In June 2013, AudiologyOnline held a series of webinars entitled, Update on Auditory Electrophysiology – Evidence-Based Clinical Applications. The webinars were organized by Dr. James Hall and presented by Dr. Hall along with Dr. Samuel Atcherson. The webinars are a wealth of the latest information on auditory electrophysiology, and there were many excellent questions from course participants that enabled us all to explore some of the finer points of the presentations. This article is a compilation of the question and answer sessions from these webinars, as well as some additional questions and answers from participants that were not covered in the webinars due to time limitations. We hope this article will add to your knowledge and understanding of auditory electrophysiology. To view the individual webinars in the series, please visit www.audiologyonline.com/electrophy2013.
Application of ABR in Objective Assessment of Infant Hearing
Questions 1 - 11 pertain to Dr. Hall's webinar, Application of ABR in Objective Assessment in Infant Hearing.
1. ABR for Infant Hearing Assessment – Sequence of Stimuli
When performing an ABR with an infant for the purpose of hearing assessment, I first use a click stimulus. To get frequency-specific information, would you recommend next using chirp stimuli or tone bursts?
Dr. Hall: In my opinion, the sequence of stimuli to use for ABR measurement in infants and young children is: 1) click, 2) tone burst (TB), and 3) chirp stimuli to refine auditory threshold.
Maybe someday we will go directly to chirp stimuli at moderate to low intensity levels, bypassing conventional TB stimuli. The problem with that approach is that chirp stimuli lose their effectiveness for intensity levels above about 60 dB nHL. If the click ABR was entirely normal (threshold of 20 dB or better), then you could go to chirp stimulation at less than 60 dB. However, if there was a moderate or severe hearing loss at any frequency by ABR measures, it would still be necessary to use conventional tone bursts or the ASSR to estimate threshold.
2. Click Stimuli and ABR Spectrum
If ABR using a click stimulus produces a response between 2-4 kHz, then why should filtering low frequencies in infants affect the ABR response?
Dr. Hall: Stimulation of the cochlea with clicks activates hair cells from the base to the apex. However, the ABR is generated when auditory neurons innervating inner hair cells in the basal region begin firing. Actually, it's possible to record a normal ABR even in patients with very poor hearing in the 2000 to 4000 Hz region but normal hearing for higher frequencies. We rarely see such patients in the clinic.
The frequency specificity of the stimulus and cochlear activity is not directly correlated with the spectrum of the ABR. In other words, the energy in the ABR (from about 100 Hz up to at least 1000 Hz) is related to neural firing in the auditory nerve and auditory regions of the brainstem, not the region of the cochlea that is stimulated. The ABR spectrum is essentially the same whether the stimulus is an 8000 Hz tone burst or a 500 Hz tone burst, or a click.
The same concept also applies to behavioral hearing. The response to a pure tone at a specific frequency is tonotopic from the cochlea to the cortex. Certain hair cells or neurons respond depending on the frequency. However, the nature of the response (e.g., the firing rate) is the same for all frequencies.
3. Click Stimulus Level
At what level should click be performed, and are you worried about waking a sleeping child?
Dr. Hall: If the child is sleeping naturally and I believe they have normal hearing based on otoacoustic emissions (OAEs) or some other findings, I will start at 70 to 75 dBnHL and go down. My motto is never wake a sleeping child, but if the child is still sleeping, I will go back and test 80 or 85 dBnHL if wave I was not clear at 70 dBnHL.
The British group recommends not going to the very highest intensities unless there are no OAEs; this is very good advice. If OAEs are absent and there are no recordable ABR tracings or cochlear microphonic up to 80 or 85 dB, I do not think there is any danger in going to the highest levels. You have ruled out auditory neuropathy, and then you can go to the very highest level. Sometimes you only start to see a clear response when you get the highest intensities, particularly if there is a sensory loss. When determining threshold, I would go down to 20 dB whenever possible, because that will rule out any hearing loss greater than 10 dB HL.
4. Tone Burst Ramping
Can you please explain tone burst (TB) ramping?
Dr. Hall: Imagine a single stimulus with an onset, an offset, and maybe a plateau or period between the onset and offset. The ramping is the shape of the stimulus from the onset to the beginning of the plateau, or how intensity is increased from no stimulation to the maximum amplitude for a single stimulus.
Linear ramping is a straight line amplitude increase. Research many years ago showed that linear ramping of stimulus onset produced unwanted energy at frequencies above and below the test frequency. Experimentation showed that it was possible to alter the frequency specificity of stimulation by modifying the shape of the ramp. This is now done with different mathematical formulae that vary the shape. Some time ago, an electrical engineer named Blackman developed an equation for a stimulus onset ramp or envelope that minimized "spectral splatter" of abrupt sound stimulation. The ramp begins to increase slowly and increases more rapidly as the maximum is approached.
