MT 1 Hearing/Brain

Sensory coding

how the auditory system encodes the stimulus

Both vision and touch are

topographical systems

frequency (hertz or cycles/sec)

sound waves = hertz; 1 kHz = 1000/s or 1 ms interval between peaks of sound waves; ½ main characteristics of sound; physical characteristic - pitch

tuning fork

musicians use it to get a pure tone, if you have one for a certain sounds then when you vibrate it back and forth it compresses/expands air molecules to create a sound wave (concentric waves of sound)

sound in a pure tone

will be a sine wave

human hearing range

20 - 20,000 Hz

speech is a couple thousand hertz

Ultrasonic sound

sounds above human range (higher frequencies)

infrasonic sound

sounds below human range (lower frequencies)

intensity (dB)

½ main characteristics of sound; perception: loudness

decibel scale (dB)

0 dB SPL is not “no sound” just the average threshold for human hearing; decibel scale comes from pressure; uses convenient numbers because human ears are really sensitive to sound (stimulus intensities); 140 dB is close to the sound intensity that will damage your hearing; covers 10 million fold range we can hear in terms of amplitude

loudest sound ever?

310 dB; 08/1887: pressure wave from fourth explosion out of Krakatoa

diffraction

process of sound bending around objects; whether or not it bends depends on frequency and the size of the object; bends more at low frequencies than higher ones

ex. human head is roughly one foot across so sounds below 2000 Hz can wrap around the head (creates a sound shadow - allows localization cues and allows sound to get to both ears); sounds above 2000 can’t wrap around the head

localization cues

interaural intensity (or level) difference (IID or ILD) and interaural timing difference (ITD)

Interaural intensity (or level) difference (IID or ILD)

especially for higher frequency, > 2k Hz; intensity difference between two ears

interaural timing difference (ITD)

sound velocity, ~ 1100 ft/s; sound heard from ear close to it than the one that it farther away

ex. if sound to the side, gets to ear 1ms before it gets to the other ear so 10s of microseconds of difference for ITD

ex. if sound in front of you, gets to both ears at the same time - 0 for both IID and ITD

“real” sounds are complex

they are not simple sine waves; while tuning fork produces pure sine wave (artificially produced) nothing in nature is pure; other sound sources produce something similar but not a pure sine wave; they will sound different even if playing same note; more complex sound waves

timbre

third sound characteristic; anything that is not frequency or intensity; produced by each individual sound source

spectral (fourier) analysis

any complex sound that is periodic has a pitch/repeats and can be decomposed into a bunch of pure sine waves; it is periodic because the major peaks come at same intervals; add all the pure sine waves together to make a complex sound, as long as periodic; power spectrum shows where the sound is contained

outer ear

shapes input with the pinna; folds in ears funnel sound over a large area and into holes in head to shape sound going in; sound location based on time difference and elevation

middle ear

needed for impedance matching; three main bones malleus, incus, and stapes (contacts the cochlea); used to vibrate the membrane on the outside of the cochlea to activate the hearing transducers; better to have the cochlea in the ear because it is filled with fluid and if it were outside then a lot of energy would be lost and we wouldn’t hear well; eardrum is larger than the oval window and connected using the three bones - eardrum vibrates the bones which allows the stapes to move the cochlea with much larger vibration than if it weren’t there

inner ear

promotes sound transduction; in cochlea

cochlea

fluid filled, contains Organ of Corti; basilar membrane vibrates with sound; oval window is pressed against stapes which moves the oval window back and forth; scala vestibuli attached to oval window so when vibrates it creates pressure wave through vestibuli goes through cochlea through the circles and comes back through the scala tympani; both scala are fluid filled

sound moves from stapes to oval window to scala vestibuli to cochlea which contains the organ of corti and basilar membrane

