Lecture 26: Auditory Evoked Potential Flashcards
What is an evoked potential?
An evoked potential is a event related potential that is synchronised with an external (sensory) event.
What are auditory evoked potentials?
Electrical potentials (voltage) recorded from the ear or scalp in response to acoustic or electrical stimulation of the auditory system
What are AEP useful for?
Useful tools to probe auditory brain function and assess pathologies underlying hearing loss.
What are the parts of AEP?
Electrocochleography (ECochG)
Auditory Brainstem Response (ABR)
Middle Latency Response (MLR)
Cortical Auditory Evoked Potential (CAEP)
I.e the neural response going through the circuits
What are the components of ECochG?
- Compound action potential (CAP)
- Cochlear microphonic (CM)
- Summating Potential (SP)
What is the subtype of ABR?
Brainstem Auditory Evoked Potential
What is the subtype of CAEP?
Late Latency Response (LLR)
AEP are either;
- Compound Action Potentials generated by nerve tracts
or
- Compound postsynpatic potentials generated by cell dendrites
What is needed to measure AEP?
- Syncronous firing in large number of neurons
- Alignment of neurons (auditory cortex is aligned, other cortices are not)
What are the characteristics of AEPs?
- Weak (peak usually less than 1microvolt)
- Readily masked by other electrical noise
- Spatially imprecise (as lots of other electrical activity)
- Temporally precise
Can the location of the AEP be determined?
Yes, using statistical and temporal analysis the location of events can be predicted
Summerise AEP based on the previous questions;
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
Whats the problem with measuring AEP?
You have a low signal to noise ratio (i.e theres a lot of background noise)
What are the solutions to target the low signal to noise ratio?
- 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
How is ECochG done?
insert probe though TM near round window.
Measures three parts (occurs in order)
- CAP
- CM
- SP
What is the CAP in EcochG?
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
What are the components of CAP in EcochG?
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.)
What is the cochlear microphonic? (CM)
- 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)
What is the summating potential?
- 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)
What does the auditory brainstem response reflect?
Activity from;
- 8th cranial nerve (auditory nerve)
- to the lateral laminiscus and inferior colliculus.
The ABR is representative of?
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
What happens with ABR and db, trace?
The higher the db the larger and clearer the waves and earlier they are.
Same reasons as ECochG
What waves are formed in ABR?
six waves
Where do the first five waves in an ABR are thought to come from?
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
Why would ARB be useful?
To determine if there are any brain tumors that would distort the recording and here they may be
Describe the trace of the late latency response:
Very characteristic 3 waves:
P1 (Peak)
N1 (null/trough)
P2 (peak)
Describe what is through to create P1,N1,P2?
P1 = Thalamus, auditory cortex and associated areas N1 = Auditory cortex to the association areas P2= Possibly non-modal areas affected (general processing)
Describe some key developmental points for fetal growth of the auditory system
Second trimester; Maturation of cochlea and cohclear nerve
Perinatally: brainstem finishing developing
12 years of age: Cortex finishes developing
How do preterm babies ABR differ?
Very hard to reproduce traces as the unmylelonated axons generate a lot of variability
What is the difference between adult and infant ABR?
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)
What factors influence the amplitude, latency and morphology of infant ABR?
- Myelination
- Cochlear and brainstem maturation
- Increased synaptic efficacy
- Orientation of pathways
How does cochlear implantation relate to P1 latency?
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.
What can AEPs be used for in development?
AEPS can be used to measure and probe hearing and neuroauditory development
