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ABR

Answer: This is a fascinating question which has occupied the audiological community for some time now, because in theory both techniques arise from the same neural generators but in practice there have been discrepancies in test performance (speed, efficiency and accuracy of threshold estimation).

The reasons for the discrepancies in the past are complex but a large part of the explanation centres on whether one or both techniques have used optimal stimulus and recording parameters. If this is not always the case, this could explain performance differences on direct comparison. Other reasons might relate to historical familiarity with the ABR, including the training materials available for audiologists and their equipment (e.g.with evoked potential platforms that have traditionally been centred on ABR testing in terms of software layout and functionality, for example).

On the other hand, ASSR offers clear advantages in terms of potential time savings by presenting test stimuli simultaneously, and by providing a fully objective outcome which is not exposed to errors of interpretation in the same way that ABR can be.

What is reassuring from the clinical perspective is that these two separate but related evoked potentials can now be used either interchangeably or in a mutually supportive manner according to the needs of the patient or other circumstances, giving the audiologist great flexibility in decision making.

Addendum, April 2018

A recently published multicentre trial (Sininger et al., 2018). has compared the 90 Hz ASSR and ABR in the clinical setting using the same CE-Chirp® stimuli. This is a crucial point as, although the stimuli rate is different for a transient versus a steady state response, the other features of the stimuli are the same between the two approaches. Other features of the recording and clinical protocol are described in detail in the paper. The results showed that average test time was around 20 minutes for the ASSR (for thresholds predicted at four test frequencies in each ear) but just over 30 minutes for the ABR to provide the same information. As mentioned in the earlier answer, it is also the case that the ASSR is fully objective whereas the ABR retains a degree of subjectivity related to tester interpretation of waveform morphology (and therefore potentially tester bias), although tools for objective waveform interpretation are available in the Eclipse ABR test protocols (e.g. the Fmp statistic, automated residual noise calculation and waveform reproducibility (cross-correlation) analysis). All in all, at least based on test time for audiogram prediction, these results would tend to support use of ASSR. Part of the reason for the lower test time is that stimuli of different frequencies can be presented simultaneously in ASSR but sequentially in ABR.

On the other hand this should not be taken to mean that the ABR is in any way obsolete or out of favour. It seems likely given the weight of research and development, the extensive evidence base and highly developed clinical protocols and the provision of training and education in ABRs, they will continue to play a central role in audiological evaluations alongside the ASSR.

What is reassuring from the clinical perspective is that these two separate but related evoked potentials can now be used either interchangeably or in a mutually supportive manner according to the needs of the patient or other circumstances, giving the audiologist great flexibility in decision making.


References and caveats
Sininger, H., Hunter, L., Hayes, D., Roush, P., and Uhler, K. (2018) Evaluation of speed and accuracy of next-generation Auditory Steady State Response and Auditory Brainstem Response audiometry in children with normal hearing and hearing loss Ear and Hearing.- Publish Ahead of Print

To learn more about ABR watch the following webinar 

To learn more about ASSR watch the following webinar 

May 2016

We tested a 7-month old baby with a bilateral conductive hearing loss. We wanted to check whether we need to mask or not. The not-masked hearing thresholds by BC were 15 dB nHL and the AC thresholds bilaterally were 40 dB nHL. We want to know what the BC threshold is on the other side?

Answer: This is potentially very complicated because, just like in conventional audiometry, a bilateral conductive loss raises the prospect of cross masking (and therefore masking dilemma) because of the need to increase the masking level in the non-test ear to overcome the attenuation from the conductive component.

The amount of masking noise depends on what you estimate the conductive loss to be in the non-test ear (ref: the BC threshold).  A masking calculator and guidance can be downloaded by pressing the link here.  You could input the values above to calculate the required masking. Depending on the estimated size of the air-bone gap, the maximum masking level possible (for the Eclipse) might be reached.

Although the patient is 7 months old, it could be worth checking back to any earlier records for ABR data in the neonatal period to help answer the question. At earlier ages the BC values are more ear specific as the plates of the cranium are not fused. This could mean the BC thresholds give ear specific information even without applying masking,


To learn more about ABR Masking please watch the following webinars 

Masking the ABR Part 1 

 Masking the ABR Part 2

 

 

Downloads 
ABR Masking Calculator 

January 2017

As the name suggests the CE-Chirp® family is not a single stimulus but instead a family of short duration acoustic stimuli which can be used in evoked potential testing. They were designed with the objective of increasing the amplitude of the auditory brainstem response. Because of this the CE-Chirp® family are primarily used when measuring hearing threshold. 

