Further electrophysiology

Question 1

What are auditory steady state responses (ASSR)?

Answer 1

Evoked potential generated in the brain in response to sound

Question 2

Why is ASSR advantageous over ABR?

Answer 2

-Allow the creation of a statistically valid audiogram for people who are unwilling or unable to participant in traditional behavioural tests
-Uses objective and statistics- based mathematical detection algorithm to detect and define hearing thresholds

Question 3

Who is ASSR typically carried out on?

Answer 3

-Newborn infants for screenings and follow-up diagnostic assessments
-Babies in the NICU
-Unresponsive or comatose patients
-Patients with suspected nonorganic hearing loss
-Ototoxicity monitoring

Question 4

How is the ASSR test carried out?

Answer 4

-Two reference electrodes are placed on the mastoid bone of each ear, an active electrode on the high forehead and ground electrode on the cheek
-Present stimulus (narrowband CE chirps or pure tones) through insert earphones
-Test frequencies of 500, 100, 2000 and 4000 Hz
-Modulate the stimulus to create a steady state response in the brain by changing the amplitude, frequency or both

Question 5

What requirement needs to be fulfilled to classify an ASSR result as a clear response?

Answer 5

The hearing response must reach a defined level of certainty (needs to be larger than the background noise and occurring at an appropriate time-locked phase)

Question 6

What happens to the amplitude of the ASSR as harmonic number increases?

Answer 6

The first spectral component has the largest amplitude and the amplitude decreases after that

Question 7

What is cortical evoked response audiometry (CERA)?

Answer 7

-Technique to measure electrical potentials from the auditory cortex in order to assess hearing abilities
-Measured using surface electrodes placed on the scalp of an individual

Question 8

Describe the main components of the CERA response

Answer 8

-Two positive peaks (P1, P2) and two negative troughs (N1, N2)
-Series of peaks and troughs
-N1: 100-130 ms
-P2: 200- 250 ms

Question 9

What factors does the waveform morphometry of the cortical response depend on?

Answer 9

-Age
-State of arousal
-Attention
-Stimulus presentation parameters

Question 10

Who is the cortical response recorded for and why?

Answer 10

-Adults
-This is because the cortical response does not mature until the late teens
-Can be attempted in older children

Question 11

What are the clinical uses of CERA?

Answer 11

-Threshold estimation in adults as an objective estimator of hearing threshold
-Medico-legal cases
-Non-organic hearing loss
-Patients who are unable to provide a reliable response during PTA

Question 12

What kind of test conditions should CERA be performed in?

Answer 12

-Quiet room
-Patient should be awake but not physically active
-Can be recorded on most auditory EP systems
-One electrode is placed on vertex instead of high forehead, other two on the mastoid bones of the left and right ear

Question 13

What kind of stimulus is used for CERA?

Answer 13

Tone burst stimulus

Question 14

What is the repetition rate for CERA?

Answer 14

1-2 seconds

Question 15

What is the number of sweeps for CERA?

Answer 15

5-30

Question 16

How many replications does CERA require?

Answer 16

2-3

Question 17

How are CERA results interpreted?

Answer 17

-Appropriate morphology, amplitude and latency- need to follow expected pattern
-Repeatable (at least 2-3 replications)
-Responses to quieter stimuli should have smaller amplitudes and longer latencies
-Sufficiently high SNR so that we are able to interpret the results with a high degree of confidence

Question 18

How do CERA results compare with results from behavioral testing?

Answer 18

-Average difference of 5-10 dB between CERA thresholds and PTA thresholds
-CAEP suggests slightly greater hearing loss than PTA which needs to be taken into account when reporting results