From the desk of Gus Mueller
We all learned in Hearing Aids 101 that's it's important to individualize the hearing aid fitting for a given patient. To do this, it's helpful to conduct some pre-fitting testing. The results of this testing nearly always help with patient counseling, and in some cases, can even alter the hearing aid style or specific features that we might select.
A common pre-fitting measure is frequency-specific loudness discomfort levels (LDLs), which can be used to provide guidance in setting the AGCo kneepoints. Our test battery often also includes a speech-in-noise test such as the QuickSIN, BKB-SIN or WIN. We might also do an ANL if we are interested in the patient's acceptance of background noise. And what else? A measure of the patient's perceptions of their understanding of speech (e.g, the PPT)? A dichotic speech test, which might help with predicting success with bilateral amplification? A measure of expectations—the ECHO maybe? Personality testing? The list goes on.
A pre-fitting consideration that seems to be discussed more and more each year is the patient's cognitive function. A test of cognitive function is especially intriguing in that unlike many other pre-tests, there is some evidence to suggest that the results could influence how we program the hearing aids. Should audiologists be conducting cognitive testing for their hearing aids patients? And what tests would we use? And what guidelines do we have for changing programming even if we do the tests? Not easy questions to answer, but we've found someone to give it a try this month here at 20Q.
Pamela Souza, Ph.D. is an Associate Professor at Northwestern University, where she not only directs research, but is actively involved in the academic program, and also works in the Northwestern Audiology Clinic. She is a Fellow of the American Speech-Language-Hearing Association, Associate Editor for the Journal of Speech, Language and Hearing Research and serves on the Psychoacoustic and Perception Technical Committee of the Acoustical Society of America. Her research in the area of hearing aids is known and respected internationally, and has been supported by the National Institutes of Deafness and Communication Disorders and the Department of Veterans Affairs.
It's understandable that Dr. Souza's many years of research related to hearing aid signal processing has led to her work related to cognition. We think a lot about cochlear distortions, but does the patient's brain care what type of compression we use? Or only certain brains? Pam's journey to cognition-related research also may have been influenced by her passion for "quilting," but we'll let her explain that path. Regardless, her research findings certainly give us something to think about, and we're fortunate to have her relate some of her most recent results and clinical impressions here at 20Q.
Gus Mueller, Ph.D.
To browse the complete collection of 20Q with Gus Mueller articles, please visit www.audiologyonline.com/20Q
1. Everyone seems to be talking about "cognition" these days. What exactly does that mean?
In general, we can think of cognition as an umbrella term for a variety of mental processes, including awareness, executive function, reaction time, processing speed, memory, and attention. I suspect, however, that you are mostly interested in the relationship between cognition and auditory processing. In a 2008 paper, Akeroyd (2008) reviewed 20 studies that measured some aspect of cognition and whether there was a relationship to speech understanding in noise. The strongest relationship was found between working memory and aided or unaided speech recognition. So maybe that's something we can talk about first?
2. Sounds good to me. I've heard of working memory, but how is it different from memory in general?
You might remember from your psychology courses that memory includes long-term and short-term storage. (Short-term storage capacity was one of the reasons that phone numbers were originally 7 digits. With phone numbers now 10 digits, plus most of us using both cell and home phones, it's no wonder that many of us can't remember anyone's number!). As new information becomes available (such as information from spoken speech), it is actively compared and integrated with information in our mental lexicon. In other words, there is an active two-way processing path between stored information (linguistic knowledge) and new information. So, to measure working memory, we need to think about measuring both storage and processing.
