Auditory Processing & Tinnitus

8/3/2019 Auditory Processing & Tinnitus

1/26

Auditory Processing & Tinnitus!

Wesley Jackson

A neural enthusiasts primer to


2/26

Some simple physics.! Although soundis a perceptual experience, any sound can

be physically modeled as a specific waveform.!

Intro!


3/26



Asinetoneisthesimplestsound,

consis1ngofasinglefrequency

component(Hz)ofagiven

amplitude(dB).

(sinetonesandothersuch

simplesoundsmaycorrelate

withhighlevelsofnostalgia)

Intro!


4/26



Asinetoneisthesimplestsound,

consis1ngofasinglefrequency

component(Hz)ofagiven

amplitude(dB).

Naturalsoundstendtobecomplex

(consis1ngofmanyfrequency

components).

TheyconsistofpaCernsofchangesin

airpressure.

Intro!


5/26

TheauditorysystemtranslatesthesepaCernsofairpressurechangeintopaCernsofneuronalfiring.

Intro!


6/26

PeripheralAuditorySystem

Auditory

processing

beginsin

cochlea

The Auditory System!


7/26

Auditory

informa1onis

senttothebrain

bytheauditorynerves.

The Auditory System!PeripheralAuditorySystem


8/26

Sound waves are translated into nerve impulses!ochlea

Specificacous1cfrequenciescause

certainloca1onsalongthebasilar

membranetovibrate.

Tonotopicorganiza5onofthebasilarmembrane.


9/26

Sound waves are translated into nerve impulses!)Innerearhaircellsaredeflectedby

vibra1onsalongthebasilarmembrane

ands1mulateauditorynervesbydepolarizingafferentnervecells.

2)Becausethebasilarmembraneis

tonotopicallyorganized,theauditory

nervesencodespecificfrequencies.

NS


10/26

frequency encoding?!Anauditorynervesfiringratewillpeakatsomespecific

characteris5cfrequency(CF).

TheFwillhavealowresponsethreshold;rela1velylow

acous1cenergy(amplitude)isrequiredforaresponse.

Ac1vity(firingrate)peaksatF

LowestdBthresholdatF

FREQUENY (KHZ)!

a.Measuring

peakresponse


11/26

frequency encoding?!TherangeofresponsesasFisapproachedistherecep5vefield.

Seehowthispar1cularfrequency

takesthelowestdBtoevokea

response?Thisisthiscells

characteris5cfrequency!

Manycellswithdifferent

Fsencode

thefull

acous1c

frequency

spectrum.

Itsrecep5vefieldistheshapeof

theresponseacrossmany

frequencies.

b.Measuring

amplitude

threshold


12/26

encoded acoustic information sent to CNS:brainstem.!

PNS NS


13/26

entralAuditorySystem

The Auditory System!


14/26

entralNervousSystem(NS)

BRAINSTEM

Hearingcanbetestedbymeasuringthe

brainstemsresponsetosoundsofvariousfrequenciesandamplitudes

AuditoryBrainstemResponse(ABR)

Whatisthe

minimumdB

threshold(i.e.

hearingthreshold)

forsucha

response?


15/26

entralNervousSystem(NS)

THALAMUS

MedialGeniculateBody(MGB)

PrimaryAuditoryortex(AI)

I

II

III

IV

V

VI

or1calLayers*

Cor3calLayersIII/IV:

25%GABAergic

interneurons

Thalamicinput

Projecttoother

cor1callayers

*No5cethecolumnarorganiza5on!

Thesecolumnsprocesssimilar

acous5cproper5es.


16/26

GABAergic interneurons!GABA*istheprimaryinhibitory

neurotransmiCer.

.ItissynthesizedfromtheexcitatoryneurotransmiCerglutamatebytheenzyme

glutamicaciddecarboxylase(GAD).

GAD65GABAglutamate

Wearepar1cularlyinterestedinthe

expressionofGAD65.Why?Because

manipula1ngtheexpressionofGAD67can

belethal!

*GABAisnottobe

confusedwiththe80ssupergroup.

Transcrip3onfactorsregulategeneexpression

2.GADisproducedbyGABAergic

interneurons,whosedevelopment

dependsonDlxtranscrip5onfactors


17/26

PV+ GABAergic interneuronsin cortical layers III/IV!

Basketcells

MostcommoninhibitoryneuroninAI

Projecttosoma(cellbody)ofpyramidalneurons

Thesecellscommonlyexpressthe

proteinparvalbumin&arethuscalled

PV+BasketCells(PV-B).