My New Handbook of Auditory Evoked Responses (Hall, 2007) book includes a more detailed explanation of this concept.
5. Differences Between Chirps and Tone Bursts (TB)
How does a chirp differ from a regular TB at 4000 Hz?
Dr. Hall: You can readily understand the chirp concept, as it is similar to a click, and you know the click is activating the entire cochlea. The ABR response to a traditional click is only produced by high frequencies. You would think that the regular TB would be rather frequency-specific, and it is. The spectrum of the chirp, click and the regular TB is the same. Let’s say the TB has a two-cycle rise and fall time. It is the onset of the stimulus that generates the ABR. There is still a lot of energy in that TB that is not used. But with the chirp, all of that energy is being used. It is similar to what we described for the click, but on a more limited scale on the cochlea.
6. Bone Conduction ABR - Stimuli
What type of stimulus should be used for the assessment of bone-conduction on an infant ABR?
Dr. Hall: It's possible to use tone bursts for bone-conduction ABR as well as for air conduction ABR measurement. The limitation is usually how much time is available for ABR recording before the child wakes up. I definitely want to estimate thresholds for low to high frequencies using air conduction.
My approach is to use only click stimulation in bone-conduction ABR recording because my main goal is to simply determine if there's an air-bone gap or the likelihood of a conductive loss. If I already have a clue about middle ear dysfunction from history or from tympanometry, the bone-conduction ABR confirms the presence of a conductive hearing loss. With that information, the child is referred to an otologist for further evaluation and possible medical management. I'll make the referral regardless of the pattern or extent of conductive hearing loss.
I suggest that you find some of the articles by David Stapells and his colleagues, like Susan Smartt, because they do attempt to record frequency-specific bone conduction ABRs.
7. Tone Burst ABR at 8000 Hz
When performing ABR with tone burst stimuli for the purpose of hearing assessment in infants, should 8000 Hz be tested? I would think this would be useful for children who have been treated with potentially ototoxic medications.
Dr. Hall: The ABR can be elicited with high-frequency tone burst stimulation at 8000 Hz and even much higher, assuming the earphones have an adequate frequency response. It would be possible, for example, to stimulate an ABR for frequencies of 10000 or 12000 Hz with the right earphones.
For years, I've been asking ABR equipment manufacturers to offer high-frequency stimuli and earphones as an option on their ABR systems for the very application you mentioned - identification or documentation of ototoxic-induced hearing loss in infants and young children. Nowadays, I use the ASSR for threshold estimation at 8000 Hz in such patients, assuming they are quiet enough.
Before using 8000 Hz tone bursts with a typical ABR system, you would want to verify behavioral threshold with the stimuli. Most supra-aural earphones roll off in the high frequencies, and insert earphones were designed to mimic the TDH supra-aural earphones. The maximum intensity level for insert earphones might be reduced for the 8000 Hz tone burst.
I think the 8000 Hz stimulus would be a good choice to use for newborn hearing screening because almost all infant hearing loss involves high frequencies, but at this point the click stimulus will probably not be abandoned for newborn hearing screening.
8. ABR Calibration
Stapells recommends calibrating the ABR using peak-to-peak equivalent SPL values instead of establishing clinical norms. What are your thoughts on this?
Dr. Hall: David Stapells is a friend and colleague, and I highly respect his writings and recommendations. Most of us do not even have sound level meters anymore, so you would have to bring in a technician to do the measurements, which is not a bad idea. Peak-to-peak equivalent SPL is a way of knowing the exact intensity of your stimuli in dB SPL. Unless you consistently perform your ABRs in a sound booth, however, you are going to be using stimuli that are not high enough in intensity in certain settings. In other words, peak-to-peak SPL is a good way to verify the intensity level in a very quiet environment, but in a typical clinical environment, it is very misleading.
Peak-to-peak equivalent SPLs will certainly tell you that your equipment is stable, that your intensity level is not changing and that your earphones are still good, but I think you still need the behavioral follow-up in the different test settings where you will use the ABR.
9. Alternating, Condensation and Rarefaction Stimuli
Is there a difference in using alternating stimulus for click and then separating out condensation and rarefaction runs?
Dr. Hall: If your equipment allows you to use an alternating stimulus and then separate out the rarefaction and condensation waves, I recommend doing that. It is very time efficient, and you can compare rarefaction and condensation ABRs for the very same conditions. In other words, they were collected at the same time, as opposed to doing 2000 sweeps with rarefaction and then 2000 sweeps with condensation. If you cannot do it that way, then you have to do it the old-fashioned way. I always start with rarefaction. If you want to be safe, always use rarefaction and condensation at a high level. Make sure there is an ABR there and then proceed.