The CE-Chirp® family of stimuli achieves larger wave V amplitudes by providing the cochlea with optimal stimulation of the basilar membrane which results in synchronous neural firing. To understand how this works it is important to understand the anatomy and physiology of the cochlea. The cochlea is tonotopically organised so that the high frequencies are detected at the base and the low frequencies detected at the apex. This means that if all frequencies are presented to the cochlea at the same time (like with a click stimulus) the high frequency components of the stimulus will be detected by the basilar membrane first followed but the mid frequency and lastly the low frequency components. This will result in neural firing across the frequency range which is not synchronous. 

A CE-Chirp® stimulus counteracts the temporal dispersion in the normal cochlea by presenting the low frequency energy content of the stimulus before the high-frequency energy. The design of the traditional CE-Chirp® is described in detail by Elberling et al. (2007a), and has the same power spectrum as a standard click ( i.e the same frequency content and the same amplitude, just a different timing relationship). In the study by Elberling and Don (2008), it was demonstrated that the CE-Chirp® ABR is up to 1.5-2.0 times larger than the corresponding click ABR in normal-hearing subjects. The practical consequence of this is that responses of a desired signal to noise ratio are obtained in a shorter test time (or that in a fixed test time, the response will have a higher signal to noise ratio) when a CE-Chirp® is used rather than a traditional click. 

Since the development of the traditional CE-Chirp® in 2007, there has been much research centred on this stimulus and it has been further optimised. Firstly was the release of the Narrowband CE-Chirps® which provides a set of frequency specific stimuli at 500 Hz 1000 Hz 2000 Hz and 4000 Hz. These can be used as alternatives to tone burst stimuli. The NB CE-Chirps has been applied in several new-born hearing screening programs and for this type of hearing testing it has been demonstrated that the NB CE-Chirps are more efficient than tone burst (Ferm et al. 2013).

Lastly in 2015 the CE-Chirp® LS family was release. LS stands for level specific. Claus Elberling, the founder of the CE-Chirp® revealed that the stimulus could be further optimized for each intensity. Therefore CE-Chirp® LS was released which provides a different stimulus for each 5 dBnHL step from 0- 100 dBnHL. This means that CE-Chirp® family of stimuli now has twenty CE-Chirp LS Stimuli in addition to  4 frequency specific NB CE-Chirp LS. 


For more information check out these 15 minute e-learning sessions on this topic 

CE-Chirp® Theory           

Narrow band CE-Chirp® Stimulus family 

 

November 2017

I want to ask a question about the article from Ferm et al., 2013 titled: “Comparison of ABR response amplitude, test time, and estimation of hearing threshold using frequency specific chirp and tone pip stimuli in newborns”

During the IA Academy presentation on youtube, she is telling that, which also exists in your CE-Chirp® overview presentation; Because the response is larger, it is seen to lower levels, so the nHL – eHL correction is 5dB less than for tone pips (Ferm et al. 2013, Ferm and Lightfoot 2015). Is this information also valid for Tone Burst Stimulus vs NB CE-Chirp®?

Answer: In the CE-Chirp® webinar presented by Inga Ferm, which describes two key articles (Ferm et al. 2013, Ferm and Lightfoot 2015), Inga is focussing on the narrow-band (NB) family of CE-Chirp® stimuli. So the direct answer to your question would be yes, the info is valid for NB CE-Chirp® stimuli.

But please note that when Ferm uses the phrase “tonepip” then we can take that to mean toneburst i.e. a toneburst of short duration (2-1-2 cycles) suitable for ABR testing. It is a small point of confusion since in ABR testing the words “tonepip” and “toneburst” are often used interchangeably. Her meaning is clear from the context of the study. I mention this as your question asked if Ferm’s tonepip versus NB CE-Chirp®  comments are also valid for toneburst versus NB CE-Chirp® – I think you’re talking about the same thing. 

I shall use “toneburst” in the below.

What Ferm does is compare the performance of tonebursts and NB CE-Chirps® by measuring the ABR first using the traditional tonebursts (Ferm et al. 2013 = 4 kHz and 1kHz; Ferm and Lightfoot 2015 = 0.5 kHz and 2 kHz), and then repeating the procedure using the NB CE-Chirps® centred at the same frequencies. The studies compare ABR amplitude, Fmp, estimated threshold and residual noises between the respective stimuli. 

One of the key findings is that the NB CE-Chirp® results at each frequency gives a greater amplitude than the equivalent toneburst stimuli (hence lower test time/fewer inconclusive findings and higher Fmp and response confidences). The higher amplitude NB CE-Chirp® ABRs also meant that the measured threshold was lower than for the tonebursts i.e. the results were closer to the true behavioural threshold, meaning the nHL-eHL correction factor can be smaller.

