C HAPTER F OUR. The Psychoacoustics of Aging: Considerations for Amplification. Pamela E. Souza. Kumiko Boike. Effect of Age on Temporal Processing

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1 C HAPTER F OUR The Psychoacoustics of Aging: Considerations for Amplification Pamela E. Souza Kumiko Boike Psychoacoustics is the study of the relationship between physical stimuli and sensations (Plack 2005). This field has a long history, dating back to at least the mid- 1800s and work by Gustav Fechner (Fechner 1966), who developed techniques still used in modern psychophysics. To most clinicians, psychoacoustics implies measuring intensity, frequency or temporal resolution using a nonspeech signal that is, something unlike the signals of interest in everyday listening. How important is it for clinical audiologists to understand psychoacoustics? From an informal survey, 27 of 30 representative AuD programs offer specialized coursework in psychoacoustics; the remaining three include some material in psychoacoustics as a component of a hearing science class. As a profession, we recognize the importance of this material. What happens once we step out of the classroom and into the clinic? Every clinical audiologist uses psychoacoustic tests, or tests designed according to psychoacoustic principles. Examples include narrow-band noise masking during puretone testing; the TEN (Threshold Equalizing Noise) test for dead regions (Moore, Glasberg and Stone 2004); the ASHA (1988) method for estimating the speech threshold at the 50% point on the psychometric function; and the on/off durations of the pulsed tone option on the audiometer. In a larger sense, psychoacoustic principles underlie all of our higher auditory functions, including speech recognition in quiet or in noise. Contact author: Pamela Souza, PhD, Associate Professor, Department of Speech and Hearing Sciences, University of Washington, 1417 NE 42 nd Street Seattle, WA 98105, psouza@u.washington.edu, phone: , Fax: Second author: Kumiko Boike, PhD, Faculty Research Associate, Department of Speech and Hearing Science, Arizona State University, PO Box , Tempe, AZ 85287, Kumiko.Boike@asu.edu. Psychoacoustic tests are used to describe how auditory abilities change throughout the lifespan (e.g., Werner 1996; Moore 2002; Schneider, Daneman and Pichora-Fuller 2002; Gross, Hall and Buus 2006). This paper focuses on psychoacoustic measures of temporal processing in older individuals. This is not to say that temporal aspects of hearing can be disengaged from spectral aspects of hearing they cannot, and the studies reviewed here include numerous examples where both temporal and spectral cues interact to determine perception. Rather, a focus on temporal processing offers a simple framework from which to consider the connection between age-related changes and hearing aid benefit. Effect of Age on Temporal Processing Although temporal processing is initiated at peripheral levels, decisions about temporal differences are thought to occur within the central auditory system (Abel 1972). Theunissen, Wooley, Hsu and Fremouw (2004) suggest that we think of this as encoding, as sound is translated into neural activity; and decoding, where neural patterns are interpreted and translated to a behavioral response. It is well known that both peripheral and central auditory physiology changes with age (Willot 1991). Beginning in the 1950 s researchers became interested in using psychoacoustics to describe age-related auditory changes. Early studies focused on frequency resolution (Meurmann 1954; Konig 1957). Temporal processing was not studied until later. As late as 1981, Marshall (p. 232) wrote that Many studies have shown that elderly listeners have difficulty in understanding temporally-degraded speech. It is thus surprising that non-speech measures of temporal analysis have received little attention. Today, temporal tasks are recognized as an important measure of the auditory abilities of older listeners. 47