10. Hearing Measurement or Estimation with ABR
Does a normal ABR indicate normal hearing or normal auditory function at the level of the brainstem?
Dr. Hall: First of all, let me differentiate between “normal hearing” and “normal hearing sensitivity.” I use the term “normal hearing” when I am referring to the whole hearing process, including auditory processing and how we use hearing for communication. With pure-tone audiometry, we are only measuring hearing sensitivity. But that can be confused with the perceptual detection of a sound. We could say “auditory thresholds,” which is what I usually say when I am being precise.
We are estimating auditory thresholds with the ABR. The ABR is not a test of hearing by any means and for many reasons. Technically, we are not measuring normal hearing sensitivity. That term would be reserved for behavioral testing. I am not a picky person when it comes to semantics, but we do need use the terms carefully.
11. Threshold Differences between ABR and Behavioral Tests
If you have differing thresholds between ABR and VRA and you need to fit a hearing aid, do you rely more on the physiological response or the behavioral response?
Dr. Hall: If I were giving a four-hour lecture on this topic, I would have some slides showing that there are several factors that influence behavioral findings that do not influence the ABR. That is one reason why we are doing the ABR; factors such as maturation, cognition, attention, motivation, and sometimes even language, in terms of instructing the child, are removed. Even though we tend to think of it as a gold standard, there are times when the behavioral test is in error and the ABR is telling us the truth.
If you are recording ABR and you get a threshold that is showing less hearing loss than the behavioral results, I would trust the ABR. If it is the other way around, I would probably trust the behavioral. That is where the crosscheck principle comes in. I would expect that child to have normal OAEs. I would expect acoustic reflexes to suggest normal hearing. We like to see agreement across our tests. This is true for ASSR and behavioral tests as well. Sometimes the electrophysiological results give us better estimates of hearing than behavioral results. Obviously, that varies depending on the child.
Clinical Applications of Electrocochleography in Audiology Today
Questions 12 - 20 pertain to Dr. Hall's webinar, Clinical Applications of Electrocochleography in Audiology Today.
12. ECochG and the Cochlear Microphonic
What do you expect to see in ECochG testing using rarefaction and condensation clicks in normal individuals?
Dr. Hall: Most normal hearing persons have very small cochlear microphonic (CM) waves on separate stimulation with rarefaction and condensation clicks. With a typical earlobe or mastoid electrode, even the ECochG AP component (wave I of the ABR) is only about 0.2 or 0.3 microvolts, so the CM is almost impossible to see unless there's no noise in the recording. Using a TM electrode, however, it's possible to see clear CM in a normal hearing subject and it's very easy to see the CM with the trans-tympanic approach (although it's hard to find normal hearing subjects who will participate in a study).
The CM in many ANSD patients is abnormally enlarged (maybe due to dysfunction of the efferent system) and often easy to detect even with an ABR recording technique.
13. The Use of TIPtrodes
What is a TIPtrode?
Dr. Hall: The TIPtrode is an insert earphone covered with gold foil. To insert the TIPtrode deep within the ear canal, pull the pinna up and back to straighten out the ear canal. You put a little abrasive liquid on a Q-tip and scrub the skin on the outer third of the external ear canal. Then slide the TIPtrode in. Do not put any gel on the TIPtrode. It often will slip right back out. The TIPtrode is used to elicit the response a bit closer to the cochlea.
14. ECochG Analysis Using the SP and AP
Is it possible to use the area ratio of summating potential (SP)/action potential (AP) over amplitude ratio of SP/AP in the diagnosis of Meniere's disease?
Dr. Hall: I've never used a ratio of the area under the SP versus the AP, although it's been suggested as a response analysis option in the literature. Theoretically, the measure would include the amplitude of each component and also an indirect measure of the duration of each component. Not all evoked response systems permit such measurements.
15. ANSD Post-Synaptic Mechanism
Would you go over the post synaptic mechanism of ANSD?
Dr. Hall: A post-synaptic form of auditory neuropathy spectrum disorder (ANSD) is dysfunction of the auditory nerve or (theoretically) the synapse between the inner hair cells and the afferent auditory nerve fibers. The term usually means that the nerve is not capable of being effectively activated with electrical stimulation. In some cases, however, the dysfunction is post-synaptic (neural), but the nerve is reasonably intact and simply not capable of responding to stimulation with abrupt and transient sounds (like clicks and perhaps tone bursts). Chuck Berlin coined the term "auditory dys-synchrony" for these patients.