As an example, imagine a patient with a true behavioural threshold of 30 dB HL at 4 kHz. We measure their ABR using tonebursts and the ABR might disappear into the noise floor at 40 dB nHL, say (not uncommon). We would then apply the nHL-eHL correction factor of 10dB, to give us a threshold of 30 dB eHL (which is a good estimation of the true threshold). If we now measure the ABR again using NB CE-chirps we would expect the ABR to disappear at a lower level, like 35 dB nHL, because of the advantages of greater response amplitude offered by the NB CE-Chirp® signal. So now since we are closer to the true threshold the correction factor should be lowered accordingly i.e. 5 dB - we apply this new correction factor to again arrive at 30 dB eHL.

But please note it is very important that the correction factor is only applied when we reach a threshold. Otherwise we could run into difficulties.

Imagine for example that someone had a normal hearing threshold, 0 dB HL. We might measure their ABR using toneburst and when we reach down to say 20 dB nHL we would probably stop further testing in many cases (as we would already have enough information to show hearing is in the normal range). If you then apply the 10 dB nHL-eHL correction factor for toneburst you would arrive at an eHL value of 10, which is not a very good estimation of their true hearing threshold (i.e. 10 dB above the true threshold). 

If you followed the same testing logic with NB CE-Chirps® and also stopped at 20 dB nHL and then applied the chirp correction factor you would arrive at an eHL value of 15, which is now a greater error than the toneburst estimation. Of course it would not be a clinically significant issue in this example since both estimations are still within the normal range, but it is worth bearing this aspect in mind. Also, remember you have not “lost” anything in terms of accuracy with the CE-Chirp® by stopping at 20 dB nHL since we chose to stop early. And, despite the greater eHL error that you could get if you chose to do this, you would still be at an advantage with in the sense that you would have got to that stage in the procedure more quickly than with the toneburst. 

You can also download a spreadsheet which automatically converts ABR threshold in nHL to the corresponding eHL value according to whether CE-Chirps® or tonebursts were used.  click here to download the sheet. 

 


References and caveats
Ferm, I. et al. (2013) Comparison of ABR response amplitude, test time, and estimation of hearing threshold using frequency specific chirp and tone pip stimuli in newborns. IJA 52: 419–423

Ferm, I. and Lightfoot, G. (2015) Further comparisons of ABR response amplitudes, test time, and estimation of hearing threshold using frequency-specific chirp and tone pip stimuli in newborns: Findings at 0.5 and 2 kHz. IJA 54: 745–750 

Igna Ferm explains more about the CE-Chirp® family in this webinar

 

November 2017

Both of these auditory evoked potentials responses arise from generators in the thalamo-cortical region of the central auditory system and so you would associate them with objectively assessing thresholds in adults e.g. medico-legal or non-organic hearing loss, or in those with learning difficulties or other challenges that preclude behavioural threshold finding techniques.

Either approach would be perfectly feasible and acceptable. At least two peer-reviewed studies have compared the techniques for time, accuracy and other clinical considerations (Van Maanen and Stappells 2005; Tomlin et al 2006). Although of similar study design, they make opposing recommendations. Van Maanen and Stappells (2005) suggest that 40 Hz ASSR would be the test of choice whereas Tomlin et al (2006) suggest that the N1-P2 response would be the test of choice. Although of similar design, some elements of the study designs may be factors leading to these conclusions. For example, Van Maanen and Stappells provide clear objective “stopping” criteria for the ASSR but not the N1-P2 responses (“by eye”).

What follows is a short list of some of the positive and negative points of the 40 Hz ASSR when set against the N1-P2 response as a way to objectively estimate hearing threshold in adults:

  • ASSR is a fully objective response detection method
    • Eliminates tester bias
    • Reduces workload
  • ASSR allows multiple simultaneous stimuli: thus theoretically faster than N1-P2 (x2-3)
  • High arousal (patient awake) is desirable for both ASSR and N1-P2 measures, but ‘adaptation’ of response is not relevant in ASSR.
  • Stimulus interleaving/switching is not necessary in ASSR, but may be useful in N1-P2.
  • CE-Chirp stimuli used in ASSR produces more synchronously active neurons, which should produce a more robust physiological response but one would expect lower frequency specificity than tonebursts used in N1-P2 measures. When differentiating sharply sloping or “notched” audiograms (common through noise exposure, for example) the frequency specificity of long-duration (e.g. 80ms) tone bursts may be desirable.
  • ASSR has (arguably) a lower legal precedent than N1-P2 in some regions, which may be relevant in medico-legal scenarios.
  • At high stimulus levels, multiple simultaneous stimuli in ASSR may lead to unwanted cochlear interactions. In N1-P2 testing multiple stimuli are not presented simultaneously so this is not a factor.
  • Although ASSR is fully objective, the tester cannot utilise waveform morphology in decision making. 