2 48 Hearing Care for Adults Using Psychoacoustic Tasks to Measure Auditor y Abilities There are many choices of temporal processing tasks, including forward or backward masking, temporal order judgment, time-reversed signals, Huffman sequences, comodulation masking release, and binaural temporal processing tasks. In the next section, we 2 3 ms for broadband stimuli (Plomp 1964; Penner 1977). Tonal or narrow-band noise signals also can be used to test gap detection, although the test must be carefully designed so that there are no unintended spectral differences (caused by turning the tone on and off) between the gapped and non-gapped intervals. In within-channel gap detection, the signals that precede and follow the gap have the same frequency. In across-frequency gap Table 1. Summary of psychoacoustic results. Upward arrow indicates relatively better performance; downward arrow indicates relatively poorer performance. Younger Older Gap detection especially if target is embedded, or gap is close to signal onset/offset Duration discrimination especially if target is embedded Modulation detection although a minority of older listeners show no deficits Modulation masking although a minority of older listeners show no deficits describe four commonly used tasks (table 1). Each of these tasks mimics, although in a simpler way, the processing needed to identify some speech cue. Gap Detection This test measures a listener s ability to detect a gap within a signal. In a traditional gap detection test, a series of broadband noise pulses is presented. One of the broadband noises is interrupted to create a gap. The listener s task is to select the alternative where the gap was present. This is called a forced choice, because the listener must select the odd stimulus from a finite set of choices. The intent is to adaptively determine the smallest gap that the listener can detect. Younger listeners with normal hearing can usually detect gaps of about detection, the preceding and following signals are different frequencies. Gap detection thresholds are about 4 5 ms for younger listeners with normal hearing (Fitzgibbons 1983; Shailer and Moore 1987; Eddins, Hall and Grose 1992), which is slightly poorer than the thresholds obtained with the broadband stimuli. With rare exceptions (e.g., Moore, Peters and Glasberg 1992), most gap detection studies show that older listeners need gaps to be about twice as long as younger listeners (Schneider, Pichora-Fuller, Kowalchuk and Lamb 1994; Schneider, Speranza and Pichora-Fuller 1994; Strouse, Ashmead, Ohde and Grantham 1998; Snell and Frisina 2000). Some studies show a larger age effect for disparate frequency tasks than for isofrequency tasks (Grose, Hall, Buss and Hatch 2001; Lister, Besing and Koehnke 2002) while others do not (Grose et al. 2001; Heinrich and Schneider 2006).

3 The Psychoacoustics of Aging: Considerations for Amplification 49 When it comes to age effects, all gaps are not created equal. The effects of age are more pronounced when the gap is inserted within a tonal sequence (Gordon-Salant and Fitzgibbons 1999). Gap detection is worse in older listeners when the gap is very close to the beginning or end of the signal, suggesting that some gaps are more confusing than others (He, Horwitz, Dubno and Mills 1999). Older listeners have more trouble detecting gaps in short pulses than in longer ones (Schneider and Hamstra 1999; Snell and Hu 1999). We could say that the faster things change, the worse the older listeners perform. depth on the next trial. The just detectable modulation depth or modulation threshold can be expressed either in decibels or as a percentage between 0% (unmodulated) and 100% modulation. We also can measure modulation threshold using a number of different of modulation frequencies (Viemeister 1979). A temporal modulation transfer function (TMTF) is used to plot these results. Duration Discrimination In a duration discrimination task, the listener is asked to distinguish between sounds of different time lengths. The difference is varied according to the listener s response. If the listener can tell which sound is longer, the difference is reduced on the next trial. The just noticeable difference depends on the overall duration of the signal. Younger listeners with normal hearing can usually detect duration changes of about 10% (Abel 1972). In other words, a listener can just detect the difference between a 100 ms tone and a 110 ms tone, or between a 1000 ms tone and a 1100 ms tone. Older listeners ability to detect changes in sound duration is poorer than that of younger listeners (Abel, Krever and Alberti 1990; Fitzgibbons and Gordon-Salant 1994). Older listeners do even worse when the target is embedded in a tonal sequence (Gordon-Salant and Fitzgibbons 1993). Younger listeners perform about the same whether the target is alone or embedded. Older subjects also have more problems with an interference duration discrimination test, where the target is followed by a tonal masker, than with simple duration discrimination (Phillips, Gordon-Salant, Fitzgibbons and Yeni-Komshian 1994). In general, the more distracters around the target sound, the more difficult it is to detect changes in duration as we age. Modulation Detection Modulation detection measures the ability to detect amplitude variations in a signal. In a traditional modulation detection task, an amplitude-modulated broadband noise is presented. The listener is asked to identify which of two or more intervals contain modulation. A correct response decreases the modulation depth on the next trial, and an incorrect response increases the modulation Figure 1. Results of a modulation detection test for three younger and four older listeners with similar audiograms. Modulation detection is expressed as the threshold in decibels, with more negative values indicating better performance. Error bars show variability for an individual across repeated tests. Boike (2004) measured modulation detection thresholds at modulation frequencies of 2, 8 and 64 Hz for younger (aged years) and older (aged years) subject groups with similar audiograms. Her results (figure 1) show the expected TMTF pattern; listeners were less sensitive to modulation at 64 Hz (i.e., modulation threshold was higher). Most of the older listeners had poorer modulation thresholds than the younger listeners. Other investigators reached a similar conclusion (von Wedel, von Wedel and Streppel 1990; Takahashi and Bacon 1992; Purcell, John, Schneider and Picton 2004). That is, we can expect older listeners to need a greater modulation depth to recognize that a signal is modulated. A minority of older listeners will perform similar to younger listeners.