The 1996 article by Starr and colleagues along with the 2008 Guidelines on ANSD provide good explanations of the post-synaptic mechanism.
16. Post-Synaptic Pattern in Clinical Treatment of Auditory Neuropathy
How would the post-synaptic pattern in ECochG in cases of auditory neuropathy spectrum disorder affect the decision of cochlear implantation?
Dr. Hall: If there is a post-synaptic pattern providing some evidence of an auditory nerve abnormality, you need to proceed very cautiously. I do not think anyone would jump to the automatic conclusion that a cochlear implant would help that patient. The first thing that would need to be done is an MRI. If the MRI showed an intact nerve, another option would be to say that we know there is a postsynaptic problem, but we do not if those nerves are excitable. They show up on the MRI, which is a good thing. So let's do an electrical mid-latency ABR or an ABR in young children because then they can be sedated to do it. You could use the same transtympanic electrode that you used to record the ECochG to present the electrical stimulation to find out if the nerve is intact.
There are otolaryngologists who have said, “I know that the inner hair cells are intact, but the patient is not developing speech and language, so it is a postsynaptic problem. It is not the inner hair cells; it is the nerve.” They think the nerve is going to be able to handle stimulation. They have proceeded with cochlear implantation, and some of the patients have done well with cochlear implants; others have not. I should point out that an ASSR can be useful. The ASSR uses a pure-tone stimulus. It is not an abrupt, transient stimulus. If you have a patient who has any ASSR activity, even if the ABR is absent, that usually means the auditory nerve is capable of firing. It is getting information to the brain, and often it means that the abrupt click or tone burst stimuli for ABR are not very efficient in activating the nerve. But it shows that the nerve can function, and that is a good sign for cochlear implantation.
17. Relevance of SP/AP Ratio
What is the relevance of the SP/AP ratio in ECochG?
Dr. Hall: If you are using ECochG to enhance wave I of the ABR or you are using ECochG in a patient to help you diagnose auditory neuropathy, you are not too worried about the SP/AP ratio. On the other hand, let’s say you have a patient with vertigo that suggests Ménière's disease, but there is no firm diagnosis. If you have a large SP/AP ratio that exceeds normal limits for the electrodes you are using, that is strong evidence of Ménière's disease.
Let's say you have a patient where there is clearly Ménière's disease in one ear, but the question is the other ear. The management of the patient may not be very successful if only one ear is treated, because the other ear will still cause problems and create vertigo. In this case, you can use ECochG on both ears. If you get the same abnormal pattern in both ears, you will know that the second apparently-normal ear is certainly not normal. The diagnosis is likely early Ménière's disease on that side, too.
18. TM Electrode with a Sleeping Baby?
Do you think it's possible to use a TM electrode on a naturally sleeping baby? I'm wondering if this might provide clarity in interpretation when ANSD is suspected in an infant.
Dr. Hall: I've never tried using a tympanic membrane (TM) electrode on a sleeping baby, but I think it would be quite challenging. It's likely the pressure of a well-placed electrode on the TM would create enough discomfort to wake a naturally sleeping baby. It certainly would be important to check and verify if the TM electrode is FDA-approved for use with infants and young children. My suspicion is that the electrode is approved only for adults.
For audiologists, a simple electrode option for ECochG in infants is a pediatric-sized TIPtrode but the ear canal would need to be prepared in advance with abrasive substance (e.g., NuPrep). Infants will not willingly tolerate that process due to some discomfort so the prep would need to be done before the child fell asleep. The TIPtrode option would be feasible with an anesthetized child.
With otology support at a children’s hospital, the best ECochG approach to use (if you really wanted test results for decisions about a CI) would be a trans-tympanic needle recording with a sub-dermal needle.
19. Insert Earphones, ABR and Electrodes
I know the importance of using insert earphones when performing ABR. However, how do you use insert earphones when using "golf club" electrodes, which are placed against the fragile round window, as opposed to a needle electrode in the promontory? I ask this question because I know of a center in Australia that uses this method as they electrically "turn around" the electrode and perform an electrical ABR to assess probable CI efficacy by also assessing the auditory nerve.
Dr. Hall: I have never used a "golf club" electrode. However, when a "silver ball" electrode is placed in the round window niche, there is no danger to the round window as it is around the corner within the niche. The silver ball portion of the electrode rests on bone. I presume the same is true of a golf-club style electrode. The Australian research group could probably answer the question more accurately.
My preference is a trans-tympanic needle electrode approach with the tip of the uninsulated needle resting on the promontory. There is then no danger in securing the electrode wire in the external ear canal with an insert earphone cushion.
20. Dendritic Potentials
Can you discuss dendritic potentials in regard to ECochG?
