Auditory Steady State Response: Interview with De Wet Swanepoel, PhD
Douglas L. Beck, AuD, speaks with Dr. Swanepoel, about the approaches and techniques of auditory steady state response (ASSR) and its similarities to ABR.
Academy: Good Morning, De Wet, thanks for your time.
Swanepoel: Hi, Doug. It’s my pleasure. Thanks for the kind invitation.
Academy: De Wet, just to be clear to the readers, I’d like to note that you are an audiologist and you’re on the faculty at the University of Pretoria, South Africa. However, at this moment, you’re also doing some work at the University of Texas at Dallas, and you’re spending a few months working and touring the United States.
Swanepoel: Yes. That’s correct, and it’s been a wonderful opportunity.
Academy: De Wet, I’ve read a few of your papers on ASSR, and if you don’t mind I’d like to take advantage of your knowledge and experience and get your thoughts as to the application of ASSR, the status quo, and your thoughts as to where it’s going?
Swanepoel: Sure. For people not familiar with ASSR, I’d like to start by saying that ASSR is similar to ABR in many ways. For example, an insert earphone is used to deliver the air conduction stimulus and the electrodes are strategically placed to record the bio-electric response. The ASSR is an evoked potential that commonly uses continuously modulated tones to evoke an auditory response, unlike the ABR that is usually evoked by transient stimuli such as a broadband click or more frequency-specific tone bursts. Further, rather than looking at the time domain as we do in ABR, the ASSR analysis occurs based on the frequency domain.
Clinically, the ASSR has established itself as a reliable measure for determining air-conduction thresholds in a variety of populations. Bone conduction ASSR has been a more recent area of investigation and is more problematic. There are a number of laboratories that continue to investigate bone conduction ASSR toward validating it as a reliable clinical method. At this time, when people have normal sensory hearing, we can get reliable bone conduction ASSR thresholds but false or “artifactual” responses may be present in mixed or sensorineural losses. These spurious responses are due to stimulus artifact, and possibly vestibular responses in the lower frequencies that may confound true threshold determination. So that still needs experimentation and analysis to best define an ASSR bone conduction protocol, but we get closer all the time.
Academy: And because the analysis is based on characteristics of the frequency constituents in the recording, this allows ASSR to use objective statistical techniques to define response versus no-response?
Swanepoel: Well yes, this approach allows for different, statistical techniques, such as a phase coherence or an F-test to be applied that allows one of two decisions to be made. The ASSR is either present or absent much like responses to behavioral audiometry is either present or absent. A typical bracketing threshold-seeking technique can therefore be used to determine thresholds that can then be plotted on a conventional audiogram format.
Academy: When is ASSR advantageous, as compared to ABR?
Swanepoel: Well that’s a core issue. To me, ASSR compliments ABR, it does not replace ABR. In other words, when one uses the two tools together, we gather more information and that helps us in our differential diagnosis and treatment plan. So as we increase the number and quality of our tools, we learn more and patients benefit from better diagnostic precision.
Academy: Reminds me of the Jerger-Hayes “Cross-Check” principle.
Swanepoel: Exactly. The ASSR is very frequency specific, in many instances even more so than typical ABR tone burst stimuli. Current findings suggest that this may in fact allow for better accuracy in predicting steeply sloping high-frequency hearing losses, which are quite typical configurations in the patients we see. The nature of the continuous ASSR stimuli also makes it better suited to presentation through soundfield speakers and processing by hearing aids than the transient ABR stimuli. This means it can be used to acquire aided ASSR thresholds. Nonetheless, the ABR remains critically important especially in view of its role in identifying and diagnosing auditory neuropathy, which has been estimated to represent 10 percent of all cases of childhood hearing loss (see Sinninger, Y.S. (2002) Identification of auditory neuropathy in infants and children. Seminars in Hearing, 23(3):193-200). In these cases, even though ASSR responses may be obtained, they are not representative of hearing thresholds and should not be used to guide hearing aid fittings. The ABR, however, is essential to diagnose these cases and will typically show absent or grossly abnormal waves with a clear cochlear microphonic response.
Academy: Yes, that makes sense. Another thing that is very impressive with regard to ASSR is the ability to test four frequencies (500, 1,000, 2,000, and 4,000 Hz) in each ear simultaneously, thus gathering eight sets of data at one time.
Swanepoel: Yes. Of course, it doesn’t always work out quite as nicely as we would like, but in many situations, such as an initial screen for normal hearing across the frequency spectrum, we can often gather eight data sets at one time, and when that happens, the ASSR is very quick and efficient.
Academy: What about the relatively new “Chirp” stimuli?
Swanepoel: Well, the Chirp is very exciting and as you note, it has recently been proposed. The idea is that the acoustic stimuli can correct for the “cochlear delay time,” which occurs due to the tonotopic orientation of the cochlea. By adjusting the phase of the stimuli one can get simultaneous displacement of the membrane, yielding a larger response. This is really good news because near threshold levels, the steady state response is very small, and if we can generate larger responses, it makes response detection faster and more accurate—which is particularly valuable when testing infants and children, where every minute is precious.
Academy: What about ASSR defining the differences between severe and profound hearing loss?
Swanepoel: Yes, this is a clear benefit. ASSR very often can define thresholds within the severe and profound range and this is very important as we counsel the moms and dads with regard to hearing aid amplification or perhaps cochlear implantation. If we know the child has thresholds at 75 dB, as opposed to having thresholds at 100 dB, the implications with respect to auditory (re)habilitation are important. It is important to note here however that some of the earlier ASSR systems utilized recording parameters that may actually have resulted in spurious, or false, responses at these very high intensities but fortunately these have been addressed in later versions and should not be a problem anymore.
Academy: De Wet, it’s been a pleasure chatting with you. Thanks so much for your time.
Swanepoel: Thank you, too, Doug. It’s been good talking with you and I’m happy to help.
Douglas L. Beck, AuD, Board Certified in Audiology, is the Web content editor for the American Academy of Audiology.
DeWet Swanepoel, PhD, is an adjunct professor, Callier Center, University of Texas at Dallas, and an associate professor, Department of Communication Pathology, University of Pretoria, South Africa.