organ of corti

what we hear with; organization level goes scala vestibuli then organ of corti then basilar membrane then scala tympani; pressure waves cause the basilar membrane to move up and down with sound wave coming into ear; causes hair cells (stereo cilia) to move back and forth which causes sound waves to be turned into neural impulses

characteristic frequency (CF)

optimal frequency (lowest threshold) for a particular point on the basilar membrane; CF doubles or halves over a fixed distance of one octave

basilar membrane mechanics (basis of topographic organ or “tonotopy”)

when you have different frequencies coming into the cochlea it sets up pressure to cause it to move up and down, structure of basilar membrane varies from end to end so a different part will vibrate maximally for a certain amount of input; pressure will go to entire basilar membrane but only one part will respond maximally; optimal frequency is lowest threshold for activating a portion of the basilar membrane; basilar membrane is doing a spectral analysis of sound - sound decomposed into component frequencies which can vibrate at different frequencies at same time

Base of basilar membrane

closer to the oval window (where sound comes in), which is narrow and stiff; stiffer end will vibrate maximally and with lower frequencies

apex of basilar membrane

closer to coiled cochlea structure (same basilar membrane), wide and floppy; floppy end will vibrate maximally and with higher frequencies

von Bekesy experiment

Fourier analysis done by basilar membrane; Extracted cochlea form dead people and brought into lab - devices at each end and measured how it vibrated in response to that based on higher or lower frequencies; Different parts of the basilar membrane vibrate maximally in response to different frequencies; explained spectral decomposition; Frequency resolution that he saw doesn’t explain spectral resolution that our perception has; He needed very high intensities to activate the basilar membranes in the dead cochlea; He proposed that there must be something in live cochlea that amplify the sound - cochlear amplifier

origin of tonotopy (“place code”)

auditory system doesn’t have topographic representation like vision and touch do; has topographical representation of frequencies not location - thus referred to as tonotopy for frequencies or the origin of the place code; system knows what frequency you’re listening to by place on the basilar membrane that is optimally vibrated; Auditory system know what frequency you're listening by the place on the basilar membrane that is optimally vibrated

organ of corti structure

bottom is the basilar membrane and above are the inner hair cells and then three rows of outer hair cells; hari cells surrounded by supportive cells; on top is the tectorial membrane

inner hair cells

focus on sensory transduction which turns acoustic information into neural impulses; move freely in fluid and with shear force they move back in forth in fluid

outer hair cells

cochlear amplifiers (springy); attached to tectorial membrane so they move it up but also back and forth freely because of the shear force

pressure waves push on organ of corti and cause the basilar membrane to pivot up and down

pivot points in two different positions - the tectorial membrane and basilar membrane; pivot points on offset so if both pivot at same time they are sliding across each other to create a shear force; pressure waves move in synchrony with basilar membrane which causes stereocilia on hair cells to move with it as well

hair cells hyperpolarization

back tip links relax and channels close

hair cells depolarization

when there are pressure waves shear force moves the stereocilia in one direction; tops of stereocilia are connected by tip links with one end of them attached to an ion channel at the tip of the adjacent stereocilia; bent over so spring tightens and pulls open the ion channel; permeable to potassium so when open potassium flows into cell and causes depolarization; depolarization producers opening of calcium channels with it going into the cell; vesicles with transmitters fuse with hair cells and excrete transmitter to create an axon potential

what happens when during depolarization of hair cell when ion channel opens to let potassium into cell

hair cells in bottom part act like neurons; top part does not so the scala media high potassium concentration so when potassium channels open potassium flows into cell

tip links

tip links are fast within microseconds and sensitive so less than a nanometer of movement will cause depolarization; ion channels respond to their mechanical movement in a microsecond time frame; very hard to find

inner hair cell receptor potential

stick micro electrode into hair cell then put probe against stereocilia; moving back in forth in 200 nm increments; membrane potential varying at same rate that stereocilia are moved by probe; no delay in stereocilia and membrane potential of hair cell