References and caveats
Van Maanen, A and Stappells, DR. (2005) Comparison of multiple auditory steady-state responses (80 versus 40 Hz) and slow cortical potentials for threshold estimation in hearing impaired adults. IJA 44, 613-624.

Tomlin D et al. (2006) A comparison of 40 Hz auditory steady-state response (ASSR) and cortical auditory evoked potential (CAEP) thresholds in awake adult subjects. IJA 45, 580-588.

To learn more about ASSR watch the following webinar 

To learn more about the N1-P2 response watch the following webinar 

May 2016

The Cochlear Microphonic (CM) has been found to be dominated by hair cells located in close proximity to the recording electrode, near the base of the cochlear, regardless of stimulus frequency. Whilst a tone will generate greatest excitation of the basilar membrane at the characteristic frequency of the stimulus, it has been found that phase-rotation for hair cells at apical regions of the cochlear causes the vector sum of their CM to be much less than if the hair cells were orientated in the same direction, as occurs at the base of the cochlea. Therefore the CM is dominated by contributions of outer (and inner) hair cells within the basal region of the cochlea regardless of stimulus frequency. For more detail regarding these concepts, please see Withnell (2001).

The Eclipse uses 30 kHz sampled sound files, so this implies a maximum of 15 kHz sound stimulation is possible without risk of aliasing.  


References and caveats
Withnell,  R.H.  (2001)  Brief  report:  The  cochlear microphonic  as  an indication of outer hair cell function. Ear and Hearing, 22(1), 75-77.

This video tutorial explains how to record the cochlear microphonic 

 

December 2016

The Eclipse can accept external triggering so this is an avenue you could pursue i.e. provide acoustic stimulation via the direct audio input and simultaneously trigger the Eclipse. Usually the triggering is associated with eABR and eALR (direct input to a cochlear implant electrode array) and so the Eclipse software has some pre-loaded protocols for these applications but there’s no particular reason you couldn’t use/adapt them to suit a direct input to a hearing aid or other device too.

The details of the triggering signal are provided in the “Additional Information – Eclipse” handbook.

December 2017

I have heard of the ‘Binaural Interaction Component’. What is this and how can it be measured using the Eclipse?

Answer: The binaural interaction component (BIC) an evoked response used to probe the binaural auditory system. Measuring it consists of a three-step process. One would need to stimulate monaurally to the left and right ears, and binaurally. The difference in response between the summed monaural responses, and the binaural response, is the BIC, and is expressed via the following equation. 

The binaural response is of smaller amplitude than the sum of the two monaural responses, and this occurs at latencies of less than 10ms, while extending to tens of milliseconds. This indicates that binaural interaction involves inhibitory processes occurring as early in the auditory pathway as the brainstem and extending through to cortical regions. For a detailed review, please refer to McPherson and Starr (1993). 

The Interacoustics Eclipse is able to stimulate monaurally (left and right) as well as binaurally and these options are available for ABR, AMLR and long latency evoked responses amongst others.

To find the BIC, the three waveforms would be exported1 in order to obtain the sum of left and right traces, and the differential between this sum and the binaural trace.


References and caveats
1 Exporting waveforms requires the Research Module, which also allows logging of sweeps and importing custom sound stimuli. 

McPherson, D.L., and Starr, A. (1993) Binaural interaction in auditory evoked potentials: brainstem, middle- and long latency components. Hearing Research, 66 (1), pages 91-98

November 2016

Yes.

Bone-anchored hearing system (BAHS) patients are typically those with a conductive (or mixed) or single-sided sensorineural deafness. Either of these two categories of patient can be tested either with conventional bone-conducted stimuli (e.g. through a Radioear B71 or B81 bone conductor), or their prosthetic device can be stimulated directly, and the evoked potential system merely measures the response (but does not provide the stimulus). This latter approach has some similarities to that of eABR (electrically evoked ABR) associated with cochlear implant patients.

September 2017

The prefix “nHL” conventionally refers to sound level (dB) of a signal referenced to the thresholds for a group otologically normal hearing people – it is the same concept as dB HL, however nHL is often associated with a short duration signal that has been calibrated using the peak-to-peak equivalent SPL method (via an oscilloscope). Short duration signals are of course used in objective hearing assessment via evoked potentials like ABR, ASSR, AMLR and ALR (as well as other applications like OAE and WBT). eHL is the estimated behavioural hearing level.

Typically, the behavioural hearing threshold is lower than the threshold given by the objective hearing assessment (e.g. ABR, AMLR, ASSR, ALR). Therefore, a correction factor is applied to the objective hearing threshold to better estimate the (all important) behavioural threshold.

