Week 7 Audition Flashcards
Audition Info
- Audition is a far sense
o Sounds can travel long distances, through and around obstacles – can hear what’s on the other side of a wall
o Gives us 3D, 360’ sensory information – can localise sounds - Audition enables us to identify, locate and react to things in the environment and
allows for verbal communication and music
o Ability to speak relies on ability to hear yourself – modulate your muscles
Auditory stimuli
- The stimulus for audition is acoustic energy = pressure changes in the molecules
(medium) around us
o Medium is most often gas but can be molecular movement in a liquid
medium or even a solid
o Steel has the fastest conductance of acoustic energy - When an object moves, it creates a disturbance in the surrounding molecules/medium
o Each molecule moves a bit, initiating movement in a nearby molecule
o Creates a ripple effect
o If this ripple of molecular disturbance reaches your ear, auditory receptors
(hair cells) can detect this acoustic energy and the perception of sound can
result - You must have some sort of medium (some array of molecules ready to disrupt) to
have sound
o Robert Boyle’s alarm in a vacuum
Suck air out of jar – sound goes away
Everything mechanically was still happening in the bell but eliminate
molecular disturbance to eliminate sound o Ridley Scott’s alien tag line
Molecules are too far away from each other in space to disturb each other – no sound created
Initial molecule moves but too far away from next one in line
Sound waves
- Acoustic energy is visually represented as a sound wave, which illustrates the
amplitude and frequency of molecular disturbance - Two physical characteristics linked to 2 perceptual characteristics
o Amplitude = displacement from baseline (y axis)
Large disturbance in air molecules = large amplitude wave
Gives loudness submodality
o Frequency = distance between crests (x axis)
High frequency disturbance in molecules = very frequent crests
Gives pitch submodality
Quantifying sound waves
o Amplitude
Humans can perceive a huge range of amplitudes
Expanded stimulus::intensity relationship
Decibel (dB) scale – moves up amplitude in a log scale
Loudness linked to amp of a sound wave given in dB
Pain threshold ~ 140 dB
Modality shift from hearing as sound to tactile pain
Danger zone begins 80 dB for hearing loss
Threshold for hearing is zero
Human range for amp detection
0 – 140 dB though is age dependent
As you get older you lose bottom end of scale
o Frequency
Humans can perceive a subset of frequencies
Hertz (Hz) – has to be in the spectrum to detect
Pitch linked to frequency of a sound wave given in Hz
We don’t hear entirety of Hz spectrum
Other animals can hear things we can’t
Sensory system is limited in detection abilities
Human range of detection
20 to 20,000 Hz for young adults
o As you get older you lose the top end
o Have to talk low and loud for elderly to hear
Emit high frequency sound the older adults can’t hear
o If loud enough is annoying to young people – move
away from area
o A noise that won’t annoy the adults – teen deterrent o 80dB bursts at ~17,000 Hz
o Use sound to exploit hearing range
Acoustic energy hits objects
- Some of it is absorbed into the object o Plaster/tile absorb ~3% o Carpet/drapery ~25% o Soft furnishings - Some of it is reflected back as echoes o Reflected echoes can be useful or annoying o Hard surfaces o Sonar/echolocation Detect where you are in environment Humans use? Boats, depth/fishing fish finding navigation systems Submarine navigation Echolocation in the blind o Medical imaging with ultrasound Reflect off bones o Poor acoustics in concert halls
Human echolocator
o An echolocator blind person listening to sound stimuli
Visual cortex lights up as well as auditory cortex
Open space when lost vision – utilised by hearing
Anechoic chambers
o All sound waves are absorbed – none are reflected
o Hear only initial original source of sound
o Used to testing sound quality of audio equipment, sound emissions of
appliances, machines, etc.