outer hair cells are the cochlear amplifier

sound induced vibration; has stereocilia but attached to tectorial membrane above; also produce membrane potential in outer hair cells which acts as cochlear amplifier

outer hair cells have unique membrane protein

prestin only in outer hair cells; plasma membrane of outer hair cells made up of 85% prestin which is source of the cochlear amplifier;

outer hair cells and electromotility

low potassium - hyperpolarization; high potassium - depolarization which makes the outer hair cells shorter, fatter, and stiffer; when depolarized the length decreases by a couple of microns because the length is sensitive to voltage; so much prestin that the entire out hair cell becomes shorter and stiffer when depolarized

if prestin were knocked out

experiment by Liberman shows that when prestin is knocked out there is no change in length and this prevent hair cells from moving - doesn’t make then deaf but causes much worse hearing because now there isn’t a cochlear amplifier

inner hair cells vs outer hair cells

inner hair cells = sensory transduction; tip links connected to ion channels to make them fast and sensitive; mechanically open ion channels; pressure waves in fluid of cochlea create neural impulses

outer hair cells = cochlear amplifier; if basilar membrane moves up the outer hair cells depolarize causing them to get shorter and stiffer; electromotlility allows the basilar membrane to move further and faster; outer hair cells stiffer allows them to bounce off tectorial membrane faster than if the cochlear amplifier wasn’t there; presence of cochlea amplifier allows basilar membrane to move farther than it would and reverse direction faster than it would otherwise; “springiness” can produce distortion making otoacoustic emissions

springiness describes what

the outer hair cells because it can produce distortions in the sound that its responding to and causes basilar membrane and organ of corti to move up and down and the cochlear amplifier adds to this movement

two tests to tell us about cochlea function

otoacoustic emissions (OAE) (DPOAE: distortion product OAE)- tests whether the cochlear amplifier works - outer hair cells - measure cochlear function

auditory brainstem response (ABR) - tests the downstream neural brain response - if inner hair cells work properly - transduction to the 8th nerve and the action potentials have to go through - tells us about the brain and everything upstream of the auditory nerve; tell you about your hearing threshold

otoacoustic emissions (OAE) (DPOAE)

to measure use DPOE so put in two sounds close in frequency and because of the mechanics of the basilar membrane this produces a distortion product but not the sound that you put in just something close to it; two speakers are placed next to the ears, one with one freq and one with another, under a microphone measures what comes out which is something of a different frequency than either of the two - the distortion that is not present; tells you whether the cochlear amplifier is working ie the outer hair cells

auditory brainstem response (ABR)

measured by putting an electrode at top of skull and measuring in microvolt range; deliver sound hundred of thousands of times before you can discern measures; measures much smaller than EFGs could be pure tone, white noise, lots of things; measures at different intensities of decibel sound pressure level

at low pressure: no response to sound so auditory system is not responding; at high pressure: they start to measure some responses; threshold will measure yours at different frequencies; each peak corresponds to neural input

synaptic ribbon

presynaptic part of the inner hair cell where transmitter is being released

ribbon counts in normal ears provide an accurate metric of the inner hair cells afferent innervation

nerves retract from the inner hair cells; spiral ganglia have auditory nerves in contact with inner hair cells and other end is in contact with the brain; one year after exposure there is a dramatic loss of cells; after noise exposure synaptic ribbons pulling away from inner hair cells and auditory nerves are pulling away

transient problem in otoacoustic emissions

high frequency regions have permanent loss of function in neural regions; recovery of outer hair cells but permanent damage to downstream things such as inner hair cells and spiral ganglia

phonemes

basic elements of speech

time waveform

display speech in temporal (timing); gives sound amplitude vs time

spectrogram

displays speech in temporal (timing) and spectral domain (sound power vs frequency) - time vs spectrum

spectrum

distribution of energy at various frequencies (cochlea and cochlear nucleus); particularly for vowels and place of articulation of consonants

