Auditory Steady State Response (ASSR)

Question 1

What are ASSRs?

Answer 1

Similar to ABR
Provides another method of auditory threshold estimation beyond typical click and tone burst ABR

Question 2

What are other terms for ASSR?

Answer 2

Steady State Evoked Potential or Response (SSEP/SSER)
Auditory Steady State Evoked Potential (ASSEP)
Amplitude Modulation Following the Response (AMFR)
Frequency Following Response (FFR)

Question 3

What is the history behind ASSR?

Answer 3

Galambos (1981)
Discovered 40 Hz auditory potential from a amplitude modulated 400 Hz tone (modulated at 40 Hz at 70 dB SPL)
Susceptible to state of arousal
Cohen (1991) - repeated this with a higher rate of stimulation (>70 Hz), found smaller responses but were less affected by arousal states
Rickards (1994) - showed that is was possible to obtain these responses in newborns
Lins and Picton (1995) - found that is was possible to obtain auditory thresholds with rates in the 80-100 Hz range
Picton (2003) - described a basic protocol for using ASSR - the way the brain follows changes in the stimulus and rate

Question 4

When did the first commercially available ASSR systems come out?

Answer 4

2001
GSI Audera
Now offered by several manufacturers

Question 5

How does stimulus of the ASSR differ from the ABR?

Answer 5

ASSR uses continuous tone with variations in amplitude and frequency modulation
ABR uses transient stimuli, frequency specific tone burst

Question 6

How does response measurement of the ASSR differ from the ABR?

Answer 6

ASSR averaging time is locked to a period of time and sustained neural activity (fluctuations in the signal over a period of time)
ABR averaging time is locked to a stimulus and peaks of neural activity over time

Question 7

How does response detection of the ASSR differ from the ABR?

Answer 7

ASSR is based on amplitude and phase in frequency domain with an objective response detection algorithm (determines threshold on it’s own)
ABR is based on amplitude and latency in a time domain with subjective response detection

Question 8

What are the similarities between ASSR and ABR?

Answer 8

Both are auditory evoked potentials
Both record bioelectric activity from electrodes arranged in similar recording arrays
Both use acoustic stimuli delivered through insert earphones (preferably)
Both can be used to estimate threshold for patients who cannot or will not participate in traditional behavioral measures

Question 9

What are the differences between ASSR and ABR?

Answer 9

ASSR looks at amplitude and phases in the spectral (frequency) domain rather than at amplitude and latency
ASSR depends on peak detection across a spectrum rather than across a time vs. amplitude waveform
ASSR is evoked using repeated sound stimuli presented at a high repetition rate rather than an abrupt sound at a relatively low repetition rate
ABR typically uses click or tone-burst stimuli in one ear at a time, but ASSR can be used binaurally while evaluating broad bands or four frequencies (500, 1k, 2k, & 4k) simultaneously
ABR estimates thresholds from 1-4kHz in typical mild-moderate-severe hearing losses
ASSR can also estimate thresholds in the same range, but offers more frequency specific info more quickly and can better estimate hearing in the severe-to-profound ranges

Question 10

What is the maximum intensity of stimuli between ASSR and ABR?

Answer 10

Stimulus intensity level is limited to about 90 dB nHL for clicks and tone bursts used to evoke the ABR
Since ASSR is elicited by steady state (sinusoid) signals with maximum intensity levels of 120 dB HL or higher (technique of choice for determining candidacy for cochlear implantation - determining viable hearing)

Question 11

Is ASSR subjective?

Answer 11

No, it uses a statistical analysis of the probability of a response (95% CI)
ABR is highly subjective

Question 12

What is ASSR measured in?

Answer 12

Nanovolts (billionths of a volt)
Smaller than microvolts

Question 13

What are the anatomical generators of slower modulation rates (40 Hz and slower)?

Answer 13

Auditory cortex
For slower rates response becomes more endogenous and affected by patient attention / arousal / awareness

Question 14

What are the anatomical generators for faster modulation rates?

Answer 14

Brainstem
For faster rates response becomes more exogenous (obligatory) and having to do more with stimulus parameters and less w/ patient attention

Question 15

Are ASSR evoked responses correlated with the rate of stimulation?