o Semi-anechoic chambers are more common due to construction difficulties o Can be quite disturbing – can hallucinate, disorientate
NASA uses to train astronauts for lack of sound in space
Process of Hearing
- Acoustic energy reaches outer ear (pinna)
o Pinna functions to resonate and localise sound waves o We have two of them to better localise sound
Can compute time difference or intensity difference between two sources – tell where sound came from
Can function with just one but spatial acuity of sound affected - Resonates down auditory canal and hits tympanic membrane
o Tautmembrane
o Where middle ear starts o Membrane vibrates - Vibration of membrane transfers to series of ossicles
o Malleusincusstapes
o Middle ear - Stapes initiates vibration on oval window
o Causes vibration in fluid-filled cochlea
o Inner ear
o Vibrations from air to fluid – moving fluid takes more energy than moving air molecules - Fluid movement disturbs basilar membrane of the cochlea
o Causes bending of hair cells (sensory neurons)
o Leads to signal transduction
o Neuronal signal relayed to next neuron (axons contributes to CNVIII, auditory
nerve) - Neural signal relayed to auditory fibres
o These form CNVIII - Synapse in cochlear nucleus of medulla
o Crosses to superior olive-inferior colliculus-thalamus-A1
Why such a piecemental process
- To amplify the acoustic energy
- The cochlear medium is liquid – initiating pressure disturbances requires more
energy
o 20::1 size ratio for tympanic membrane::oval window enables concentration o Size differential
o Ossicles together form a level that enables amplification - In order for this amplification process to work pressure in middle ear must be equal to outside air pressure
o Have Eustachian tubes to help maintain appropriate pressure
Auditory reflex
- If high amp acoustic energy hits your tympanic membrane – and this is then
concentrated and amplified – wouldn’t this damage your oval window - You have an acoustic reflex in place to prevent this
- Tensor tympani muscle on tympanic membrane and stapedius muscle on stapes
o When high amp acoustic energy arrives, these muscles contract and resist the movement of the tympanic membrane and ossicles
o Dampen noise by ~30dB - 2 pitfalls of the reflex
o Primarily works for low-frequency sounds
Sensitive to only low end – if you have high amp sound wave this
doesn’t work as well and can damage hearing
o Takes 50ms to initiate
There is a delay before the muscles tense - So high amp, abrupt stimuli can damage the middle and inner ear o Not common in nature
o Guns and cars and missiles are higher frequency - If acoustic energy hit your oval window with no tympanic membrane
o If you damage tympanic membrane – break it mechanically, or pop it due to unequalised pressure, or infection
o Reduces hearing capabilities – won’t have amplification process o Reduced by ~30dB
o Normal conversation sounds like a whisper
o Is able to fix itself unless repetitively damaged
the Inner ear
Cochlea has 3 compartments o Scala vestibuli Top bit – where stapes hits into oval window o Scala tympani Bottom bit o Midline compartment Where sensory neurons (hair cells) are Has connection to tectoral membrane and CNVII
The Inner ear (Inner hair cells)
o Close to the inside of the compartment
o Majority of signal transduction – when they bend, release glutamate to the
next cell (auditory nerve fibre) into CNVIII
o Have heaps of afferent fibres going to the brain
o If you make a mouse that lacks functioning in IHC = deafness
The Inner ear (Outer hair cells)
o Further away from inside of compartment
o Help to amplify the cilia bend so that signal transduction can occur o Cilia have direct connections to tectorial membrane
Facilitates movement of membrane
o Can purposefully bend OHC and move membrane more by having efferent
connections – from the brain
Mostly from superior olive – tells OHC to bend more or less
Functionally important for high sensitivity hearing
In a quiet room – send efferents to help hear better
o Help generate pain signal from the inner ear
Mouse without IHC still exhibits nocifensive behaviour to high dB
noises via OHC pathway
Still responds to loud noises in a pain type behaviour
Other ways to initiate vibration in cochlear fluid
o Can move fluid by moving – spinning around o Vibrate bone
Get acoustic energy from pressure change from mouth while you talk
Vibrate the bone which moves the fluid