sound sources

periodic sounds (voicing) from vocal folds in larynx - vocal cords beating together, anything with pitch - resonators

aperiodic (noisy) sounds from stops or friction in vocal tract including lips, tongue, and palate - stops of friction in vocal tract which stop the air flow and closing the airway - articulators

vocal fold sound source

produce voiced sounds that are periodic and have pitch includes vowels (/i/, /u/). diphthongs: vowel combinations (/ai/), nasals (/m/, /n/) - vocal folds beat together at periodic rhythm to elicit particular pitch

vocal tract sound source

produces unvoiced sounds that are aperiodic (noisy) and lack pitch includes unvoiced plosives, also known as stops (/t/, /p/, /k/) and unvoiced fricatives (/s/,/sh/,/f/) - not periodic because the airway stops then opens again, closing airway and blowing air through it are fricatives

vocal fold and tract sound source

mixed periodic and aperiodic includes voiced plosives (/b/,/d/,/g/) and voiced fricatives (/z/, /v/) - unvoiced “see” plus voicing makes voiced “zee”, close things off and then open again in voiced plosives, voiced fricatives - tuned on voicing as well

vowels have pitch

periodic opening and closing of vocal folds in larynx produces a periodic pulse train; pitch corresponds to the fundamental frequency which is the rate at which the vocal folds are beating together; any periodic sounds has harmonies at integer multiples of the fundamental frequency; modified by positions of tongue and lips to form resonances (formants) to enhance particular ranges of harmonies

formants

enhanced spectral ranges made when position of tongue and lips form resonance cavities to enhance a particular frequency range of harmonics

resonators in vocal tract enhance some harmonies and suppress other, periodic sound energy at each harmonic; enhanced frequencies of the vocal tract; controlled primarily by position of the tongue and lips; distinguish vowels by the relative frequencies of formants; same pitch but different formants

cochlea good at spectral analysis

cochlea uses its mechanics to understand high frequencies at base and lower frequencies at apex

rate place code

cochlea represents frequencies; frequencies of formants are represented in the cochlea by maximum displacement at particular places on the basilar membrane and maximum spike rates of auditory nerve fibers that innervate those places

1. formant frequencies for recognition of vowels and place of articulation

time code

pitch of a vowel transmitted primarily by the timing of action potentials in the auditory nerve; neural codes

fricatives

sound source is friction in the airway, voiced or unvoiced; distinguished by frequency and bandwidth

high freq - /s/ lower - /sh/ and wide band /f/ and /th/

voiced fricatives have pitch due to vibration in the larynx and unvoiced fricative have no pitch

neural mechanisms for detection of fricatives

neurons that are sensitive to broad ranges of high frequencies; voiced fricatives could be detected by neurons that are sensitive to combinations of high and low frequencies

plosives or stops

sound source is broad-band noise burst (explosion) produced by release of air pressure from an articulator; acoustic cues for place of articulation are given primarily by glides in formants, changing resonators which causes glides in formant; various consonants are produced by varying places of articulation

voice onset time (VOT)

time between release of pressure at the articulator and onset of voicing; difference between voiced and unvoiced plosives

neural mechanisms for discrimination of voice onset time

detect time delays on the order of 40-70 ms; octopus cells in the cochlea nucleus are specialized for marking the onsets of sounds, neurons in auditory cortex respond to onset of sound with a burst of spikes followed by the period of inhibition

octopus cells

in cochlear nucleus are specialized for marking the onsets of sounds, they can see when the sound starts and where the voicing comes on

resonators

the placement of the tongue and lips determines frequencies of formants

source of sound

voicing from the larynx or friction in the airway

articulators

for stops and fricatives

two things discriminate phenomes

spectra and timing of sounds

frequency (spectral) analysis

recognize sounds largely by their spectra (distribution) of frequencies; frequency analysis begins with mechanics of the basilar membrane in the cochlea, then projects up the auditory pathway as tonotopic organization; particularly important for recognition of vowels and place of articulation of consonants