Temporal integration is one factor that gives rise to the nHL-to-eHL difference. Temporal integration is the way in which the auditory system integrates energy over time and is particularly relevant for short duration sounds below about 200ms. (This encompasses most sounds used in objective hearing assessment.) However, the pure tones used in behavioural pure tone audiometry are typically much longer than 200ms (e.g. several seconds in duration), so these sounds produce greater temporal integration which acts to increase loudness and lower threshold at an equivalent level (perceptual threshold improves by about -3dB per doubling of sound duration up to 500ms according to the Zwislocki model).  There might be other factors influencing the nHL-to-eHL correction too. For example, the signal presentation rate in the EP recording (which also influences temporal integration, particularly at faster rates), the amount of residual noise in the EP recording, and calibration differences in the ear of the patient. This last feature is of particular relevance when using insert phones amongst infant populations, since the behavioural hearing level is referenced to that of otologically normal adults. Other relevant factors might include the electrode montage, the anatomy of the head and orientation of the neural generators, and arousal (particularly in auditory evoked potentials arising from the cortex).

June 2016

There are indeed several approaches. The fully objective and perhaps easiest method is to use the automated residual noise calculator. This feature analyses the sweep-to-sweep variability of the measured and provides a numerical and graphical display of the residual noise. The lower the variance, the more “stable” the averaged waveform becomes.

Here is an example of the graphical and numerical display. The y-axis shows residual noise (0-200 nV), and the x-axis shows number of sweeps (0-8000). A target value of 40 nV is shown by the arrow.

As the number of sweeps increases the variability decreases (more noise is averaged away), showing a curve that decreases rapidly, but then begins to asymptote. This is perfectly normal and the reason for the asymptote is because the remaining noise in an average trace decreases as a function of the square root of the number of trials.

This popular statistical method to evaluate noise is available for the most commonly used evoked potential application – threshold estimation in infants using the Auditory Brainstem Response.

There are other methods also in common use, and could be used during ABR measures but would be relied upon in applications where the automated residual noise calculator is not available, such as the Auditory Middle Latency Response, the Auditory Late Response and Neuro Latency/Rate exams (i.e ABR for differential diagnosis of retrocochlear pathology).

One approach is the “average gap” method, demonstrated below for an N1-P2 complex of the Auditory Late Response.

The lower (blue) trace is an average response of 50 presentations of a 2 kHz toneburst signal; 25 condensation sweeps and 25 rarefaction. These two sub-averages are shown above (A+B curves), with their baselines overlaid. If the noise was zero and these two sub-averages contained the evoked potential only (a hypothetical scenario) then they would overlay perfectly. Any differences between them therefore represents the residual noise. The “average gap” method involves estimating (“by eye”) the difference between the trace by observing the gaps between the trace. The average gap of these A+B curves is shaded for illustration. It is a semi-objective approach since the results are dependent on the interpretation of the observer, although they are based on objectively obtained data.

 

A related method is via the differential (A-B) curve. This subtraction curve will be smaller amplitude (a “flat” trace) when the noise is low and greater in amplitude (a “wavy” trace) when the noise is high. The amplitude of the waveform (e.g. how much it deviates from the baseline) thus gives an objective indication of the residual noise. For example, below is the same data as the previous example but instead of the A and B curves displayed overlaid, the differential curve (A-B) is displayed overlaid onto the average trace. It may be noticed that where the “gap” is large in the previous example, the differential curve deviates far from the baseline whereas when the “gap” is small, the differential curve remains close to the baseline.

In the final example, below, we see an ABR measured in an infant at 40 dB nHL using the CE-Chirp stimulus in the left ear. The child has a mild hearing loss and the response here is close to threshold. However, a clear wave V is nonetheless present, as demonstrated by the waveform markers showing a faint but clear wave V; the data have an Fmp value of 11.05.

Crucially the residual noise is low with an automated indication of 7nV, very little gap between A+B curves and a “flat” differential curve.

 

To learn more about the Residual noise calculator in the Eclipse we recommend you watch the following webinar

October 2017

The HiLo CE-Chirp® is a stimulus which is used in automated ABR or ABRIS (auditory brainstem response infant screening).  Hearing screening programs typically recommend to screen hearing initially using a broadband stimulus such as a click or a CE-Chirp®. A problem with such stimuli is that although they are effective for eliciting an evoked response they are limited with respect to their frequency specificity. 

The HiLo CE-Chirp® provides clinicians with a more frequency specific stimulus which can be used in new born hearing screening. Its design is based on the CE-Chirp® which has a frequency range of 200 – 11,000 Hz. However the HiLo CE-Chirp® is actually two separate stimuli; a low frequency dominated stimulus and a high frequency dominated stimulus. 