3. Wait a minute. I'm mostly interested in the effects of hearing loss. Where does cognition come in?
As clinicians, we spend a lot of time measuring the auditory periphery. We usually don't measure higher aspects of the system in a direct way, but they are important. If you think of a typical situation, such as a restaurant, there's a lot of processing going on. The listener first needs to attend to the signal and to ignore other talkers. They may also be switching attention between different talkers. During the conversation, they are processing and interpreting the ongoing and rapid flow of information. If there's background noise (which is likely), they may be assembling "glimpses" of the target signal as it's masked out by fluctuating background noise. At the highest level of processing, they're assigning meaning to what they heard. If you think about the situations your patients have difficulty in (and complain about!), you'll realize how important cognition is for everyday listening.
4. OK, that makes sense. Can you tell me a little more?
Sure. Let me explain more about how working memory is involved in speech understanding, using a model called the Ease of Language Understanding (ELU) Model (Ronnberg, Rudner, Foo, & Lunner, 2008). The basic idea is that the listener receives an incoming signal that includes a stream of multi-modal linguistic information. That information includes phonology, semantics, syntax, and prosody. If the listener can see the speaker, it also includes visual cues that need to be integrated with the auditory information. Under ideal conditions, such as speech in quiet to a listener with normal hearing, there is a rapid match to stored representations in long-term memory. So if you say, "hit the ball" I quickly picture a baseball and bat (or a tennis racket, etc.) and I don't need to work very hard to assign meaning to that statement. Under distorted conditions (such as noise, reverberation, or distortion from a hearing aid or cochlear implant), or degraded input from an impaired auditory system, the stream of phonological information is incomplete and the stored representations cannot be matched. That mismatch triggers a need for effortful processing, engaging working memory. So another way to say this is that effortful cognitive processing of speech is activated only in situations where the auditory input cannot be easily matched to a phonological representation stored in long-term memory.
5. Well, that seems reasonable...but is there any evidence this really happens during difficult listening situations?
If that model is correct, and working memory is important when listening in difficult or noisy situations, we would expect to see a relationship between working memory and the ability to understand speech in noise. Specifically, we expect that patients who have poor working memory will not be able to recognize speech in noise very well. Data from our lab and others show that to be true. As one example, we tested a group of older listeners with mild-to-moderate sensorineural loss using a version of the Reading Span Test and the QuickSIN (Killion, Niquette, Gudmundsen, Revit, & Banerjee, 2004). You might not be familiar with the Reading Span Test—I can tell you more about it later if you like. Our data showed a clear relationship: patients with good working memory had better QuickSIN scores (Souza, et al., 2011). There was also more variability among the patients with poor working memory than those with good working memory. We can interpret these data to say that if a patient has good working memory, he is likely to be able to understand speech in noise to the extent possible according to his pure-tone audiogram. If the patient has poor working memory, he's likely to have poorer-than-expected speech recognition in noise, but his performance will also be less predictable.
6. So, is cognition completely separate from the degree of hearing loss?
Yes and no. You can have good or poor cognition regardless of how much hearing loss you have. One connection emerges when we consider that peripheral auditory damage prompts greater reliance on cognition. The auditory signal is only the beginning--there's a lot of sophisticated processing happening "downstream" that is necessary for good communication. If the auditory signal is degraded, the downstream processor has less to work with. For example, in a recent study (Rossi-Katz & Arehart, 2009), the ability to segregate simultaneous speech signals and attend to and recognize a target talker was predicted by both cognitive ability and high-frequency auditory thresholds. That helps explain why the consequences of cognition are more pronounced when listening conditions are challenging because of background noise, hearing impairment, or both (Pichora-Fuller, Schneider, & Daneman, 1995; Stenfelt & Ronnberg, 2009; Schneider, Daneman, & Pichora-Fuller, 2002).
