20Q: The Importance of Psychoacoustics in Clinical Audiology

From the Desk of Gus Mueller

Over the years, there haven’t been too many “Gus’s” in the fields of audiology or hearing science, so as you might expect, we tend to stick together. It’s only natural, therefore, that whenever I think of psychoacoustics, the first name to come to mind is Gus Fechner. And it’s not just me. Each year, on October 22^nd, Fechner Day is celebrated with psychophysics conferences at many sites in the U.S. and Europe. Why October 22^nd?

Gus Fechner (1801-1887), was a German philosopher, physicist, and experimental psychologist. When he was only in his 40s, he developed very poor vision and poor overall heath, and as a result, lost his job at the university. Living on a small pension, he soon became reclusive and rarely left his home. He suffered from depression, was suicidal and spent most of his time in bed. But . . . as the story goes, on a Tuesday morning, October 22, 1850, he awoke with new revolutionary ideas about the non-linear relationship between psychological sensation and the physical intensity of a stimulus. These ideas resulted in work that he later published. Because of this morning discovery, he is known as the founder of psychophysics.  

Simply stated, Fechner’s discovery relates to the relationship between the actual change in a physical stimulus and the perceived change by humans. This is something that applies to much of what we do in clinical audiology and the fitting of hearing aids—probably to a greater extent than we realize. To tell us more about this relationship, it seems only fitting to bring in someone who has recently written a book on the topic. Our guest author this month at 20Q is Jennifer Lentz, PhD.

Dr. Lentz is a professor of Speech, Language and Hearing Sciences and is currently the chair of the department at Indiana University, where she has served on the faculty for the past 18 years. You have probably read her publications related to auditory perception in listeners with normal hearing and hearing loss, assessment tools to diagnoses and characterize tinnitus, and the application of physiological and psychological models to auditory perception.

She is a fellow of the Acoustical Society of America, and recently served as an associate editor for their journal JASA, as well as the Journal of Speech, Language, and Hearing Research. She currently serves on several committees for the Acoustical Society, including Women in Acoustics.

As mentioned earlier, Dr. Lentz has written a psychoacoustics textbook geared toward students in AuD programs, Psychoacoustics: An introduction to the perception of normal and impaired hearing with audiology applications, published by Plural Publishing.

Going back to our friend Gus Fechner, he is known for the following quote:

Man lives on earth not once, but three times: the first stage of his life is his continual sleep; the second, sleeping and waking by turns; the third, waking forever.

I’m not entirely sure what this means, but I’m quite certain that if you are in either Stage 2 or Stage 3, you’ll enjoy this great psychoacoustics review from Jenny.

Gus Mueller, PhD
Contributing Editor

Browse the complete collection of 20Q with Gus Mueller CEU articles at www.audiologyonline.com/20Q

20Q: The Importance of Psychoacoustics in Clinical Audiology

Learning Outcomes 

After reading this article, professionals will be able to:

• Discuss the role that psychoacoustics has in modern audiological assessment.
• Describe the perceptual deficits experienced by listeners with sensorineural hearing loss and how amplification affects perception.
• Explain how psychoacoustics might be influential in audiological rehabilitation.

       Jennifer Lentz

1. Didn’t you recently publish a textbook on psychoacoustics? 

I did! It was published in the fall of 2018 and was directed at students studying clinical audiology. It reviews the basics of psychoacoustics in listeners with normal hearing, but also covers the effects of sensorineural hearing loss on auditory perception. I also included a discussion on the clinical applications of psychoacoustics, including diagnosis and hearing aids.

I have been teaching psychoacoustics to clinical audiology students at Indiana University since 2002. Every year it was difficult to find a book that addressed material at an appropriate level and had the information that clinical audiology students needed. There are great hearing science and ssychoacoustics books out there, but they do not really address how sensorineural hearing loss impacts perception, or how the fields of audiology and psychoacoustics are integral to each other. I felt that this book could make psychoacoustics more accessible to the audiology student, as I designed it to specifically target the aspects of psychoacoustics most important to audiology.

2. You mention that these two fields are tied together?

A phrase that I like to use is “every audiologist is a psychoacoustician.” I say this because every audiologist uses the foundation of psychoacoustics for clinical practice. Of course, a major component of audiologic assessment, the audiogram, is a psychoacoustic test. It underlies your decisions to fit a hearing aid, facilitate interpreting speech test results, use for prescriptive targets in hearing aid fittings, help patients understand their communication difficulties during counseling, and is critically important for establishing sensorineural versus conductive hearing loss. There is currently nothing more powerful within audiometric assessment than the audiogram, and even after being in place for over 100 years, it remains the “gold standard” test of hearing. The audiogram is the most sensitive test available that can describe the detection capabilities of the auditory system. Other psychoacoustic tests also are in the audiologist’s arsenal, but are not as useful to the audiologist as the audiogram.