Use this to test hearing loss
Your voice sounds different to you
We get two different sources of acoustic energy that gives the perception of our voices
Auditory pathway
- Cochlea-auditory nerve- cochlear nucleus in medulla- superior olivary
nucleus in medulla-inferior colliculus in midbrain - MGN of thalamus - A1 - From the cochlear of both your ears into cranial nerve VIII
- Synapse into the medulla at the cochlear nucleus
- Medulla synapses to another nucleus – inferior colliculus in midbrain
o Also sends projections across to another nucleus in the medulla – superior olive
Then to midbrain - Ascending up to the thalamus and to auditory cortex
Localisation (Auditory Pathway)
o Pathway decussates but also sends info up the same side
o Info to cochlear nucleus – from goes across and some ascends up the same side after it goes to the superior olive
o Get spatial integration of sounds coming from the left and right
o Gives you localisation – integrate information from the two ears
o This is why we have the circuitous route going through the superior olive
Why info is sent up both sides
Gives spatial localisation
Integrate info from two ears which gives spatial resolution – where the
acoustic energy is coming from
Binaural neurons (Auditory Pathway)
o Within the superior olive
o Receive information from both sides of the CN coming in – from both ears
o Compute the interaural time difference and interaural intensity difference
between ears
Which side did the acoustic energy enter first
Is it higher amp on one side or the other
Together helps to optimise sound localisation L/R
Where stuff is happening in the environment
High intensity spatial information
Primary auditory cortex A1 (Cortical processing)
o Tonotopically organised – areas segregated by tone
Overrepresentation of Hz frequencies that are low-mid ranges
Hear a lot of speech so deveop more space devoted to processing that information
This is the range that predominates human speech o Concentric, hierarchical processing
Central Core Area (Cortical processing)
o Primary auditory region (A1)
Responds to specific frequencies, simple tones – basic info
Has minimal adaptation
Always fires when acoustic stimuli are present
Don’t adapt to it when the sound continues – try to ignore but will still hear it
Surrounding Areas (Cortical Processing)
o Secondary auditory regions (belts)
More complex info
o Tertiary/association regions (parabelts)
Even more complex info
o Features of both
Respond to complex sounds, sort features of sound
Both exhibit adaptation
When enviro is more cluttered and mixed – can adapt a bit better to
that and stop hearing
Hierarchy of processing info (Cortical processing)
o Starts basic and adds on information as your move from A1 out into the belts and parabelts
Three basic levels of auditory processing
- Spatial localisation
o Where did it come from – survival aspect – useful in alerting you o The posterior/dorsal belt area provides ‘where’ info
Spatial info – where sound is coming from
If damaged – can hear a person calling you but won’t know what
direction it came from
o Binaural integration - Sound recognition
o Which sound belongs to which thing
Can parse out based on recognition ranges
o The anterior/ventral belt area provides ‘what’ info – recognising different sources of sounds
Patterns of voices vs drinks clatter
If damaged – can hear something yelling from behind but won’t
recognise the voice, or perhaps even recognise it as a voice
o Sorting different sources of sound via:
Localisation, common spectral content and time course, familiarity
Optimise that signal vs everything else - Signal to noise optimisation
o Usually try to listen to a signal of interest and declutter background noise – something you want to pay attention to and other signals you don’t
o Also anterior/ventral belt
o Overcome amplitude/frequency masking
If everything is the same amp – hard to pick out signal from noise
Make signal larger amp to overcome masking effect – talk louder
If someone is talking at similar frequency to background – even
increasing amp, it can still be hard to hear them
o Additional level of binaural unmasking
The 3 levels of process allow:
o Parsing – separate out which noises belong to which thing
Attend to distinct sounds within a scene
Separate into different streams of info
o Binding – identify that all the sounds together belong to a party for e.g.