7. How do you clinically measure working memory?
The idea of measuring working memory has been around for decades. Early working-memory tests relied more on storage, and less on processing. More recent work in this area used a version of the Reading Span Test (Daneman & Carpenter, 1980). The Reading Span Test was designed to measure simultaneous storage and processing capacity. In studies concerned with speech recognition, it's usually given as a visual (reading) test rather than an auditory test, so that results will reflect cognitive capacity and not be contaminated by hearing loss. During the test, the participant views a sequence of sentences which are shown one to two words at a time on a computer screen. Examples of sentences include "The snail crept slowly" and "The fish drove a car". After each sentence, the participant is asked whether the sentence made sense. (Although that response can be scored, it often isn't considered further--its primary purpose is to engage cognitive processing by forcing the listener to assign meaning to the sentence.) After several sentences have been presented, the participant is asked to recall the first or last words of those sentences. The number of sentences presented prior to recall increase during the test, and the final score is the percentage of correctly recalled words.
8. If I give the Reading Span Test, how do I interpret the results? Does a patient "pass" or "fail"?
There are no norms, as such, but there is a lot of information about what to expect. In a sample of 173 hearing-impaired patients, Lunner (personal communication) found a mean score of 42% correct and a range (+/- two standard deviations) of 9%-76%. A different sample of patients tested in our lab had a mean score of 40%, and a score range (+/- two standard deviations) from 17%-72%. From these data, we expect the average score for hearing aid candidates to be around 40%. (When we tested a group of college students in their 20s, the average score was about 70%.) Most research studies have divided patients into groups according to how they scored on the test. So, one simple way to interpret scores in a clinical context is to consider patients with scores below 40% to have poor working memory, and patients with scores above 40% to have good working memory.
9. This sounds like a test I'd like to try out with my patients. Can you recommend a working memory test that I can use in my practice?
A computerized version of the Reading Span Test is available. That version has proved very informative in research studies (Hallgren, Larsby, Lyxell, & Arlinger, 2001; Ronnberg, Arlinger, Lyxell, & Kinnefors, 1989; Lunner, 2003), but it does take about 20 minutes to administer and score. We still need a more convenient test format as well as more research on what this tells us about speech understanding. Several labs are working on this, so expect other clinically-appropriate cognitive tests to be available in the near future.
10. You said something about working memory being important in cases where a particular type of hearing aid processing might not be appropriate. What did you mean by that?
I'm glad you asked, because I think this is one of the most interesting and potentially valuable reasons to think about working memory. There is evidence in the research literature that working memory affects the way an individual may respond to hearing aid processing. Let's focus on two different aspects of hearing aid processing: fast-acting wide dynamic range compression (WDRC) and frequency lowering. Both of these are sometimes used in digital hearing aids, and both offer potential for improved speech-sound audibility and recognition. But both also alter the acoustic properties of the signal in a way that may not be suitable for every patient.
11. OK, I'm interested. I sometimes fit products that have a fast compression release time. Should I be thinking about working memory when I do?
First, let's think about what a short release time (with a low compression kneepoint) will do to speech. The hearing aid gain is adjusting very quickly, so the hearing aid improves consonant audibility but at the expense of more alteration of the signal. The longer the release time, the more similar the amplitude variations (or "envelope") of the output signal will be to unprocessed speech. To understand where working memory comes in, consider one study in which the experimenters measured working memory, attention and reaction time (Lunner & Sundewall-Thoren, 2007). They fit their patients with two-channel WDRC with two different compression settings: a fast WDRC with a 40 ms release time and a slow WDRC with a 640 ms release time. Patients wore each compression setting for 10 weeks and speech recognition was measured in unmodulated and modulated noise. For the modulated noise, patients with better working memory did better with fast-acting WDRC. In fact, the cognitive score explained 39% of the variance in performance, while the amount of hearing loss explained only 3%! So, this really supports the idea that in a complex listening task (in this case, the noise is modulated so that you need to "glimpse" the signal of interest through the noise) and when fast WDRC is altering the signal, cognition matters. That reinforced earlier work which showed a similar relationship between cognitive ability and release time (Gatehouse, Naylor, & Elberling, 2006).
12. Well . . . okay, but that's just one study. Do you have more evidence that this really matters to my hearing aid patients?