3. How exactly does the audiogram reflect the field of psychoacoustics?

The audiogram has been around since the early 1900s, and as you know, represents absolute threshold, or the lowest sound level that a person can hear. Measurements are based on the principle of detection, which within Erber’s (1982) hierarchy of perception, is the lowest level of perception. The audiogram is surprisingly powerful, considering that it does not reflect abilities at the three higher perceptual levels: discrimination, identification, and comprehension. However, one can expect that if a person has difficulty with detection, they will also have difficulties with the higher levels of perception.

The modified Hughson-Westlake up-down method, used to measure threshold, is an adaptation of a classical psychophysical procedure, the method of limits, and the parameters of that method are based on the psychometric function. This function describes the ability of a person to detect a stimulus as a function of the intensity level. Responses go from 0 percent (“I cannot hear the stimulus”) to 100 percent (“I can always hear that stimulus”) over a range that is very similar regardless of a person’s age or hearing status (Marshall & Jesteadt, 1986). This feature allows us to use the same procedure for all adult patients who come to the clinic for hearing tests. The step sizes used for the adaptive portion of the procedure (5 dB up and 10 dB down) are chosen to sample the psychometric function efficiently. As you have experienced, though, there are people who always respond at one presentation level but never to the presentation level that is only 5 dB lower. There is evidence that some of these people may have a steep psychometric function (Arehart, Burns, & Schlauch, 1990), and the step sizes used are small enough that we can estimate thresholds from these people as well. The method also allows for measuring thresholds for a person with a shallower psychometric function, who may not respond 100 percent of the time to specific presentation levels. Here, you may have to keep track of responses in order to get two responses out of three on the ascending trials.  

4. Makes sense. When conducting audiometric measures, we’re often doing masking too. Is this also linked to psychoacoustics?

Absolutely! The principles of masking, effective masking levels, and the characteristics of the noise bands used in masking for pure tones are all based on psychoacoustics. As you know, when you present maskers during measurement of the audiogram, the maskers will raise the threshold of the ear being masked. You present these maskers in terms of decibel effective masking level (dB EML), a term that indicates what the threshold of a tone would be if you measured it in the presence of that noise. The acoustic characteristics of the sound, the size of the critical band, and a conversion from sound pressure level (SPL) to hearing level (HL) all determine dB EML. The critical band, initially quantified by Fletcher (1940), is really important because the critical band describes which frequencies mask other frequencies. To ensure that your masker is effective, it has to include all of the frequencies that can mask the pure tone you are using for testing. The conversion from dB SPL to HL also came directly from the work on thresholds being conducted by Bell labs in the 1930s (Sivian & White, 1933). 

Honestly, I am frequently amazed at how well this procedure works, particularly in light of modern studies which show that the critical band theory has significant limitations. In the 1980s, David Green showed that spectral components far away from the tone can influence its detection (Green, 1988; Kidd Jr., Mason, Brantley, & Owen, 1989). His work has greatly challenged the critical band theory. There is also inter-individual variability in how listeners with sensorineural hearing respond in other psychoacoustic tasks (Lentz, 2018), yet dB EML does a great job of allowing us to obtain accurate masked thresholds across a wide range of people and degrees of hearing loss.

5. I get why masking is so important for audiology, but why can’t we rely on physiological tools in place of the audiogram?

Physiological tests, such as the auditory brainstem response (ABR) or otoacoustic emissions (OAEs), may not be nearly as sensitive, easy to administer, or comprehensive as the basic audiogram. For example, the ABR only provides a measurement of the neural response at the level of the brainstem, and its results can be difficult to interpret. On the other hand, the audiogram reflects the general status of the entire auditory pathway from the outer ear to the brain. Other physiological tests may measure neural function of higher auditory centers, but they also have their limitations. In the end, no physiological tests measure hearing. As a result, they do not inform the audiologist whether a patient heard the stimulus or not. Only the patient can say that, and we don’t have any test to date that can measure hearing without a behavioral response.

6. But the audiogram does have its limitations, right?

Yes, indeed. The audiogram only provides a measurement of the lowest level of auditory perception. While detection thresholds are expected to correlate with simple speech measures, such as the speech recognition threshold, the audiogram cannot stand on its own. For example, the audiogram cannot independently be used to establish the presence or absence of a retro-cochlear site-of-lesion, and it is also not very sensitive to speech comprehension difficulties or cognitive issues. Other tests are necessary in order to get a more complete picture of auditory function. Word recognition testing, especially when conducted with background noise, can be important to help you with counseling. Modern audiology relies a lot on physiological assessment for retro-cochlear assessment, but psychoacoustic tools once were the only ones available. In certain (albeit rare) cases, you might find it helpful that these tests are still available.