Interneuronsprovidelocal

connec1onswithincortexandare

primarilyinhibitory(GABAergic)

Winer&Schreiner(2011).TheAuditoryCortex


18/26

PrimaryAuditoryortex(AI)

AuditoryPathwayOverview


19/26

Hearing Damage!

Damagedhaircellsnolongerrespond

toenvironmentalfrequencies.


20/26

Hearing Damage!. Haircellsresponsivetohigher

frequenciesarentasresponsiveas

before.

2. Thismeanstherewillbelessthalamicinputtothecortexforthesefrequencies.

3. Socor1calneuronstunedtothesefrequencieswillhavereduced

thalamicinputandbecome

deafferented;theynowprimarily

respondtolowerfrequenciesthan

before.

(Yangetal.2011)

Seehowhighfrequencythresholdsdontrecovera[erhearingdamage?

ABRshowsimpairedhigh-frequency

hearingaerhearingdamage:

Socor3calresponseemphasizes

lowerfrequencies:

Eff f A di D ff i


21/26

S

Effects of Auditory Deafferentation:1) Cortical Reorganization& Map Disruption!

Sensory deafferentation causes many changes to the Central Nervous System!Thetonotopicorganiza1onofAIbecomes

reorganizedasneuronsnowrespondprimarily

tolowerfrequencysoundsthanbeforehearing

damage.

)ThisneuronpreviouslyhadaclearhighF

2)Nolongersopreciselytunedfor

(previously)highfrequencyneurons!

Hencethemapdisrup1on

(Yangetal.2011)

Seehowthehigh

frequencyareaofthe

cortexisnolonger

tonotopically

organizedaer

hearingdamage?

Thisisanexampleof

mapdisrup5on!

Eff t f A dit D ff t ti


22/26

Effects of Auditory Deafferentation:2) Morphological Changes!

Sensory deafferentation causes many changes to the Central Nervous System!or1calmapdisrup1onisaccompaniedbymorphological

changestodeafferentedpyramidalneurons.

Eff f A di D ff i


23/26

Effects of Auditory Deafferentation:3) Changes in Synaptic Plasticity!

Sensory deafferentation causes many changes to the Central Nervous System!Deafferentedneuronsdontfireasoenbecause

theyhavelostmuchthalamicinput.

Therefore,tomaintainoveralllevelsofexcitability,

tonicinhibi3onisreducedbydecreasinglevelsof

ambientGABA.Thisisachievedbydownregula1onofGAD.

WheredidalltheGADgo?

(Yangetal.2011)

Eff f A di D ff i


24/26

Effects of Auditory Deafferentation:3) Changes in Synaptic Plasticity!

Sensory deafferentation causes many changes to the Central Nervous System!Tonicinhibi3oniscausedbyambientGABA

extrasynap1cGABAwhich

modulatesbaseline

neuronalexcitability.Extrasynap5cmeansfar

fromthesynapses.

RememberhowGABAergic

basketcellssynapsetothe

somaofpyramidalneurons,

ratherthanthedendrites?

Eff f A di D ff i


25/26

Effects of Auditory Deafferentation:4) Tinnitus!!

Sensory deafferentation causes many changes to the Central Nervous System!Thisdeafferenta3on-mo3vatedincreaseinexcitabilityisthemost

likelycauseof3nnitus,astheseoverexcitedhighCFneuronswillnowfireveryoen!

Otherneuronslisteningtotheac3vityoftheseneuronswillinterpret

thisincreaseinac3vityasthepresenceofthatfrequencyinthe

environmenthencethepercep3onofsound,or3nnitus!

Note:ContrarytoGoogleimagesearch,theaboveresponseisnotactuallyareliable

characteris5cbehavioralcorrelateof5nnitus.

Eff t f A dit D ff t ti


26/26

Effects of Auditory Deafferentation:4) Tinnitus!!

Sensory deafferentation causes many changes to the Central Nervous System!Thisdeafferenta3on-mo3vatedincreaseinexcitabilityisthemost

likelycauseof3nnitus,astheseoverexcitedhighCFneuronswillnowfireveryoen!

Otherneuronslisteningtotheac3vityoftheseneuronswillinterpret

thisincreaseinac3vityasthepresenceofthatfrequencyinthe

environmenthencethepercep3onofsound,or3nnitus!

Note:ContrarytoGoogleimagesearch,theaboveresponseisnotactuallyareliable

characteris5cbehavioralcorrelateof5nnitus.

Auditory Processing & Tinnitus

Documents

Transcript of Auditory Processing & Tinnitus