We do. In a recent study (Souza & Sirow, 2012), patients seeking hearing aids in a private practice clinic were asked if they would complete some extra tests. It's important to note that these patients were not originally seeking opportunities for research participation, so they are a good sample of the type of patients each of us might see in an audiology clinic. The group had a median Reading Span Test score of 35% with a range (+/- 2 standard deviations) from 14% to 53%. We classified patients who scored below the group median as having poor working memory, and patients who scored above the group median as having good working memory. We also measured aided speech-in-noise testing using the QuickSIN, with a variety of hearing aid release times ranging from 12 ms to several seconds. The group with good working memory performed well with all the different compression settings, with a slight (although not statistically significant) tendency for better speech-in-noise scores with fast WDRC. The group with poor working memory patients performed better with the slow WDRC aid than the fast WDRC aid. The average difference in QuickSIN score with the slowest versus fastest release times was about 4 dB.
13. Does a 4 dB SNR advantage really matter?
In many listening situations, yes. Remember that a 4 dB difference in QuickSIN score is enough to move from the "moderate difficulty in noise" category to the "mild difficulty in noise" category. And a 4 dB improvement in SNR is about the same improvement as you can obtain from good directional microphone technology, which translates to as much as a 25% improvement in overall speech understanding (Killion, 2004). Of course, not every patient is guaranteed to show that much difference, so we still need to study what individual factors are involved. But the conclusion we drew is consistent with other data (Lunner & Sundewall-Thoren, 2007; Lunner, 2003; Gatehouse, Naylor, & Elberling, 2006): patients with low working memory tend to do better with slow-acting compression. I think an important point is that the Souza and Sirow patients were "regular patients" fit with typical hearing aids under relatively uncontrolled conditions, letting the hearing aids operate in their default mode and without doing any special manipulations to conduct a "research" test. That should give us a pretty good idea of what will happen in real listening environments.
14. Good point. What about other types of hearing aid signal processing?
An early study in this area found that patients with good working memory were better at identifying differences in noise reduction parameters than patients with poor working memory, suggesting those two groups were responding differently to that processing (Lunner, 2003). We have also looked at working memory and frequency lowering; specifically an algorithm using frequency compression. As you probably know, the goal of frequency compression is to shift high-frequency cues down into a range where they are more audible. This particular study (Arehart, Souza, Kates, & Baca, submitted) was a laboratory study in which frequency compression was applied to a set of sentences in noise. All of the patients had sloping hearing loss, and all of them received frequency-gain responses appropriate to that loss. So, these individuals were listening to sentences in noise, which is already a difficult situation; plus we were manipulating the signal by compressing it in frequency. We knew a lot about these patients: we knew how much hearing loss they had; how old they were; what their working memory was (using the Reading Span Test and dividing patients into good and poor groups); and how well they could understand the test sentences. We found that patients with good working memory had significantly better scores than patients with poor working memory. Patients who were older or had worse high-frequency thresholds had poorer intelligibility. The listeners with poor working memory always did worse, but especially when the amount of signal distortion increased (either more noise, or more frequency compression).
15. Does this mean that I shouldn't use frequency compression when patients have poor working memory?
Well first, for adults with long-standing hearing loss, I'm not sure that there are sufficient data to support the use of frequency compression for those with good working memory. But also, remember that this was a laboratory study where the frequency compression was not the same as that used in wearable hearing aids. For one thing, frequency compression was combined with frequency shaping, but not with other features usually found in wearable aids like multichannel WDRC and noise reduction. Second, the effect occurred with lots of frequency compression, probably more than you would choose to use for all but the most unusual audiograms. But when you consider these data along with the fast-acting WDRC studies we talked about earlier, what it suggests is that we need to be conservative when manipulating amplification parameters for patients with poor working memory. With both fast WDRC and frequency compression, we may be improving signal audibility but altering the signal in a way that doesn't suit listeners with lower cognitive ability.