Dr. Hall: I've never recorded a dendritic potential (essentially a response from the synapse). Indeed, they are a little controversial in that not all experts think that it's possible to detect an electrical response that reflects synaptic transmission as it reaches the dendrites of the auditory nerve fibers.
A very small subset of auditory neuropathy spectrum disorder (ANSD) patients may have a synaptic abnormality, yet normal inner hair cells and normal auditory nerve fibers. I think they are usually identified by the process of elimination. That is, there is evidence of normal inner hair cell function and normal 8th nerve function, but the ABR is still absent, so that limits the possibilities to the synapse. Genetic consultation might also help to identify children with neurotransmitter abnormalities.
Neurodiagnostic Auditory Evoked Response Applications
Questions 21 - 39 pertain to Dr. Atcherson's webinar, Neurodiagnostic Auditory Evoked Response Applications.
21. Electrophysiology Terminology
There are so many terms and acronyms used in electrophysiology. Do you think we should all be using consistent terms for various auditory evoked responses?
Dr. Atcherson: I agree with you. We probably should be using consistent terms. Perhaps some of the issues are related to all of the different types of measures within the same family of responses. We should also consider that it is not just the audiology discipline using those measures; psychology, medicine and neuropsychiatry use very similar waveforms as well. Perhaps we should have a symposium to discuss what exactly we should be naming these things. It comes down to whether we call them potentials or responses. Very often a conference or a book will say “auditory evoked potentials” and then immediately start talking about “auditory responses.” We could talk all day about that.
22. Assessing Diffuse Brain Involvement
What is best way to assess diffuse brain involvement?
Dr. Atcherson: There is no standard method of assessing diffuse brain injury. Diffuse brain injuries are typically those that cannot be seen on imaging scans and are microscopic injuries due to tearing, shearing, and even some small vascular injuries. When imaging offers no explanation for perceived difficulties, sometimes evoked potentials can. The difference, in principle, is that imaging may offer great spatial resolution for anatomical structures, but they have poor or no temporal information. Evoked potentials, on the hand, are just the opposite, and offer some functional information in millisecond units.
In addition to routine audiologic evaluation, this is where one might consider using ABR, MLR, and LLR (and/or P300). Even if auditory nuclei are preserved and unaffected, fiber tracts and white matter may be affected in such a way to delay latencies or reduce amplitude compared to what one might expect. In my clinical practice, I’ve seen how diffuse brain injuries results in asymmetries in MLR with normal ABR and normal P300.
23. Equipment with Limited Stimuli
What can we do when our recording equipment does not have the choice of using a speech stimulus?
Dr. Atcherson: If your system does not permit the choice of speech stimulus, will it allow you to import a speech stimulus? I have both the IHS SmartEP system and the Bio-Logic Navigator Pro system, and both will allow me to import custom stimuli. You will need to follow the manual for how to format your stimuli in terms of sampling rate, file extension, etc. You may be able to use a naturally-spoken speech stimulus if you have a digital recorder or a high-quality microphone and computer.
However, some systems do not give you that option and, therefore, there would be nothing you can do. Some clinicians and researchers have figured out how to use another stimulus generator to present stimuli and that same computer sends a trigger to the evoked potential system. In this case, you should consider speaking with that system’s manufacturer.
24. Online and Offline Filtering
What is the difference between online and off-line filtering?
Dr. Atcherson: Online filtering is when you are collecting responses and running the average with preset filters. For the LLR, I would set the filter at 0.1 to 100 Hz. After the average has been collected, I will go back in and review the results after the patient has gone home. I may do a little more filtering off-line after the responses have already been collected and I no longer have contact with patient.
25. Defining Mismatch Negativity
What’s mismatch negativity?
Dr. Atcherson: The mismatch negativity (MMN) is another cortical event related potential (CERP) that is considered “pre-conscious.” Like the P300, it is another evoked potential that can be used to discriminate between two different acoustic stimuli. To elicit the MMN, you would use the oddball paradigm (1000 vs. 2000 Hz, or /ba/ vs. /da/), similar to that described for P300. However, the MMN is a difference wave, so you would need to subtract the target stimulus average from the standard stimulus average, and if the MMN is present, you should see a negativity around 200 msec. Unfortunately, the MMN is not always present in all individuals with normal hearing and normal auditory processes. Therefore, it has been viewed as clinically unreliable at this time.
26. Disorders Impacting MLR and LLR
Do MLR and LLR results vary depending on the disorder?
Dr. Atcherson: Unfortunately, this is a highly unanswerable question. Both the MLR and LLR have widely variable and widely distributed generators, with both exogenous and endogenous characteristics. Different disorders may have different effects, but the more important issue is that the same disorder may have very different patterns of abnormality. No two brains are alike, hence the variability.