7. Psychophysical tests for retrocochlear assessment?

As you are aware, we typically do not use these tests for retrocochlear assessment anymore because the physiologic tests yield much better hit rates and lower false positive rates than psychoacoustic tests do. Also, the MRI has better diagnostic properties than immittance testing or ABR. Yet, you might find cases where you cannot administer these tests (e.g., maybe you can’t get a seal for an immittance probe due to an outer ear anomaly), or your tests are inconclusive. A lot of older people cannot get an MRI, so modern audiometers still have the ability to implement many of these tests and can be used when needed. You have to be careful to not make strong conclusions based on these tests, however, as most of these tests do not have robust sensitivity or specificity.

8. What are some examples of these tests?

Perhaps one of the oldest tests was developed by Fowler (1936), and it was designed to establish cochlear versus retrocochlear hearing loss. Fowler had noticed that some patients with unilateral hearing losses needed higher sensation level sounds in the poorer ear to match the loudness of sounds in the better ear. By adapting a classical matching procedure, he developed the alternate binaural loudness balancing (ABLB) test to measure how patients judged the loudness between their two ears. This test was then used to measure whether a patient experienced recruitment, or faster growth of loudness, that is expected to be associated with sensorineural hearing loss. If the ear with hearing loss experienced recruitment, the growth of loudness measured using the ABLB would be much faster in the ear with hearing loss than in the better ear.  On the other hand, if the growth of loudness looked similar between the impaired and good ears, there was good reason to expect that the loss was retrocochlear (assuming that conductive pathology had been ruled out).

Other psychoacoustic tests to establish cochlear/retro-cochlear site of lesion came 15-20 years later; some of these were based on the intensity difference limen, or the just noticeable difference in intensity. At that time, patients experiencing recruitment were expected to have smaller (e.g., better) difference limens than patients without recruitment. In theory, faster growth of loudness would make it easier for a person to tell whether two sounds had different intensity levels.

9. Wasn’t there a popular test based on this concept?

Yes, there was. The Short-Increment Sensitivity Index (SISI) was based on this principle (Jerger, Shedd, & Hartford, 1959). In this test, a continuous pure tone was presented to the patient, and the tone’s level was randomly increased in a 1 dB step. The original SISI test consisted of 20 of the 1 dB blips, and the patient would indicate whenever they heard the change in loudness.  In theory, the patient with recruitment (cochlear pathology) would notice more level increments than one without. Unfortunately, this test was hard for some patients and did not do a good job of establishing site-of-lesion.

10. Well, thanks for the history, but are there any psychoacoustic tests that are currently used in clinical practice?

Oh, of course.  Right now, we see psychoacoustic tests being used in a variety of ways. 

Examples would be in measuring tinnitus, pseudohypacusis, and auditory processing disorder.  Tinnitus is typically characterized by pitch and loudness, both of which can be easily measured using an audiometer using psychoacoustic procedures (Tyler, Noble, Coelho, Roncancio, & Jun, 2015). The typical clinical method to establish tinnitus pitch is based on a matching procedure, but more recent work applies pitch ratings to yield a more complex picture of the pitches that are involved with tinnitus (Roberts, Glasberg, & Moore, 2008). I can see this kind of testing playing a role in clinical audiology once more validation of the technique is conducted. 

Another classic example of how psychoacoustics is involved in diagnostic audiology is the Stenger test, which, as you know, is used to identify unilateral pseudohypacusis. This is a procedure that really has withstood the “test of time,” as it was introduced in 1907 (using matched tuning forks, not an audiometer). It is an extremely powerful psychoacoustic test that is based on binaural lateralization: A binaural sound will be lateralized to the ear with the louder stimulus. In the Stenger test, you cleverly play a sound to the poorer (presumed feigned) ear at -10 or -20 dB sensation level (SL), and simultaneously present a 10 or 20 dB SL sound to the better ear. A patient feigning hearing loss should hear the sound in their poorer ear as louder than the sound in the better ear (because the measured threshold is higher than true), but because they are feigning the loss, they won’t respond to it. The audiologist knows, however, that the patient can hear the sound because it is audible in the better ear, and therefore detects the pseudohypacusis.  It is an extremely clever technique based on psychoacoustic principles. 

