ABR Lecture

Question 1

Define Event Related Potentials

Answer 1

Changes in voltage (potential) that occur at a particular time before, during or after (‘related’) something that happens in the physical world +/or some psychological process (‘event’).

Question 2

Exogenous evoked potentials

Answer 2

<100ms

Exogenous potentials are evoked by stimuli in the world
outside of the brain
Determined by physical properties of the evoking
stimulus

Question 3

Endogenous evoked potentials

Answer 3

>100ms

Evoked by psychological processes within the brain
Reflect psychological significance of the stimulus

Question 4

Outline the difference between EPs, ERPs and Emitted potentials

Answer 4

ERPs are Changes in voltage (potential) that occur at a particular time before, during or after (‘related’) something that happens in the physical world +/or some psychological process (‘event’).

EPs are ERPs that follow a physical stimulus

Emitted potentials occur in the absence of any evoking stimulus

Question 5

In AEPs, which electrode is positioned for near-field recording?

A. Ground

B. Reference

C. Active

D. Silver-silver chloride

Answer 5

C. Active, positioned over a high response area

The reference is positioned over a low response area - far field recording

Question 6

What happens to the amplitude of the ABR waveform with adaptation ?

A. Increases

B. Decreases

C. Stays the Same

D. None of the above

Answer 6

B. Decreases with increased rates of stimulation due to adaptation

Question 7

Electrode contact impedences in ABRs should be below:

A. <2KΩ

B. <5KΩ

C. <8KΩ

D<12KΩ

Answer 7

B

Question 8

Match electrode to its position:

1. Active electrode A. Mastoid/Earlobe
2. Reference electrode B. Contralateral mastoid
3. Ground electrode C. Vertex l

Answer 8

1C

2A

3B

Question 9

Describe Common Mode rejection

Answer 9

Signal + noise is recorded from the active electrode. Noise is recorded from the reference electrode. The pre-amplifier/differential amplifier in the system removes what is common to both inputs to get a cleaner system = “common mode rejection”

Question 10

What is recorded within a bandpass?

Answer 10

Potential between the non-inverting (positive) and inverting (negative) electrodes

Question 11

Which is NOT a way an Evoked Response is distinguished from background noise?

A. Synchronous Averaging

B. Pattern Recognition

C. Common mode rejection

D. High pass and low pass filters

Answer 11

B

Question 12

What does the early latency reponse NOT include?

A) Cochlear Microphonic (CM)- AC receptor potential
representing response from many individual hair cells mainly at basal turn of cochlea. OHC’s contribute most to the response.

B) Vestibular action potential (VAP) - response as the vibrations transition into the semi-circular cannals

C) Summating Potential (SP)- DC receptor potential arising
mainly from hair cells excited by non-linear vibrations of
BM within cochlea.

D) Compound Action Potential – Not generated in the organ
of corti (the others are). The AP is the overall negative
potential charge arising from several spike discharges in
the cochlear nerve.

Answer 12

B is completely fictional :P

Question 13

What 2 methods can be used for Electrocochleography recording?

Answer 13

Trans-tympanic method- recording electrode consists of a thin stainless steel needle which is insulated except for the tip. Placed through TM to rest on promontory in middle ear. LA for older children and GA for younger (INVASIVE). Need medical backup.

Extra-tympanic method: Recording sites for this technique are either the wall of the EAM or TM.

a. For EAM placement surface recording electrode (Silver) is positioned on inferior wall of EAM and held in place by a light ‘V’shaped spring.
b) For TM placement single use CE marked hydrogel electrode recommended and causes minor discomfort. ( Non-invasive)

Question 14

When do true neurogenic potentials occur?

Answer 14

First 10ms after auditory stimulus

Question 15

Define ABR

Answer 15

ABR is a far field recording of the electrical activity from sequentially activated neural tracts and nuclei in the ascending auditory nerve and brainstem pathways (Moller 1999).
True neurogenic potentials, occurring in the first 10ms after the auditory stimulus and comprise a family of peaks and troughs.