27. MLR and Threshold Reference
What effect does MLR have on determining threshold?
Dr. Atcherson: This webinar did not focus on threshold estimation applications for MLR, but I can provide you with a good resource for more information. I would recommend the paper written by Nousak & Stapells (2005) for information on this topic.
28. MLR and Condensation
Some experts recommend using condensation for MLR recordings. Do you agree?
Dr. Atcherson: I am not familiar with a difference in MLR recordings in response to rarefaction or condensation recordings. The MLR does not depend on neural synchrony in the same way that the ABR does. While it has not been my clinical experience that stimulus polarity affects click-evoked MLRs, this is not to say that others have not found an issue. I am simply unaware of it.
29. Formula for Computing Percentage of Amplitude Difference
Could you tell me the formula for computing percentage of amplitude difference?
Dr. Atcherson: (Larger Na-Pa amplitude minus Smaller Na-Pa amplitude)/Smaller Na-Pa amplitude, then multiply by 100 to get percent. If >50%, this is considered a positive finding.
30. P300 Testing – Eyes Open or Eyes Closed?
Why should the eyes be open during P300 testing?
Dr. Atcherson: Closing the eyes during CERPs has the effect of producing large alpha EEG waves, and as the patient drifts into even light sleep, the cortex becomes less active and thalamo-cortical rhythms associated with sleep begin to emerge. This, in effect, would reduce or even eliminate the evoked potential of interest, particularly if the goal is to discriminate between two acoustic events.
Secondly, P300 tests are generally quite boring and long, and if the patient has their eyes closed, the likelihood they will fall asleep during an active button-pressing or counting task will be quite high.
31. P300 – Electrode Montage
Do you think full-skull (cap) montages produce more reliable P300 responses?
Dr. Atcherson: Caps, in an of themselves, do not produce better P300s. Rather, caps allow for a broader coverage of the head surface and allows one to record the P300 across the head. Cz may not be the most optimal site on the head; it may be Pz. It depends on what your goal is with measuring P300.
32. LLR Without Eye Blink Monitor
What is the risk of using LLR without an eyeblink monitor?
Dr. Atcherson: You can certainly record the LLR without an eyeblink monitor, but you will need to ensure that the patient does not blink excessively, and you will need to make sure that the artifact rejection is effective in rejecting sweeps where an eyeblink has been generated by the patient. The problem with this method is that small eyeblinks may not be rejected and can contaminate the LLR, especially if they are excessive.
To minimize eyeblink artifacts without an ocular channel, you might be able to lightly tape a small ball of cotton over the upper lid of each eye. They can still blink, but the magnitude of the blink will be reduced. Another technique might be to have the patient purposely blink repeatedly and set the artifact rejection levels to the level that would reject that patient’s eyeblink. The bottom line is that you can certainly record LLRs without an ocular channel, but you need to take care not to misinterpret the LLR when, in fact, it may be an averaged eyeblink or a few very large eyeblinks that have contaminated the entire average.
33. P300 Testing – Monaural or Binaural Stimulation?
Why conduct the P300 test with monaural stimulation instead of using bilateral, simultaneous stimuli?
Dr. Atcherson: One can record P300 using either monaural or binaural stimulation. My recommendation of monaural presentation is to look for ear-related and hemispheric differences. If one performed only binaural stimulation, then it is possible that the stronger auditory pathway might mask a weaker ear and give the impression of normal results. After all, we should be evaluating each ear separately.
34. P300 Testing- Absence of Stimulus for Oddball Paradigm
Instead of using an odd stimulus in the P300, can the absence of a stimulus be used in the oddball paradigm when recording P300?
Dr. Atcherson: Absolutely. You can make the absence of sound the odd stimulus. You should still see an endogenous potential if you have /dah/dah/dah/dah/dah/ /dah/. You should have been able to hear when I did not say “dah.” You should get a P300 response. You might have to import a stimulus with no amplitude or a completely silent stimulus in your clinical equipment, but you should be able to do that.
35. CAEP and ABR thresholds
Can you explain CAEP and how they relate to ABR thresholds?
Dr. Atcherson: Due to lack of time, I did not go into any great length about the comparison between ABR and CERA. Lightfoot and Kennedy's paper did not specifically examine ABR, but they did note that the time it took for CERA was comparable and not worse than ABR. They also briefly talked about Nousak and Stapells (2005) ABR paper showing ABR thresholds within 5 dB of behavioral threshold; however, they needed a minimum of 8000 stimulus presentations to achieve that level of accuracy.