We also see psychoacoustics used in tests of auditory processing disorder, particularly temporal processing and frequency pattern sensitivity. In the temporal processing domain, gap detection (the ability to detect a quiet “gap” in a stimulus) measures are strong indicators of central auditory dysfunction (Musiek et al., 2005). Duration pattern tests, which measure how well a person can tell that sounds have different durations, also can be informative (Musiek, Baran, & Pinheiro, 1990). In both cases, people with neurological dysfunction have difficulty perceiving the gaps or the difference in stimulus duration. Musiek (1994) has also shown that deficits in the perception of frequency (pitch) patterns can be associated with central auditory dysfunction.

11. That’s all really interesting. Do you see a future for more psychoacoustic testing in diagnostic audiology?

Indeed. I can think of a lot of ways this might happen, but we are still a long way from that. One example that comes to mind applies to hidden hearing loss. This essentially refers to the idea that a person might experience difficulty with understanding speech, often in noise, but audiometric test results reveal hearing within normal limits. This is a problem I’m sure you have encountered, particularly among people in their 20s and 30s.  Of course, this problem could arise because a person’s hearing thresholds have changed, but perhaps by 5 or 10 dB from when that person was in their teens. An emerging idea is that some of these people might have cochlear synaptopathy, or damage to their auditory nerve synapses (Kujawa & Liberman, 2015). Yet, this hypothesis is extremely controversial. There is a convincing commentary by Frank Musiek and his colleagues, which argues that difficulty understanding speech in noise is more likely a central auditory processing disorder than hidden hearing loss (Musiek, Chermak, Bamiou, & Shinn, 2018).

12. Why doesn’t the audiogram detect this kind of damage?

The audiogram is a threshold detection test, and the coding of low-level, near-threshold sounds is done by only one type of auditory nerve fiber, the low-threshold fibers. Yet, there are two other auditory nerve fiber types involved in coding supra-threshold sounds, and these fibers are the ones thought to be involved in synaptopathy. Psychoacousticians are currently working on techniques that might identify deficits in supra-threshold abilities. Rather than using threshold-in-quiet tasks, they are using paradigms such as amplitude modulation detection (the ability to detect small modulations imposed on a stimulus), detecting tones in noise, and binaural detection/discrimination tasks (Plack, Barker, & Prendergast, 2014; Bharadwaj, Masud, Mehraei, Verhulst, & Shinn-Cunningham, 2015; Bernstein & Trahiotis, 2016).  Should these tools have sufficient diagnostic power, we may see more psychoacoustic testing in your future!

Even if we don’t use psychoacoustics to diagnose something like hidden hearing loss, measuring the perceptual deficits of individual listeners with sensorineural hearing loss also has the potential to benefit rehabilitation strategies, from counseling to assistive device selection.

13. That’s a pretty bold statement, and I’d like to hear more. But first, how does sensorineural hearing loss impact perceptual abilities?

Hearing loss, by definition, causes sounds to be more difficult to hear, but listeners with sensorineural hearing loss also experience degradations in the way sounds are represented by their ears. Psychoacoustic studies reveal that listeners experience deficits in a variety of auditory abilities, including the representation of spectral, temporal, and binaural information. We commonly assume that people with sensorineural hearing loss experience deficits in all of these domains. Yet the research in this area shows incredible variability in the severity of these deficits. Some patients demonstrate perceptual abilities akin to listeners with normal hearing, whereas others may have no ability to represent that particular acoustic cue at all. The audiogram is not a good predictor of how poor these deficits might be, and the only way to know how much perceptual degradation has occurred is to measure it. 

14. Should I test for any of these deficits?

At this point in time, probably not. First, these tests can be time-consuming and we don’t have clinical versions of most of them. Allocating your valuable appointment time to gather data probably doesn’t make a lot of sense from a diagnostic point of view. An important goal is to make these tests more efficient for clinical application (Shen & Richards, 2013; Shen, Sivakumar, & Richards, 2014). Second, it is unlikely that your rehabilitation approach would change based on those results. You might modify your counseling approach to a small degree, but you wouldn’t do anything differently with respect to assistive device selection. Hearing aids do not restore these supra-threshold deficits, and we need more research to know which algorithmic modifications might be helpful to address the specific deficits of an individual patient.

15. Why can’t amplification help with any of these deficits?

As a general rule, all amplification does is increase the level of sound presented to a listener albeit in a nonlinear and sophisticated way. Amplification cannot rectify the perceptual distortions associated with sensorineural hearing loss. Further complicating the problem is the nature of the amplification algorithms themselves. For example, compression greatly helps with sound comfort and hearing aid satisfaction. Yet, compression distorts the spectral and temporal cues that are needed to decode the information present in speech. Hearing aids can also reduce important binaural cues needed for sound location. With some products, the digital filtering and processing within the hearing aid produces different delays in different frequency bands, affecting interaural time differences. Compression algorithms can also result in different amounts of gain at each ear (e.g., for sounds to one side of the body), and so they also can change interaural level differences.