The lower stimulus (Lo) covers the frequency range from 200 Hz - 1.5 kHz. Whereas the higher stimulus (Hi) covers the frequency range 1.5 kHz – 11,000 Hz.  In the software the screener will see two graphs which are generated for each ear. One for the Hi stimulus and one for the Lo. 

The above graph shows the display of the HiLo Stimulus in the Titan Suite. The Hi stimulus is shown in the column labelled high and the Lo stimulus is shown in the column labelled low. 

Typically when using the HiLo CE-Chirp® the Hi stimulus will pass quicker than the Lo because it less vulnerable to the effects of ambient noise.  

November 2017

This is a useful aid in interpreting the quality of evoked responses that are seen after gathering and then averaging together a series of epochs in order to cancel the noise. Examples include ABRs, long latency responses, ECochGs and OAEs.

The recorded EEG signal (or acoustical signal, in the case of OAEs) consists of the response of interest (this is time locked to the stimulus) and the randomised noise. Averaging enough epochs will cause the noise to cancel, leaving the signal of interest.

A key question is how reliable is the averaged data. Does it accurately reflect the underlying evoked response of interest, or is it too contaminated with noise to be a reliable reflection? Of course, the more reliable it is, the greater confidence one has with its interpretation and therefore clinical decision making such as when estimating threshold or making other diagnoses. 

The wave reproducibly measure helps establish the reliability of the measurement. Individual epochs are allocated alternately into two separate memory banks; the A and B buffer. A mathematical calculation known as cross-correlation between the responses in each buffer is then performed, and this provides a measure of the similarity of the two sub-averages; A and B.  If a response is reliably present, and the residual noise is sufficiently low, the similarity between A + B data i.e. the wave reproducibility will be high. 

If there is no signal present, and/or the residual noise is not sufficiently low, then the wave reproducibility will be low.

Please note that in this sense, when the waveform reproducibility is high it helps increase tester confidence in interpreting a response as being present. However, when the waveform reproducibility is low then this information alone is not enough to differentiate between an absent response and an inconclusive result whereby the residual noise is too high to make a firm conclusion on presence or absence. 

Another feature must be used in conjunction with wave reproducibility to differentiate between an absent evoked response or inconclusive findings. Checking the differential (A – B) curve is one method. This provides an indication of the residual noise whereby a flat curve indicates low residual noise. Simply “eyeballing” the difference between A + B traces provides a similar, albeit subjective, indication.

More precise indications of residual noise are available when performing ABR measures where a precise value of residual noise is obtained.

For more information please refer to the Interacoustics Eclipse user manual and Additional Materials handbook.

February 2017

Impedance is the opposition to flow of alternating current, like that generated by the bioelectrical neural activity of the brain. 

It has two components, resistance and reactance. With regards to electrode impedance, the main interface between equipment (i.e. electrode) and human is the skins surface. The voltage fluctuations that we measure are transmitted (or one might say conducted) through the fluid of the body. The current can also relatively easily flow through the wires and circuitry of the instruments that we use. However, the interface is where the challenge lies – the surface of the skin. When we place the electrode on the surface of the skin, the current flow is impeded by dead skin cells and other impurities.

In order to overcome this, what we do is scrub the skin clean and use conducting gel which acts as a capacitor/resister…and we ‘lower’ the impedance i.e. we bridge the interface between fluid filled tissues of the body, and the instruments.

Impedance is usually measured by passing a known, low-level current through the circuit (including the interface with the skin) and measuring the opposition to the flow of this current.

October 2016

Let us try to unpack some of the issues raised by this interesting question. 

The first thing to bear in mind is that the units for masking ABR stimuli have to be in SPL – this is because a white noise masker is required to effectively mask the relatively wide bandwidth ABR stimuli (e.g. click sounds are broadband, as are CE-Chirp. Other signals like toneburst that are more frequency specific exhibit spectral splatter due to their short duration, and the bandwidth of narrow-band CE-Chirps cover several hundred Hz). Since there is no nHL (or even HL) correction for white noise the unit of level is SPL. 

The level of masking noise required depends not on comparative loudness in relation to the stimulus (which is presented to the test ear), but on the level of masker in the non-test ear required to mask out any cross-heard sound in the non-test ear. Based on the study by Lightfoot et al (2010), we know what these levels are.


References and caveats
Lightfoot, G., Cairns, A., and Stevens, J. (2010). Noise levels required to mask stimuli used in auditory brainstem response testing. International Journal of Audiology, 49:, pp794-798.

December 2016

I wanted to know whether the Eclipse can be used to do ABRs on children that are not sedated (e.g. a 4-year old looking at an iPad). Is sedation essential with children when using the Eclipse? 

Answer: The Eclipse can do ABR and this procedure can, in principle, be applied to patients of any age.