auditory nerve and brainstem function

time analysis (highly specialized for processing temporal information; speech reception relies on temporal information on all time scales)

spatial hearing (identifying sound sources; aid in attending to single talker amid competing voices; brainstem processing of particular spatial cues; organize streams of phonemes form a particular talker into utterances)

functional parameter that vary among AN fibers

spontaneous rate, threshold, and dynamic range

auditory nerve (cochlear nerve)

only pathway from ear to brain; part of the vestibulocochlear nerve (8th cranial nerve)

contains vestibular nerve for balance and posture (afferent fibers from periphery to the brain)

vestibular and auditory nerves pass through the internal auditory canal

vestibular nerves have cell bodies in Scarpa’s ganglion and auditory nerves have cell bodies in cochlea

human auditory nerve has 30,000 afferent fibers

10% are type 2 only innervate outer hair cells

90% are type 1 (myelinated) run from cochlea to the brain

type 1 afferent nerve fibers

cell body (spiral ganglion cells): live in spiral ganglion of cochlea - do housekeeping, genetic material, maintenance of intracellular components, metabolism

peripheral process: forms afferent synapse on one inner hair cell; form as many as 20 spiral ganglion cells can synapse on each hair cell

central axon: 90% of the fibers of the auditory nerve, each of the 30,000 fibers have large diameters and myelination that begins on the peripheral process and covers the cell body and entire axon; forms 3 major branches before synapsing in cochlear nucleus

spike-rate-vs-sound-level functions from 3 auditory nerve fibers (firing properties)

spontaneous rates (spikes/sec): how much it spikes even in absence of sound , rate with no sound; The rate of neural action potentials (spikes) in a nerve fiber present in the absence of sound

threshold (dB SPL): minimum sound to elicit change in firing neuron; The minimum sound level that causes an increase in the spike rate of a nerve fiber.

dynamic range (dB): range from threshold to where it doesn’t fire any faster; The range of sound levels over which changing level results in change in spike rate.

dynamic range of sound-evoked spikes

fluctuations in sound level (sound envelopes) in range of 2-16 Hz which are critical for speech reception; dynamic range of a nerve fiber is the range of sound levels over which changing level results in change in spike rate

Winter and Palmer experiment

60% of fibers have high spontaneous rate, low threshold, narrow dynamic ranges - allow us to detect faint sounds

40% have low spontaneous rates, mid to high thresholds, wider dynamic ranges - allow us to understand speech at high sound levels with noisy backgrounds (lost when have hidden hearing loss)

“rate/place” code definition

fiber that innervates place along cochlear basilar membrane is most sensitive to a particular characteristic frequency (CF); neuron’s firing rate increase in proportion to the energy at its CF; information is given by the firing rate in a fiber innervating a particular place in the cochlear

Each auditory nerve fiber innervates a single inner hair cell. The fiber inherits the frequency sensitivity of that hair cell.; The firing rate of each fiber varies with the velocity of the basilar membrane at that cochlear location

To a first approximation, a higher spike rate signals a louder sound.; Firing rate sensitive to energy of CF

Each auditory nerve fiber responds with a maximum rate of action potentials when a sound elicits maximum velocity of the basilar membrane at the place that the fiber innervates. This is a rate/place code for sound frequency.; High freq - base active; Low freq - apex active

timing code definition

neurons fire spikes are synchronized “phase-locked” to the envelopes of sounds and to cycle by cycle fine structure up to a few kHz

The tuning curve encloses the range of frequencies and sound levels that elicit action potentials in the fiber -- point of turning curve is the CF

Low sound - hardly any action potentials

Loud sound - strong response more action potentials and bandwidth increases

Each fiber responds to a minimum sound level at a particular “characteristic frequency” (CF)

Frequency ranges expand at higher sound level

Low CF - broader freq

Higher CF - sharper freq

rate/place code benefits

Frequency specificity is very precise at near- threshold sound levels

Tip of tuning curve is precise freq

Frequency organization of cochlea is preserved as tonotopic organization throughout the auditory pathway

Sound spectra can be represented as patterns of activity in a tonotopic map

Combinations of frequencies

rate/place code limitations

Tuning broadens at moderate sound levels, where we do most of our hearing.