• Click stimulus mainly used- transient (main energy of click between 2-4kHz)
• Occurs in the first 10ms after a stimulus (Analysis window – 10-20ms)
• Not frequency-specific, particularly in the low-mid frequencies
• Can use short duration tone pips, with energy concentrated at one frequency (but response is not so clear at threshold)

Question 16

Which of the following is ABR reliability NOT determined by?

A. Wave reproducability

B. Waveform Morphology

C. Waveform Latency

D. A+B

E. A+C

Question 17

What are the normal latencies for peaks I, III and V for 50, 65 and 75dB respectively?

Answer 17

50dB - 1.7, 3.9, 5.7

65dB - 1.6, 3.8, 5.5

75dB - 1.6, 3.7, 5.6

Question 18

Which is the most robust wave in the ABR waveform?

A. I

B. III

C. V

D. VI

Answer 18

C

Question 19

What decision levels are used to score ABR waveforms?

Answer 19

1. Response definite (++)
2. Response probable (+)
3. No response (-)

Question 20

Which is the most important wave in Neurological/diagnostic ABR and how does it differ?

Answer 20

Many diagnostic criteria based on wave V latency- Most Robust and arises from top of the brainstem, hence sensitive to lesions within brainstem and tumors of CP angle (e.g.- acoustic neuroma)

Compare wave V latency recorded from one ear with that of the other ear. In normal subjects the inter-aural latency difference of wave V (ILD-V) is close to zero.

Abnormally large rate-induced latency shifts in wave V are often seen in retro cochlear disease.
Amplitude of wave V smaller than wave I if generators of wave I are unaffected and wave V impaired.

Question 21

What is maturation and what are its effects in the ABR?

Answer 21

Whereas cochlear function is fulyl developed at birth, axons and synapses undergo a process of maturation (level of myelination of nerve fibres

I, III, V delayed and broadened

Question 22

Describe what effects to the ABR waveform this type of hearing loss has:

Conductive hearing loss

Answer 22

Prolongs the latencies of all components of the ABR, including wave V.
Bone-conducted stimuli can be used to evoke an ABR and recommended protocol suggested by Stevens(2002). BC-ABR is recorded using similar stimulus and recording parameters as for AC-ABR.

Question 23

Describe what effects to the ABR waveform this type of hearing loss has:

Cochlear hearing loss

Answer 23

Flat Cochlear loss: Threshold latency of click-evoked ABR shorter than observed in normal hearing. Loudness recruitment

High frequency Cochlear loss: Will introduce a general increase in response latency, compatible with time taken for traveling wave to traverse inactive basal region. If recruitment then latency will still tend to shorten towards normal values at high stimulus levels.

Question 24

Describe what effects to the ABR waveform during:

Auditory neuropathy

Answer 24

Auditory neuropathy is characterised by the presence of an OAE ( CM/SP) and an absent or poorly defined ABR (Starr etal,1996; wood etal, 1998; Rance etal, 1999)

Likely causes are dysfunction of the synaptic connections b/w dendrites and hair cells, selective loss of inner hair cells, demyelination of Aud. Nerve combined with intact OHCs, Hence now known as ‘Auditory dysnchrony’ rather than ‘Pathology’.

Question 25

What are Middle Latency Responses (MLRs)?

Answer 25

10-60 ms after a stimulus
Thought to arise mainly from the thalamus and primary auditory cortex
During this middle section, muscle activation occurs in response to the sound:
Post Auricular Myogenic Response (PAM): method of
threshold estimation. However, not always present, even in
normal subjects, so not a good clinical test

Question 26

What are Late latency responses?

Answer 26

50-500ms

Originates higher in auditory cortex

Slow Vertex Response (SVR)/ cortical responses and consists of exogenous (obligatory) and endogenous (event related) potentials.
P1/N1/P2/N2 are essentially obligatory responses determined by physical properties of evoking stimulus.

P300- Event related and Evoked by psychological processes within the brain

Mismatch Negativity (MMN). is a response reflecting detection by the brain of a change in the stimulus.