This is fine, except that the vast majority of patients we see are not in the age group where awake ABRs are necessary. We typically see either infants who sleep naturally, or we see adults who can be relied upon to cooperate and lie still (and perhaps nod off). The children in the age group we are talking about here (let us say 2+ years) are less frequently seen for ABRs because most can perform behavioural testing quite easily (like VRA, play audiometry) and behavioural testing is always preferable to ABRs. 

With ABR testing it is always easier to have a quiet (asleep) patient and far harder with an awake patient because of the greater myogenic noise (through movement and so on) which impacts negatively on the signal-to-noise ratio. If an ABR is definitely needed in a child at this age then one option is to do the test during periods of natural sleep (e.g. pay them a home visit in the evening and test the child when they are in natural sleep at home in bed) or if that does not work then it might be necessary in some cases to resort to sedation.

Aside from these ideas, there are a number of features on Eclipse that help to make recording in adverse recording conditions more practical.

If the noise is aperiodical, like movements when you have a patient who is not always lying completely still throughout the recording process then perhaps the most important feature to aid recording is the Bayesian weighted averaging. This will reduce the influence of sweeps that contain movement artefacts. In addition of course, you have the artefact reject options, which can be fine-tuned at any point during testing. Do not forget also that using the CE-Chirp® stimuli will maximise SNR, but through increased signal rather than decreased noise.

There is also the option of performing tests that do not rely on the patient being asleep, such as 40 Hz ASSR or the ALR (N1-P2 cortical evoked potentials) although this procedure is not usually applied to children below around 10 years old for threshold estimation as the response goes through a long maturational period through childhood and up to young adulthood.

September 2017

It is absolutely key that any masking needs are met for accurate diagnosis and management. The principles of masking ASSR are much the same as the ABR  in terms of the rules of masking and the potential for interaural cross-over of sound (see ABR masking webinar below).

The main difference is that in ABR we only test at one frequency at a time, whereas in ASSR we might test up to 4 frequencies simultaneously. So, the potential for cross-hearing at all frequencies need to be monitored simultaneously, not just one at a time.

The other issue to bear in mind is that typically we test binaurally with the ASSR (by AC), and only monaurally with the ABR. This does not alter the fact that if there was an interaural asymmetry that was large enough for cross hearing to occur, the better hearing ear should be masked. In that case we would revert to ASSR with monaural stimulation i.e. we would continue to stimulate the test ear in the normal way, but now instead of stimulating the other ear with test stimuli, we would input masking noise to that ear instead.

For instance if we have selected white noise masking of the right ear (Number 6 in image), and so we could continue to test the left ear ASSR monaurally while the right was masked out. The masking is presented by the insert phone. The level of masking needed is chosen from the down menu in the temporary settings (Number 4 in image) (and you can work out the level of masking needed by using the ABR masking calculator (see webinar…). Of course, that tool is intended for the ABR but there is no reason why it can’t be used for ASSR too…we still use the chirp in both ABR and ASSR, the main difference being that the rate of stimulation is different, but that doesn’t affect the interaural cross-over. 

 

So, consider using this tool to set the masker to the level necessary to cover whichever frequency shows the biggest interaural asymmetry…all the other frequencies in your ASSR test would then receive enough masking automatically. If in doubt, use the masking calculator to check all frequencies with an interaural asymmetry one by one, then set the masking level to the highest masking level recommended.

Please note that BC ASSR is also feasible, and rules of masking are also applied in the same sort of logic as above, but taking into account that BC ASSR isn’t performed at 500 Hz. Masking with BC ASSR is likely to be needed more frequently than AC ASSR, similarly to in ABR (and Pure Tone Audiometry). Please see Hansen and Small (2012) for more info.


References and caveats
Hansen, E.E., and Small, S.A. (2012)  Effective Masking Levels for Bone Conduction Auditory Steady State Responses in Infants and Adults With Normal Hearing/ Ear& Hearing, 33 (2), pages 257–266 

Webinars mentioned in the answer: 

Masking the ABR Part 1 

 Masking the ABR Part 2

 

February 2017

My feeling is that there won’t be a huge difference in the nHL-to-eHL for adults depending on whether you use inserts or supra-aural headphones. The reasoning is as follows: there are two considerations when deriving the nHL-to-eHL correction. The first is the age-related transducer correction (this is a calibration correction and is applied for infants under 6-months of age and is much greater with insert phones due to small ear canals in infants and less with the supra-aural headphones due to the larger overall enclosed volume). The second consideration is the actual underlying difference in ASSR threshold and behavioural threshold (and this effect should be much the same irrespective of the transducer type). 

So here the first of these two considerations doesn’t apply as we’re talking about patients over 6 months of age while the second one is not dependent on transducer type.