Tuning too broad

“Rate” is ambiguous. Increasing spike rate can mean that a stimulus is approaching a fiber’s CF or that the sound level is increasing.

Ambiguity to rate and freq

Spectral patterns tend to flatten out at mid-to-high sound levels

The Rate/Place Code - action potentials (rate) basilar membrane (place)

The cochlea is tonotopically organized, meaning that each sound frequency (tono-) elicits maximum motion at a particular place (topic) along the basilar membrane.

Inner hair cells at that place are tuned to that particular characteristic frequency

Each spiral ganglion cell innervates a single inner hair cell, thereby inheriting the frequency tuning of that hair cell. - every inner hair cell innervated by about 30 ?

The firing rate of each spiral ganglion cell (and auditory nerve fiber) signals the amount of sound power at its particular cochlear place.

That is the rate/place code

Temporal Code

The high temporal precision of ribbon synapses in the cochlea can transmit the timing of fluctuations of hair-cell receptor potentials to the timing of auditory-nerve action potentials

Neural spikes tend to fall at a particular phase of the sound – this is phase locking.

At frequencies below ~1.4 kHz, neurons can synchronize (phase lock) to the cycle-by-cycle temporal fine structure of sounds

At higher frequencies, neurons phase lock just to sound envelopes

Temporal components of sound: envelope and fine structure

Envelope frequencies ~2-16 Hz carry important speech information (e.g., syllable segmentation, voicing, voice onset time)

Temporal fine structure is needed for normal pitch perception and source localization. Useable up to about 1400 Hz ( how fast vocal cords are beating together)

Receptor potentials of inner hair cells

At low frequencies, the receptor potential follows the cycle-by-cycle fine structure of the sound waveform.

At high frequencies, the potential follows the envelope of the sound (upward deflection - creates distortion; no temporal fine structure but can see the envelope)

Neurotransmitter is released in proportion to the depolarization.

At low frequencies, auditory nerve fibers phase lock to the temporal fine structure of the sound

At high frequencies, auditory nerve fibers phase lock to the sound envelope

Auditory nerve fibers phase locked to temporal fine structure of a tone

Each fiber fires only on some fraction of the cycles. The aggregate response across multiple fibers can code the entire waveform. This is called “the volley principle”.

Basilar membrane flattened out; hair cells turned on - no firing when stimulus is low; spikes fire up at a particular stimulus - need multiple fibers to transmit temporal fine structure = volley principle

Auditory nerve fibers phase locked to the envelopes of speech sounds

Vowels lot of energy at low frequencies

Synchrony in some speech sounds

stop/fricatives - at high frequencies

Phase locking to cycle-by-cycle temporal fine structure

important for pitch perception and for coding the time of arrival of sounds at the two ears for spatial hearing

Phase locking of nerve fibers or brain neurons can code the envelopes of sounds

For speech sounds, important for signaling syllable segmentation, voicing, and voice onset time (4 to 16 Hz is most important)

On to the brainstem: Four key principles for central auditory processing

tonotopic organization; parallel processing; binaural organization; timing is critical

Tonotopic organization

Sound frequency (tono-) is mapped as brain location (topic)

This is the brainstem version of the rate/place code that originates with basilar membrane mechanics

Parallel processing

Auditory tasks are distributed among multiple specialized cell types and nuclei

Dividing stuff up into different regions - MSO and LSO - analyzing two different aspects of hearing LSO (higher frequencies nucleus) MSO (low frequencies)

(all important aspects of hearing and separated into the brain before they come back together)