Answer 15

Yes
It is considered a positive response if there is a significant increase in neural activity following the tonal stimulus

Question 16

What is the carrier frequency and modulation frequency?

Answer 16

Carrier frequency - continuous signal (ex. 2000 Hz)
Modulation frequency - signal within the carrier (ex. 100 Hz)
*look at change in result of the modulation to generate response

Question 17

Was the main push for ASSR to give estimated thresholds of hearing?

Answer 17

Yes
Attempts to estimate threshold of hearing using electrophysiologic measures similar to behavioral results would be (may be dB eHL for estimated HL)
Typically represented as an “estimated audiogram” with confidence range
Does not require clinician experience in waveform analysis (statistically based)

Question 18

Is there normative data for ASSR?

Answer 18

Yes, for different factors such as age and arousal
Plot of probability (95-99% then response confirmed and not random EEG)

Question 19

What are the benefits of ASSR?

Answer 19

For infants & young children can be useful for determining candidacy for cochlear implants and hearing aids
Together with tone burst ABR it can provide clinically significant information for assessment of infants or difficult to test patients
ASSR provides better estimation of thresholds for high freq HL than tone bursts (due to better frequency specificity

Question 20

Are tone bursts better at estimating low frequency thresholds?

Answer 20

Maybe

Question 21

What is the normative data for air conduction in infants?

Answer 21

500 Hz – 50 dB
1000 Hz – 45 dB
2000 Hz – 40 dB
4000 Hz – 40 dB

Question 22

What is the normative data for air conduction in adults?

Answer 22

Responses to 40 Hz stimulus on awake adults are within 10 dB of behavior thresholds
Reponses to 80 Hz stimulus on sleeping adults also correlate with behavioral thresholds – better for high frequencies than low frequencies
Correlates better to those with hearing loss than those with normal hearing (estimation varies for HL between 5 and 20 dB, estimation for normal hearing varier between 10 and 25 dB) - variance is a little less on those with HL

Question 23

What factors affect ASSR?

Answer 23

Will see maturational affect on ASSR for the first 12 months of life (immature system)
Don’t want to use low rate on young children because the earlier stuff matures faster than cortical
40 Hz stimulus should not be used on children below age of 14 (40 Hz cortical response, longer maturation than brainstem)
Premature infants have higher thresholds caused by immature auditory system
High intensity stimulation at 500 Hz responses should be interpreted with caution due to artifact potential – may be a myogenic (somatic) response similar to VEMP
Sleep
Anesthesia
Attention
Internal noise

Question 24

Is there limited data available for bone conduction ASSR?

Answer 24

Yes
Spurious responses occur due to stimulus artifact and possibility of vestibular response at frequencies below 1000 Hz (SAL technique recommended to overcome effects)
High frequency BC thresholds improve with age; Low frequency BC thresholds worsen with age
Much smaller signal than ABR so it is less sensitive

Question 25

Can you use ASSR in the soundfield?

Answer 25

Yes, because you are using both ears Better stimulus for sound field – less distortion with speakers and amplification Benefit of sound field ASSR for infants with hearing aids when behavioral data is not possible Studies support reasonable correlation between aided ASSR thresholds and behavioral data

Question 26

What is the montage for ASSR?

Answer 26

The same or similar to traditional recording montages used for ABR recordings are used for the ASSR Two active electrodes are placed at or near vertex and at ipsilateral earlobe/mastoid with ground at low forehead If collecting from both ears simultaneously, a two-channel pre-amplifier is used When single channel recording system is used to detect activity from a binaural presentation, a common reference electrode may be located at the nape of the neck

Question 27

What transducer is preferred for ASSR?

Answer 27

Insert earphones Can use headphones, bone oscillator, or SF

Question 28

What high pass and low pass filters are used for ASSR?

Answer 28

The high pass filter is approximately 40 to 90 Hz and low pass filter might be between 320 and 720 Hz with typical filter slopes of 6 dB per octave (unlike ABR)

Question 29

What is the common gain settings of ASSR?

Answer 29

10,000 Needs to be very large because these are very small responses

Question 30

Is the artifact reject left on for ASSR?

Answer 30

Yes, don't typically need to touch it It is thought to be advantageous to have manual "override" to allow clinician to make decisions during test and apply course corrections as needed

Question 31

What are some tips and tricks?