I encourage you to explore this page on Dr. Lightfoot's website:
Personally, I would love to have the opportunity to play with Lightfoot's system. I think it has an important place in diagnostic and rehabilitative audiology.
36. Measuring Amplitude of Late Responses
How do you measure the amplitudes of late responses: peak to trough or peak to a baseline?
Dr. Atcherson: With the MLR, you could measure peak to baseline if you have a pre-stimulus baseline. It is not generally required to have a pre-stimulus baseline, however. I do not see it much in the literature, but it is certainly something that you can do. In terms of evidence-based practice from the literature, the overall peak-to-trough amplitude or trough-to-peak amplitude for Na and Pa seems to be the most consistent and reliable measure.
The method for measuring LLR amplitude depends on what you are looking for. You must have a pre-stimulus baseline to achieve this. You have to feel comfortable that what you are getting is not going to be a voltage drift over the course of your particular measure. In the event that you do have a fairly flat pre-stimulus baseline on a clinical system, you could certainly do peak to baseline measures or you could consider P1 to N1 peak-to-peak or N1 to P2 peak-to-peak. There are lots of different ways to do this, and I am not sure what is best to recommend. If you are going to collect your own normative data, you might as well start taking into account all of those different things so that you can use them to your advantage.
37. Eyes Open or Closed?
We know that closed eyes stimulate alpha brain activity, which could mask the cortical responses, so which testing condition do you recommend: eyes open reading, eyes open watching a movie, or keeping the eyes closed?
Dr. Atcherson: This is one question with which I have wrestled, both clinically and in research. Here is my thought. If you choose to allow patients to read something during testing, it depends on what they are reading. If they are reading a magazine, it is likely that the patient is going to be flipping through pages, making a lot of noise and generating excess muscle activity. That would not be advised. Reading a novel where they occasionally turn a page might be okay.
In my opinion, keeping the eyes closed would not be advised for MLR, LLR or P300. Just as you suggest, the alpha-band rhythm can come in and mask the response. Watching a movie may actually be the best bet. It should possibly be muted, however. If the patient can read, you could have subtitles or captions on. Some studies show that a low intensity, 40 dB SPL or lower, might not have that big of an effect on those particular measures.
That is a good question, because we typically have no way of measuring the sleep level directly. Of course, the answer might be different if you are talking about doing these measures in a newborn infant.
38. Recording in the Presence of Hearing Loss
When a hearing loss is present, is there a guide for using sensation level (SL) instead of a fixed intensity level in dB nHL or SPL?
Dr. Atcherson: Although I have not personally dealt much with sensorineural hearing loss in my research, I think this is an important question. To paraphrase your question in a different context, “If I am seeking out individuals who may have hearing loss, should I use a fixed level that is consistent with conversational speech or should I adjust the intensity level to the sensation level?” There is not much research on that topic, so I do not have a complete answer for you today.
You might want to start by identifying your purpose for doing the evoked response assessment. Is it to provide an objective measure of how a person would hear conversational speech or is it to get the optimal response out of the patient under ideal circumstances to evaluate how the brain processes auditory information? Of course, you can record auditory evoked responses with stimuli presented by loudspeakers rather than earphones from patients who have a cochlear implant or hearing aids. That issue becomes more complicated in those patients.
39. Ear Effect v. Electrode Effect in CAEP Testing
How could I correlate ear effect and electrode effect? For example, is the left-ear stimulation supposed to activate right auditory cortex?
Dr. Atcherson: My experience has been that you see either the ear effect or the electrode effect and not both. But you bring up a good point. It is well known in the literature that the contralateral auditory pathway can lead to a stronger response, especially for truly cortical responses such as the CAEP. For example, right stimulation to left hemisphere may have a stronger response than right stimulation to right hemisphere. In this way, the response over the ipsilateral hemisphere may be slightly smaller than the contralateral hemisphere. This begs the question, "How much smaller should a CAEP be before you can call it abnormal?" Unfortunately, that is not a question for which I have an answer. My clinical experience with ear and electrode effect is with the MLR.
In the same vein, please also consider the overall morphology and not just the overall amplitude. The amplitude between hemispheres may not differ, but it may be clear that one hemisphere has an abnormal morphology compared to the other hemisphere.
Cortical Response Applications
Questions 40 - 45 pertain to Dr. Atcherson's webinar, Cortical Response Applications for Audiometric and Audibility Assessment.
40. Time Commitment for CAEP
Approximately how long does it take to record a cortical auditory evoked potential (CAEP)?
Dr. Atcherson: If you are using your own clinical system, presenting a minimum of 20 stimulus presentations, you might be able to get a solid waveform in a couple of minutes. Every waveform should be replicated, so that will double your time.
