Lecture 26: Auditory Evoked Potential

Question 1

What is an evoked potential?

Answer 1

An evoked potential is a event related potential that is synchronised with an external (sensory) event.

Question 2

What are auditory evoked potentials?

Answer 2

Electrical potentials (voltage) recorded from the ear or scalp in response to acoustic or electrical stimulation of the auditory system

Question 3

What are AEP useful for?

Answer 3

Useful tools to probe auditory brain function and assess pathologies underlying hearing loss.

Question 4

What are the parts of AEP?

Answer 4

Electrocochleography (ECochG)
Auditory Brainstem Response (ABR)
Middle Latency Response (MLR)
Cortical Auditory Evoked Potential (CAEP)

I.e the neural response going through the circuits

Question 5

What are the components of ECochG?

Answer 5

• Compound action potential (CAP)
• Cochlear microphonic (CM)
• Summating Potential (SP)

Question 6

What is the subtype of ABR?

Answer 6

Brainstem Auditory Evoked Potential

Question 7

What is the subtype of CAEP?

Answer 7

Late Latency Response (LLR)

Question 8

AEP are either;

Answer 8

• Compound Action Potentials generated by nerve tracts

or

• Compound postsynpatic potentials generated by cell dendrites

Question 9

What is needed to measure AEP?

Answer 9

• Syncronous firing in large number of neurons

- Alignment of neurons (auditory cortex is aligned, other cortices are not)

Question 10

What are the characteristics of AEPs?

Answer 10

• Weak (peak usually less than 1microvolt)
• Readily masked by other electrical noise
• Spatially imprecise (as lots of other electrical activity)
• Temporally precise

Question 11

Can the location of the AEP be determined?

Answer 11

Yes, using statistical and temporal analysis the location of events can be predicted

Question 12

Summerise AEP based on the previous questions;

Answer 12

Rely on arrays of neurons, aligned and firing synchonously

Only roughly associated with structures nad nerve fibre tracts

Have a reliable temporal and amplitude (db related) relationship to stimulus

Question 13

Whats the problem with measuring AEP?

Answer 13

You have a low signal to noise ratio (i.e theres a lot of background noise)

Question 14

What are the solutions to target the low signal to noise ratio?

Answer 14

• Reduced electrode impedance; Reduce resistance (<5kohms)
• Differential amplification; signal used is the difference between positive and negative electrodes. (one electrode gets noise, other gets noise and AEP, so if subtracted should be left with AEP)
• Filtering; Band pass is 100-3000Hz
• Amplification; 100-500,000times
• Averaging; removes unreliable information
• Repetitions; eyeball comparison of reliability

Question 15

How is ECochG done?

Answer 15

insert probe though TM near round window.

Measures three parts (occurs in order)

• CAP
• CM
• SP

Question 16

What is the CAP in EcochG?

Answer 16

Two negative potentials (N1 and N2)
- Associated with the auditory nerve, not sure what though, seems to be the passage in information through the spiral ganglion

Question 17

What are the components of CAP in EcochG?

Answer 17

Higher stimulus frequency and level lead to shorter latency (because high hz encoded as base of cochlear, so latency shorter)(level = because more auditory nerve fibres are brought into action faster)

Higher stimulus frequency and broader spectrum lead to higher amplitudes (because broader spectrum activates more neurons and high hz = base cochlear, higher amplitude as closer to electrode.)

Question 18

What is the cochlear microphonic? (CM)

Answer 18

• Response of cochlea to sound
• Amplified signal matches incoming sound
• Hence microphonic
• largely produced by OHC (as more of them, also larger potential difference at rest)

Question 19

What is the summating potential?

Answer 19

• Cells depolarise and hyperpolarise in response to condensation and rarefaction in the stimulating sound waves (movement of basilar membrane)
• The effect of depolarisation is greater than hyperpolarisation because from rest, fewer ion channels can close than can open
• The difference in these can lead to the summating potential: a change in the endochochlear potential due to the population of firing cells

Graphically this leads to an offset (change in potential with hyperpolarisation (from rest) is small, while depolarisation is large (from rest)

If you average these potentials it is slightly negative from baseline therefore on an AEP it is slightly less (start trace of AEP will be at rest)

Question 20

What does the auditory brainstem response reflect?

Answer 20

Activity from;

• 8th cranial nerve (auditory nerve)
• to the lateral laminiscus and inferior colliculus.

Question 21

The ABR is representative of?

Answer 21

The ABR is representative of the synchronous discharge of onset-sensitive neurons in the auditory pathways of the peripheral and central auditory CNS to a click or brief tone

Not a test of conscious hearing

Unaffected by degree of attention

Question 22

What happens with ABR and db, trace?

Answer 22

The higher the db the larger and clearer the waves and earlier they are.

Same reasons as ECochG

Question 23

What waves are formed in ABR?

Answer 23

six waves

Question 24

Where do the first five waves in an ABR are thought to come from?

Answer 24

Wave One: Distal end 8th CN
Wave two: Proximal end 8th CN
Wave Three: Cochlea nucleus (but multiple generators)
Wave Four: Superior olive complex and lateral laminiscus
Wave Five: Laminal laminiscus (contralateral) inferior colliculus

More important about when they occur not where

Question 25

Why would ARB be useful?

Answer 25

To determine if there are any brain tumors that would distort the recording and here they may be

Question 26

Describe the trace of the late latency response:

Answer 26

Very characteristic 3 waves:

P1 (Peak)
N1 (null/trough)
P2 (peak)

Question 27

Describe what is through to create P1,N1,P2?

Answer 27

P1 = Thalamus, auditory cortex and associated areas
N1 = Auditory cortex to the association areas
P2= Possibly non-modal areas affected (general processing)

Question 28

Describe some key developmental points for fetal growth of the auditory system

Answer 28

Second trimester; Maturation of cochlea and cohclear nerve

Perinatally: brainstem finishing developing

12 years of age: Cortex finishes developing

Question 29

How do preterm babies ABR differ?

Answer 29

Very hard to reproduce traces as the unmylelonated axons generate a lot of variability

Question 30

What is the difference between adult and infant ABR?

Answer 30

Amplitude:

• Infants: Wave 1 larger than wave 5
• Adults: Wave 5 larger than wave 1

Latencies:
- Infants have longer latencies than adults (lack of myelon)

Question 31

What factors influence the amplitude, latency and morphology of infant ABR?

Answer 31

• Myelination
• Cochlear and brainstem maturation
• Increased synaptic efficacy
• Orientation of pathways

Question 32

How does cochlear implantation relate to P1 latency?

Answer 32

Prior to 3 years p1 latency is in normal range for a cochlear implant. After this the latency tends to be out of the normal range for a cochlear implant.

Question 33

What can AEPs be used for in development?

Answer 33

AEPS can be used to measure and probe hearing and neuroauditory development