Generate a unified auditory scene
Info from all direction at all times but able to make a unified
experience
o Also allow you to function in audibly cluttered environments and detect
important sounds
Loudness (encoding)
o Larger amp sound waves cause larger displacements in the basilar membrane more hair cells bend more neural potentials generated
o The more neural activity (population activity) sent up the auditory relay, the higher the intensity/loudness of the percept
o Loudness is frequency encoded (rate encoded)
Higher firing rate encodes louder sounds – how many action potential correlates with how loud or soft the sound is
Larger amp = louder noise
Pitch (encoding)
Temporal theory
Basilar membrane vibrate at a frequency that matches the frequency of the sound wave
50 Hz sound wave causes a 50 Hz vibration in the stapes
This transferred to the cochlear fluid, producing a 50Hz
oscillation in fluid
Hair cells bend at 50Hz interval
Afferent fibre APs are phase locked to 50Hz
o Sometimes neurons can’t fire that fast, especially at frequency gets higher – need to have rests between
o Fibres fire at the crests at sound wave – miss some
o Maybe another fibre nearby fires at the missed ones
Across the population of fibres/neurons, a 50Hz signal is
generated and propagated upstream
Perception of 50Hz pitch results
2 shortcomings
Neurons can’t fire in excess of 1000 Hz
o Yet the human range of pitch perception ranges 20 to 20,000 Hz
o Even if there are multiple neurons firing together the range will only be about 3000 Hz
Basilar membrane doesn’t vibrate uniformly
o Differs in width and flexibility
o Some parts are stiff and might move every second one o Tips might vibrate at 50Hz but not all of it
Pitch (encoding)
Place theory
Different regions of basilar membrane vibrate in response to different
frequencies of sound wave
Georg von Bekesy studied post-mortem cochleas and discovered that
basilar membrane is:
Not uniform in shape through cochlea
o Shape and thickness varies
o Starts thin and gets wider – is not going to move at
same speed through cochlea
Tonotopically organised – different areas of membrane
respond maximally to different frequency soundwaves
o Different frequency ending in the membrane at different points
o 100 Hz is deflected earlier on compared to 25 Hz
Shortcomings
At low frequency <50Hz there is a universal deformation
At lower frequency temporal theory kicks in
Auditory fibres
- Each region of basilar membrane responds maximally to a specific frequency of
soundwave - Thus auditory fibres connected to that portion of membrane will response maximally
to a specific frequency of soundwave - Each fibre exhibits a characteristic frequency
o The hair of the hair cell has a frequency so the corresponding fibre has a specific frequency it responds to - There is a range for the fibre where it will respond even when dB are low
o When out of the range you need massive dB to have it respond
o All across membrane there are all sort of fibres responding to different
frequencies
Each auditory fibre exhibit a unique intensity threshold
o Recording from a population of hair cells – all have a characteristic frequency ~900 Hz but intensity thresholds differ
Fibre 1 = 2dB
Fibre 2 = 10dB
Fibre 3 = 20dB
Fibre 4 = 80dB
o At 80dB all four of them fire because all of their thresholds for firing are met o They recognise a specific frequency and within that population of frequency
specific cells there are amplitude specific cells Allow to distinguish volume
o If you only have place theory you wouldn’t be able to distinguish volume differences within a pitch
- The combo of characteristic frequency and intensity threshold is important o It enables you to distinguish loudness within a pitch
Encoding
- The submodality of pitch is mostly spatially encoded (place theory)
o Exception: very low frequencies (<50Hz) are temporally encoded (temporal theory) - The submodality of loudness is encoded by the level of population activity sent through the auditory system
o Rate encoding/frequency encoding
Testing hearing
- Threshold tests
o Detection – can you hear that o Method of constant stimuli
o Method of limits
o Method of adjustment - Improves objective threshold tests
o Forced-choice test
o Signal detection test
o These give confidence – make sure they are actually hearing - Quantification