16. You’re almost making it sound as if hearing aids aren’t very useful?

No, that’s not the intent—just pointing out what they can’t do.  As you know, right now nothing works better to improve speech understanding than a hearing aid. We know from the speech perception literature that the primary factor in hearing aid benefit is the restoration of audibility (Humes & Roberts, 1990). If a person can’t hear speech, supra-threshold deficits are irrelevant. So, even though hearing aids might have some limitations, everyone who needs amplification should be fitted with them. Since a hearing aid can’t help with perceptual distortion, techniques to reduce noise and provide directionality are used to achieve better speech perception. I expect that we will continue to have improved algorithms that will help patients more and more. I also hypothesize that if we knew which auditory abilities were the most affected by a specific patient’s hearing loss, we could, in principle, use that information to better fit hearing aids.

17. If I hypothetically could make these measurements, which auditory abilities do you think would be best targeted to assist with hearing aid fittings?

Right now, I think the best bet is binaural auditory perception. There is a wide range of binaural abilities in listeners with sensorineural hearing loss (SNHL). Notably, Hawkins and Wightman (1980) showed deficits in the perception of ITDs in most listeners with SNHL, and Jerger, Brown and Smith (1984) measured binaural deficits using the masking level difference, a task that measures a reduction in masking due to the binaural system. However, both studies had participants with SNHL who had near-normal binaural abilities! Along with technologies already in place (e.g., digital pinna compensation algorithms), bilaterally matched hearing aids that preserve spatial cues could go a great way to help those patients who have intact binaural hearing. I would love to see a test of binaural hearing included in the clinic that would provide a functional description of a specific patient’s binaural ability (with and without hearing aids). You could also use this information in counseling to better inform a patient about realistic hearing aid outcomes.

18. You mentioned earlier that we do have tools to measure temporal processing. What might I be able to do for a patient whose temporal processing is poor?

Although there is no universally accepted method for testing auditory processing disorder, common tests included in its diagnosis are based on temporal processing: gap detection (such as the gaps in noise test developed by Musiek et al., 1990) and temporal duration pattern tests (Musiek et al., 2005). If you had a patient who performed poorly on these tests, you could use that information in your counseling by telling that patient to focus on good communication strategies, lip reading, and asking their communication partners to speak slowly and clearly. You can also speak with this patient about the realities of hearing aids and how they alone cannot solve this patient’s communication problems. While you already counsel a patient in this way, you could be more targeted in your approach. You may also be able use this information in your device selection. For example, you might want to choose devices that do not yield a lot of temporal distortion of the acoustics, such as hearing aids that use slow-acting compression, although there are numerous other factors that should be considered in this decision.

19. Ultimately, do you see a future in the connection between psychoacoustic tests and hearing aid fittings?

To date, very little hearing aid fitting is conducted using psychoacoustic tests to influence the selection process, with most fittings being based on the audiogram for the amplification targets and a patient’s personal needs. Yet, there is an emerging body of work that suggests patients with different types of perceptual problems might benefit from different hearing aid algorithms including the type and speed of compression, the frequency gain function, and the type of algorithm or technology used for directional processing and noise reduction.  An individual’s specific perceptual deficits or their cognitive abilities might be important for these kinds of decisions (Souza, Jenstad, & Boike, 2006; Oxenham & Bacon, 2003).

20. What do you think needs to happen before we reach this goal?

At this point in time, science has not connected specific psychoacoustic deficits with speech perception outcomes or treatments. Our laboratories still have to work on these ideas so that science can inform clinical practice. Right now, it is really only speculation that this knowledge could be used to make changes to hearing aid algorithms and ultimately improve speech perception. Basic psychoacoustic tests might be important, but this work needs to be conducted in conjunction with speech perception studies and physiological assessment. I would speculate that using hearing aid simulators in the laboratory will greatly facilitate the ability to connect auditory deficits with specific rehabilitation needs. Precision medicine is likely on the horizon for audiology, and it is a noble goal to connect specific auditory deficits to technology.

References

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Tyler, R.S., Noble, W., Coelho, C., Roncancio, E.R., & Jun, H.J. (2015). Tinnitus and Hyperacusis (pp 647-658). In J. Katz, M. Chasin, K. English, L. Hood, & K. Tillery (Eds.), Handbook of clinical audiology, seventh edition. Lippincott Williams & Wilkins.