Answer 31

Montage: Adult: Fz – neck (or inion to avoid muscle artifact) midline placement and/or ipsilateral mastoid Infant: Fz – ipsilateral mastoid Patient should be asleep or comfortable as possible Approach like audiogram – 500 Hz, 1kHz, 2kHz If threshold questionable, repeat w/ intensity increase of 5 dB

Question 32

How does analysis take place?

Answer 32

It is mathematically based and dependent upon the fact that related bioelectric events coincide with the stimulus rep rate The specific method of analysis is based on the manufacturer's statistical detection algorithm It occurs in the spectral domain and is composed of specific frequency components that are harmonics of the stimulus repetition rate Early systems only relied on the first harmonic

Question 33

Are there correction tables for converting ASSR thresholds to estimated HL audiograms?

Answer 33

Yes Found to be within 10 dB to 15 dB of audiometric thresholds Correction data depends on variables such as: equipment used, frequencies collected, collection time, age of subject, sleep state of subject, stimulus parameters

Question 34

What are the clinical advantages of ASSR?

Answer 34

Frequency-specific signals are employed for estimation of thresholds at frequencies from 250 Hz to 8000 Hz Frequency-specific auditory thresholds can be estimated with air conduction and bone conduction signals Stimulus intensity levels up to120 dB HL can be used to elicit frequency specific thresholds The ASSR is, therefore, useful for assessment of severe to profound degree of hearing loss in infants and young children ASSR detection and analysis is automated and statistically based. Clinician experience in analysis is not necessary

Question 35

What are the clinical disadvantages of ASSR?

Answer 35

ASSR requires a very quiet state of arousal. Movement artifact and interference may preclude testing or may invalidate results with overestimation of actual auditory threshold levels in young children who are not sleeping ASSR usually requires that the patient sleep naturally or with sedation. Anesthesia is sometimes necessary for valid ASSR assessment of hearing sensitivity The influence of deep sedation and anesthesia on the ASSR evoked by high modulation frequencies (e.g., > 60 Hz) requires further investigation. Sedation and anesthesia invalidates threshold estimations for ASSR evoked with slow modulation frequencies (e.g., < 60 Hz) Modest discrepancies between ASSR thresholds and either behavioral and/or ABR thresholds are reported in the literature Discrepancies between ASSR thresholds and behavioral thresholds are possible for patients with conductive hearing loss Estimation of ear-specific thresholds with bone conduction signals requires the use of masking to the non-test ear Unlike ABR, there is no biological marker for test ear with ASSR There is little site-specific information for patients with hearing loss since the ASSR waveform cannot be analyzed ASSR cannot be used to differentiate sensory versus neural auditory dysfunction Absence of ASSR does not differentiate between profound sensory hearing loss versus ANSD (cannot see the waveform, cannot determine site of lesion) In the USA there is no current procedural terminology billing code specifically for ASSR

Question 36

What are common misconceptions?

Answer 36

ASSR and ABR are not competitive - the decision clinically is not to record tone burst ABR OR to record ASSR The two procedures are complementary

Question 37

Why were ASSRs made?

Answer 37

To get another objective test to use on our patients

Question 38

What are limitations of acoustic reflexes?

Answer 38

Difficult to obtain in patients below 6 months of age Can predict the presence or absence of a disorder Not useful in severe-to-profound hearing loss

Question 39

What are the limitations of OAEs?

Answer 39

Can predict only the presence or absence Confounding influence of ear canal and middle ear disorders Not useful in assessing losses greater than mild Only assesses peripheral, may be cortically deaf

Question 40

What are the limitations of auditory evoked potentials (ECochG, ABR, and Middle and Late Responses)?

Answer 40

Generated by a transient acoustic stimuli (click or tone burst) This results in reduced frequency specificity The above are not present in severe-to-profound hearing loss

Question 41

What stimuli does ASSR use?

Answer 41

Pure tones that are amplitude and/or frequency modulated When modulation frequencies of greater than 60 Hz are used, the ASSR characteristics are similar to those for the ABR

Question 42

What are the current ASSR systems called?