If you are going to use the CAEP for threshold purposes, you will start building on from there. From my readings, it seems that the CAEP procedure may not be any longer than the ABR for threshold estimation. You really should optimize your system and become familiar with it so that you know what you are doing and can be quick about it.
41. Using Electrode Jumpers for CAEP Recording
Could you please explain the effect of linking earlobes using jumpers or other linking electrodes on CAEP recordings?
Dr. Atcherson: Using a jumper cable allows an electrode setup where there is one electrode on each earlobe, and they go into the same common reference port on the electrode box. The jumper number may vary depending on the manufacturer. If you do not have a jumper that will allow you to electrically link and plug into the same reference port, you might need to talk to your manufacturer. Linking will simply allow you not to have to unhook one ear and hook the other up for clinical efficiency.
42. CAEP Findings in Single-Sided Deafness or Cochlear Implantation
What are the CAEP findings in those with single-sided deafness or with a unilateral cochlear implant?
Dr. Atcherson: It is clear from the literature that if you have a unilateral hearing loss and you were to record monaurally separate CAEPs, the CAEP on the deaf side will look very different from the normal ear on the other side. What you might consider doing, however, is putting a little masking noise in the normal ear so that you do not have any crossover. It is seems to be the case that even with severe to profound hearing loss, you can still get a cortical potential.
43. Attention and CAEP
Does attention affect the sensitivity of CAEP and auditory threshold estimation?
Dr. Atcherson: Absolutely. The patient needs to be awake and alert, but then you immediately start thinking how you can accomplish this with infants. The HEAR lab developers have a number of different recommendations for how to work with infants. An infant, for example, would be sitting on a parent’s lap and may be watching something softly on TV or they may be able to play with an iPad or something like that. You would give them anything to minimize activity. The bottom line is that have to be awake. If they are sleeping, you run into the risk of having alpha rhythms, which means that the cortex seems to deactivate and you end up with large, rhythmic, cyclic-looking EEGs that can obscure the cortical potential.
44. CAEP in Auditory Neuropathy
What do CAEPs look like when recorded from cases of auditory neuropathy?
Dr. Atcherson: I do have examples, not included today, of adults with auditory neuropathy. In a normal case, you can clearly see P1, N1, and P2. A significant case of ANSD had a virtually indistinguishable, unidentifiable CAEP. Some individuals with moderate to severe ANSD had what looked like smaller amplitudes and longer latencies. I cannot tell you one particular configuration that applies to all individuals with ANSD because it varies from person to person. I would be very interested in seeing what the HEAR lab can do with some of those individuals.
45. Inconsistencies to Watch for in CAEPs
You showed a case where there was no response at 80 dB, but there was a response at 75 dB. Can you clarify how that is?
Dr. Atcherson: That was my student’s project. If there is not a response at a higher intensity level, but there is a response at a lower intensity level, you need to investigate that. In that situation, I would have gone back up to 80 dB and then repeated 75 dB to see what I got. Nevertheless, my student went in with the mindset that she already knew what the behavioral results were and she wanted to find threshold with CAEP just to see how well-correlated they were. For that particular project, the focus was on correlation as opposed to detection. I thank you for bringing that up. That is certainly a flaw in that particular study, and I certainly would not advocate that you delete waveforms; certainly you should replicate all waveforms in a clinical setting. You need to continue investigating things when they do not make sense.
Guidelines for identification and management of infants and young children with auditory neuropathy spectrum disorder. (2008). Retrieved from the Bill Daniels Center for Children’s Hearing, The Children’s Hospital, Colorado, USA at http://www.childrenscolorado.org/pdf/Guidelines for Auditory Neuropathy - BDCCH.pdf
Hall, J.W. (2007). New handbook of auditory evoked responses. Boston, MA: Pearson.
Lightfoot, G., & Kennedy, V. (2006). Cortical electric response audiometry hearing threshold estimation: Accuracy, speed and the effects of stimulus presentation features. Ear and Hearing, 25(5), 443-56.
Nousak, J. K., & Stapells, D. R. (2005). Auditory brainstem and middle latency responses to 1 kHz tones in noise-masked normally-hearing and sensorineurally hearing-impaired adults. International Journal of Audiology, 44(6), 331-344.
Starr, A., Picton, T. W., Sininger, Y., Hood, L. J., & Berlin, C. I. (1996). Auditory neuropathy. Brain: a Journal of Neurology, 119(Pt 3), 741-753.
Cite this content as:
Hall, J.W., & Atcherson, S.R. (2013, August). FAQs on AERs. AudiologyOnline, Article 12060. Retrieved from: http://www.audiologyonline.com