16. What is it about poor working memory that makes you more susceptible to signal alteration?
We don't know the answer to this yet, but if we go back to the idea of the ELU model--that working memory is activated when there's not an immediate template match to the lexicon--we could hypothesize that manipulating the acoustic signal creates a mismatch compared to the expected (familiar) speech signal. Making the match requires working memory, and the patients with poor working memory are at a disadvantage. Consider that with fast-acting WDRC, the soft components of speech (usually consonants) will be given more gain than the louder components (usually vowels). Because the hearing-aid gain is changing so rapidly, the consonant audibility is improved but now the consonant/vowel ratio within a word is different than it was before the person wore hearing aids. It's possible that given time they would learn to easily match this "new" representation of the word to the lexicon (that is, they would acclimatize) but so far that hasn't been demonstrated.
An interesting point was raised by Cox and Xu (2010). Like some of the studies described earlier, they grouped patients by good or poor cognitive ability and compared speech-in-noise recognition for long and short release times. However, their low-cognition group performed better with a short than with a long release time—the opposite of what other researchers have found. Cox and Xu suggested that the explanation lies in the type of speech material. Their test used sentences that had a lot of contextual information. They argued that if the listener can use that information, he may not be so dependent on signal acoustics. In that case, access to contextual cues might minimize the negative effect of a short release time; or even reverse it, because now the listener can combine improved consonant audibility with more certainty about what the word is likely to be. It really reinforces the idea that listening is a complex task and the way we perform that task is different in different situations.
17. So I have to ask, should a clinical audiologist really be measuring cognition?
This is not something most of us were trained to do in our audiology coursework, but I wouldn't be at all surprised if it's something we will be doing clinically in the relatively near future. It is beginning to be acknowledged as within our scope of practice. In fact, the American Academy of Audiology guidelines published in 2005 (Valente, et al., 2006) already recommend that we consider non-auditory tests including cognition and personality as part of the hearing aid evaluation process.
18. What should I tell my patients about this testing?
We've found that if we explain to patients how working memory is involved in listening, especially noisy listening, it makes sense to them. In fact, they may have noticed this connection themselves. Patients often tell us that listening in difficult environments makes them tired; that they have to work harder; and that they miss parts of ongoing conversations because they are still trying to figure out what they just heard. One caveat: when we give these tests, we've found it sometimes causes patients to worry about memory loss and dementia. There is no evidence that patients with low working memory scores today are going to develop dementia in the future, so this is an important fear to allay.
19. Suppose I've figured out that a particular patient has poor working memory. Is there anything I should do differently, besides setting my hearing aid parameters conservatively?
Yes. Remember that there is a strong association between low working memory and difficulty understanding speech in noise. This means that your patient is going to struggle in many everyday listening situations, even with the best hearing aids. Other treatment options include assistive listening devices, such as personal FM systems; offering supportive aural rehabilitation such as speechreading; or offering extra counseling to encourage realistic expectations and suggest communication strategies. Although there is no evidence yet that providing such options for patients with poor working memory can better address their issues, it seems reasonable to target extra clinical services to those who need them the most.
20. What can I expect to happen next in this area?
As clinicians, we're not going to spend our precious time measuring something that's only of academic interest. We are only interested in incorporating a new test if it tells us something--in this case, if it directs the decisions we make--and if that test is convenient and efficient to do in the context of a clinical appointment. There are a number of researchers in both the audiology and psychology domains who are working to better define the consequences of reduced cognition and how it should direct clinical choices. Watch for a more convenient and time-efficient working memory test and also for more research to confirm whether patients with poor working memory should be fit with different hearing aid processing. It's an exciting time for audiologists as we improve our focus on the whole system—ear and brain together, and gain the diagnostic tools to do it.
The author thanks collaborators Kathryn Arehart and Lynn Sirow; and Thomas Lunner and Jerker Ronnberg for helpful conversations about working memory. Preparation of this article was supported by the National Institutes of Health (R01 DC60014).
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