Answer 42

Steady-state evoked potentials (SSEPs) or amplitude-modulation following response (AMFRs) Because the acronym SSEP can be confused with “somatosensory evoked potentials” the term “Auditory Steady-State Response (ASSR) has been adopted *but typically referred to as auditory steady state responses

Question 43

What is the spectrum of stimulation with ASSR?

Answer 43

Major energy at the carrier frequency (CF) and side bands at CF minus the Modulation Frequency (MF) and at CF plus MF Ex: the spectrum of a 1 kHz tone modulated at 100 Hz maintains a peak at 1 kHz with side bands at 900 and 1100 Hz Creates harmonics that are detected by the system - also makes the signal longer (making it more frequency specific)

Question 44

How does frequency modulation affect the responses?

Answer 44

Lower or higher frequency modulations can drive where that response will reach (more toward he cortex or brainstem) Higher modulation rates will make it more like ABRs

Question 45

Is there some spread of acoustic energy no matter how you modulate it?

Answer 45

Yes When Frequency Modulation (FM) is used in combination with Amplitude Modulation (AM), there is some spread of acoustic energy encompassing the amount of FM That is what is used for detecting the signal

Question 46

What do modulation rates of higher than 20 Hz but lower than 60 Hz result in?

Answer 46

Response characteristics that are similar to those found in middle latency responses (MLR) Generators though to be in the auditory midbrain, thalamus, and primary auditory cortex

Question 47

What do modulation rates of higher than 60 Hz result in?

Answer 47

Dominated by evoked potential from brainstem sites, including wave V of ABR

Question 48

What are generally the clinical modulation rates that are used for ASSR?

Answer 48

Higher than 80 Hz This makes the response comparable to the ABR Wave V response This allows for us to estimate hearing sensitivity with more frequency specificity

Question 49

What are the typical carrier frequencies of ASSR testing?

Answer 49

500 Hz 1000 Hz 2000 Hz 4000 Hz

Question 50

Does ASSR also have a limitation of subjective nature of response detection like ABR?

Answer 50

No The techniques used to detect the ASSR rely on measures phase coherence, magnitude-squared coherence, of Fast Fourier Transform (FFT) of the response subjected to a variance ratio test

Question 51

What is phase coherence?

Answer 51

Related to signal to noise ratio The basic concept is that the phase of the response is measured relative to the phase of the Modulation Frequency (MF) These responses stack on top of each other (successive signals) What raises the signal over the noise floor

Question 52

Does the modulation stimuli create some changes in the EEG?

Answer 52

Yes The computer can pick up these variations and analyze them Calculates probability of the response *no change with carrier tone and modulation itself, but can pick up the change when they are presented together

Question 53

What does low coherence and high coherence mean?

Answer 53

When the sampled EEG phase distributions are out of phase (random), there is low coherence, with values close to 0 (No Response) When sampled EEG phase distributions are in phase, coherence values will be closer to 1, indicating that a response is present The computer software calculates this for you and gives you the phase coherence values for each test frequency and intensity used

Question 54

May ASSR be better at estimating thresholds than toneburst ABR?

Answer 54

Maybe Tends to be in some of the research

Question 55

Is ASSR useful for electrophysiological assessment of severe to profound HL in infants and young children?

Answer 55

Yes Automated response detection and analysis (i.e., experience in waveform analysis is not necessary)

Question 56

Does ASSR require a very quiet state from the patient?

Answer 56

Yes Movement artifact (interference) is likely to produce invalid results, or overestimation of actual auditory threshold levels Sedation is required with infants and young children The influence of deep sedation and anesthesia on the ASSR evoked by high modulation frequencies (e.g., > 60 Hz) requires further investigation Sedation and anesthesia clearly affect the ASSR for slow modulation frequencies (e.g., < 60 Hz)

Question 57

Is the max effective intensity level of ABRs limited due to their very brief duration?

Answer 57

That is, due to their short duration, less energy is delivered to the auditory system The short duration can also reduce frequency specificity However, it is largely solved by the use of sophisticated equations (e.g., Blackman) for ramping or shaping of the signal onset

Question 58

Do you have to worry about ramping with ASSR?

Answer 58

No As its name implies, the ASSR is generated with steady state (ongoing sinusoidal rather than transient) acoustic signals The inherent limitation of maximum intensity associated with the transient signals does not, therefore, apply to ASSR signals