic maps (primary auditory cortex (A1), rostral field (R), and rostrotemporal field (RT))

belt contains...

several areas, at least some are tonotopically organized

parabelt representation of frequency is...

not well characterized/less clear

subset or neurons in auditory cortex are...

sensitive to harmonic sounds (musical sounds with harmonics), more specifically they respond best to harmonic complex tones (HCTs)

core responds best to...

pure tones, sounds of one single frequency

belt responds best to...

band-passed noise, sounds of intermediate complexity between pure tones and vocalizations

parabelt responds best to...

complex sounds, species-specific vocalization

pathway from core auditory cortex, through belt regions, to inferior frontal cortex

auditory ventral "what" pathway, processes auditory objects

pathway from auditory cortex, to parietal cortex, to frontal cortex

auditory dorsal "where/how" pathway, processes sound location

sounds are assigned categories in...

the ventral "what" pathway

superior temporal gyrus (STG) is important for....

processing phonemes

STG has ____ organization

the STG has anterior-posterior organization for slow vs fast speech, posterior represents fast speech (on phonemic time scale) and anterior represents slow varying speech sounds (on syllabic or prosodic time scales)

Vestibular labyrinth

structure of inner ear that includes semicircular canals and otolith organs

vestibular labyrinth use

uses hair cells like cochlea, is important for balance, equilibrium, posture, and eye movements, semicircular canals sensitive to head rotation and otolith organs are sensitive to gravity and tilt

Otolith organs

detect linear acceleration and change in head angle

macula

sensory apparatus in otolith organs, it's hair cells respond to tilt (depolarize when cilia bends towards kinocilium and vice versa)

otolith

calcium carbonate crystals, when head and macula are tilted gravity pulls otolith crystals which deforms gelatinous cap and bends cilia

macula arrangement

one macula in both saccule and utricle, macula oriented vertically in saccule and horizontally in utricle (when head is upright)

direction preferences of ear hair cells

preferences vary systematically, hair cells in macula cover a range of directions

how macula detects direction of linear movement

saccule and utricle on each side of head are mirror images of each other, when cells on one side of head depolarize, cells on other side at corresponding site hyperpolarize

Semicircular canals

detects angular acceleration (nodding, shaking head)

ampulla

where hair cells are contained in the semicircular canals, ampulla is a bulge along the canal

cilia on ampulla

cilia protrude into gelatinous cupula, they hyper/de-polarize together because kinocilia of hair cells are similarly oriented

bending of cilia

they bend when canal is rotated, endolymph in canal lags behind due to inertia, which exters force on cupula causing it and the cilia to bend, which polarizes them

push-pull mechanism for semicircular canals

three canals on each side of head combine together for sensitivity for all rotation angles, horizontal canal, anterior vertical canal, and posterior vertical canal, each are functionally paired with canal on opposite side of head

left and right horizontal canals are sensitive to...

rotation in horizontal plane

left anterior and right posterior canals are sensitive to...

rotation in vertical plane that are ~45 degrees anteriorly to the left

right anterior and left posterior canals are sensitive to...

rotation in vertical plane that are ~45 degrees anteriorly to the right

head rotation...

has opposing effects on canals in functional pair, depolarizes hair cells in one canal while hyperpolarizing cells in opposite canal, i.e. the "push-pull" mechanism, this optimizes detection of rotation

Vestibular nerve

hair cells connected to cells forming the vestibular nerve, cell bodies of vestibular nerve cells are located in Scarpa's ganglion

depolarized hair cells release...

glutamate onto vestibular nerve cells, which increases the excitability of the vestibular nerve cells

when head starts to rotate...

vestibular nerve activity changes, on one side it increases and decreases on the other side

long lasting head rotation leads to...

adaption, after about 12-30 seconds the endolymph and canal move together (no more delay) and cupula straightens, vestibular nerve activity adapts and returns to initial level

when head rotation stops after adaption...

cupula bends in other direction, causing opposite response from hair cells, vestibular nerve activity changes which causes temporary sensation of counter-rotation and dizziness

vestibular pathways

vestibular information combines with other sensory information in the cerebral cortex to plan and execute actions/movements

parieto-insular vestibular cortex (PIVC)

contributes to heading (self-motion) perception and receives multi-sensory input, neurons in PIVC respond to 3D rotation and translation