o Quantifying perception of loudness and pitch is not always easy
Psychophysics of audition
o Controlled, reproducible auditory stimuli of known amplitude and frequency easy to produce
o Experimental confound
For threshold detection, pitch and loudness are interlinked in terms of
psychophysics
o Ramifications of this confound
To do an accurate psychophysical assessment for hearing, your stimuli
needs to span a large range of frequencies and amplitudes
Different dB stimuli in variety of frequencies
Map out audibility function – map where things become audible,
what is detectable
Anything below the line is inaudible
Compare the dip to standard curves to determine if they have
hearing loss
Once you get to 1000Hz
stimulus only has to be low
dB
This is why some stereos
have a bass boost or loudness button to amplify low frequency so you can hear low frequency at low volume
Take the low frequency signals and boost the dB to let you perceive them
o Participant variables
Identification of pitch is highly variable between individuals
Familiarity and practice with certain pitches will influence A1
tonotopic map
Tonal language fluency, musical training
Perception of loudness and pitch decreases with age
Hearing Loss
- The most common sensory disability
- Deafness = no detection under 82 dB
o Profoundly deaf is no detection at all
o Still tactile detection if loud enough - Impairment = any loss in audition relative to normal
young adult - Can be isolating, frustrating, scary
o Isolated from others and void of a background din - On the curve
o Has to be 80dB or above to hear 1000Hz and up
o Maybe a person with hearing loss or elderly
Four types of hearing loss
- Conduction loss
o Impairment in outer/middle ear’s ability to transmit/amplify sound
o = reduced sensitivity to all frequencies
o Could be due to ear wax, infection, tympanic membrane perforation/rupture,
Eustachian dysfunction, otosclerosis of ossicles (lose ability to move over
time)
o If can’t transmit signal to cochlea all frequency will be affected - Sensorineural loss
o Impairment in inner ear signal transduction and transmission
o = reduced sensitivity to specific frequencies or total loss if no functioning hair
cells
o Use bone conduction to determine if hearing loss is up-stream or down-
stream of the cochlea
If can hear is on bone conduction then cochlea is working - Age related hearing loss
o = presbycusis (old, hearing)
o Men lose more auditory capabilities than women
o At15,<20kHz
o At30,<15kHz
o At50<12kHz
o Low dB ad higher frequency loss
o Possible reasons
Loss of cochlea elasticity?
Loss of nutrients to maintain cochlear health?
Cumulative exposure to loud noise throughout industrialised life –
socio reasons
A 70 year old African elder has normal young adult hearing
Damage to auditory system progressively – just part of the
society you live in - Noise exposure
o = socioacusis
o Loud, abrupt noises can damage auditory system
o Sustained, long term loud noise can damage also
o Day to day social life stuff – cumulative loud day stuff
o Music: loud earphones, concerts, clubs, musicians
o Occupational: factories, blenders, hair dryers, lawn mowers o Transport: cars, trains, motorcycles
o Crowds: bars, sporting events, lecture halls
Compensating for hearing loss
- Amplify the acoustic energy via a hearing aid
- Implant a prosthetic ossicles
- Cochlear implant
o Implications for plasticity
If auditory cortex is set up a certain way and all of a sudden send
signals the brain has never had before – can it cope?
o Psychosocial implications
Parents part of deaf community
Want child to be part of community
With an implant – won’t be deaf but won’t be able to hear perfectly
either - Regenerate damages/dead/missing hair cells?
o Stem cells?
o Can’t fix hair cells – don’t grow back
Functions of audition
- Identify, localise and react to things in the environment
- Communicate
o Non-speech vocalisation – screaming, crying o Speech vocalisation
o Noisemakers – fire alarm has meaning
o Music – emotion in a song, or the words - Audition is key for receiving and producing communication
o We hear and simultaneously process the sound of our own voice
o If listening to incongruent audio input of distorted speech sounds, we alter
our articulations to try and correct
o Make adjustments while you speak – constant sensorimotor feedback
Week 7 Audition