We should also bear in mind that you can still make an accurate determination of normal hearing, reliable PTA/ unreliable PTA etc… without any correction factors at all. You can still make those determinations (at least in the interim, until your inserts are available) in nHL without switching to eHL. 

All in all I’m therefore inclined to suggest basing your conclusions/reports on the nHL thresholds where possible, unless needs specifically require eHL (e.g. need to fit a hearing aid) and in that case use the insert values due to lack of a readily available alternative, but rationalised using the above explanation. When your insert transducers are replaced then you can of course revert to normal practice.

The origin of the nHL to eHL correction factors is noted below for insert phones. You will find this information on p140 of the Additional Information Eclipse manual installed on the Eclipse computer (it should be at the following location C:\Program Files (x86)\Interacoustics\Eclipse)

May 2017

This is a pretty interesting question for several reasons but the answers to your question is not easy. The direct answer is we do not know what the rationale is for any advantage of CE-Chirp® over tonebursts or clicks (the conventionally used stimuli). We don’t think the answers have been established in the literature yet. I will elaborate:

Here below is a tuning curve for oVEMP and cVEMP taken from Sandhu et al (2012).

n.b. there are several key VEMP parameters (amplitude, latency, interaural asymmetry, morphology) but these comments just refer to amplitude for now.

The obvious thing to note from these tuning curves (which reflect the same pattern as other such published curves) is that the amplitude is greatest in the low frequencies, peaking at 500 Hz, but drops off markedly above 1000 Hz for both cVEMP and oVEMP. The frequency range of interest for obtaining the largest VEMPs are 250-1000Hz – perhaps partly related to the middle ear transfer function (for AC VEMPs) and partly related to the fluid impedance properties of the vestibule.

As you are perhaps aware, we have currently two conventional stimuli. The click, broadband, would theoretically produce the largest response. Also we have the tonebursts and although frequency specific, their longer duration (several ms depending on frequency, compared with the 0.1 µs click) produces a greater intensity stimulus (energy per unit time). So, the 500 Hz toneburst is often preferred as this frequency corresponds to the peak in the tuning curves above. 

The interest in the CE-Chirp®  (e.g. centred on 500 Hz) is that it should cover the frequency range where the tuning curve peaks (500 Hz NB CE-Chirp®  is 360-720 Hz).

On the other hand this is slightly narrower than the 250-1000Hz range, and so recently a special VEMP Chirp has been developed (Walther and Cebulla designed a 250-1000Hz Chirp ).

Walther and Cebulla found some advantages to the tonebursts in amplitude but even still, what is not well understood is why a rising Chirp should be relevant for the utricle and saccule. The rising Chirp  i.e. temporally separating the frequency components by starting at the low frequency end of the spectrum and progressively moving to higher frequencies) is relevant to compensate for the tonotopic arrangement of hair cells in the cochlea; but this is not a factor with the otoliths as they are not (thought to be) arranged tonotopically. So, it is not immediately clear why the temporal delays of the Chirp should be relevant. It is worth speculating why researchers haven’t turned towards a narrow-band noise instead of the CE-Chirp® (e.g. a standard 1/3 octave wide NBN centred at 500 Hz, or designed more specifically with VEMPs in mind e.g. 250-1000Hz NBN) 

This would have the broadband advantages together with a stimulus duration long enough to generate a large response, but it would not have the unnecessary complication of the rising Chirp. 

Perhaps one potential limiting factor is loudness. The auditory response isn’t needed for the VEMP but you still want your patient to have a comfortable test. If the stimulus is too loud or likely to exacerbate tinnitus/hyperacusis then any advantages of NBN would be negated.

We do not know if this has been investigated. If not, it might be a useful research question to answer.


References and caveats
Sandhu, J., Low, R., Rea, P., and Saunders, N. (2012) Altered frequency dynamics of cervical and ocular vestibular evoked myogenic potentials in patients with Ménière’s disease. Otol Neurotol 33. Pages 444-449.

Walther, L., & Cebulla, M. (2016) Band limited Chirp  stimulation in vestibular evoked myogenic potentials. European Archives of Oto-Rhino-Laryngology, 1–9.  

To learn more about cVEMP testing we recommend you watch this webinar

 

May 2017

Reaching the 99% value will allow you to be more certain that you have reached the correct answer, when you see an ASSR recorded at a particular level with a particular stimulus. To reach a higher degree of certainty requires more time – this is the time needed to acquire more data i.e. keep averaging to reduce the residual noise and increase the signal-to-noise ratio.

One of the key advantages of ASSR is the statistical approach to interpration of the data, removing tester bias. However, statistics themselves are at risk of error and the 99% value minimises this risk. However, most people would agree that even a 95% certainty regarding the response is still high, and this criteria will be met in a shorter period of time.

August 2016

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