4 50 Hearing Care for Adults Modulation Masking In modulation masking, the listener s task is to detect a modulated target signal in the presence of a modulated masker. As with modulation detection, the modulation depth of the target is varied. Results are expressed as the amount of modulation masking (masked modulation threshold minus unmasked modulation threshold) as a function of masker modulation frequency. Multiple studies (Takahashi and Bacon 1992; Boike 2004) found that elderly listeners had more difficulty with modulation masking than younger listeners. In the Boike study, one older listener performed similarly to the younger listeners; the remainder required larger target modulation depths. In the Takahashi and Bacon study, five of seven older listeners could not complete all conditions. Thus, despite good (equivalent to young) performance by those older listeners who could complete the task, older listeners were more likely to have difficulty with modulation masking. These psychoacoustic results are consistent with reports by Turner, Souza and Forget (1994) who noted an age effect in a test in which older listeners were required to recognize a speech-modulated noise in a background of noise. In that case, the target was not regularly modulated as it would be in a modulation masking test, but contained speech envelope modulations. Nonetheless, the task required the listener to attend to one set of (target) modulations while ignoring background modulations. Some subjects commented that once the background noise was modulated, they could no longer tell what was the target and what was the masker. The same older listeners performed as well as younger listeners when the noise was not modulated. It is not surprising that older listeners who cannot detect modulation in quiet would be at a similar or greater disadvantage in background noise. In the next section, we consider the relevance of these findings to speech recognition. Connecting Psychoacoustics to Speech Recognition Those of us interested in hearing and aging are usually trained either as psychoacousticians or as clinically focused researchers. Consequently, there are few studies that bridge the gap between pure psychoacoustics and consequences for real life hearing aid use. We can certainly see the similarities between psychoacoustic tasks and speech. For example, to understand speech (figure 2), we must distinguish between shorter and The ear is an important sense organ Figure 2. Top panel: time waveform for the sentence The ear is an important sense organ spoken by a female speaker. The bottom panel shows the amplitude envelope for the same sentence. The words at the top of the figure are aligned according to the respective segment on the time waveform. longer sounds (duration discrimination), detect variations in speech amplitude (modulation detection), identify pauses or gaps (gap detection) and extract a speech signal from a background noise (modulation masking). On the other hand, psychoacoustic stimuli are relatively simpler, usually involving focused listening for a specific attribute; may be limited to a narrow frequency region rather than requiring us to listen across auditory filters; and do not offer the same redundant, contextual or visual information as real speech does. Under those circumstances, what is the relationship between performance on psychoacoustic tasks, and recognition of unamplified or amplified speech? Some experiments examining the relationship between simple temporal tests (e.g., gap detection in quiet) and complex speech recognition tests (e.g., in reverberation or background noise) have found weak or no relationships (Dubno and Dirks 1990; Takahashi and Bacon 1992; Divenyi and Haupt 1997; Strouse et al. 1998; Snell and Frisina 2000). It may be that only those factors relevant to speech recognition in the test environment will be related, and that the selected tasks tapped abilities that were not required to recognize speech, or were redundant with other abilities. For example, Gordon- Salant and Fitzgibbons (1999) found that recognition of reverberant speech was related to gap detection, but not to duration discrimination. Pichora-Fuller, Schneider, Benson, Hamstra and Storzer (2006) found that older listeners had more trouble detecting gaps for both

5 The Psychoacoustics of Aging: Considerations for Amplification 51 speech and nonspeech stimuli, and that the magnitude of the age difference was greater for complex stimuli. An alternative hypothesis is that speech is a complex signal, requiring complex auditory processing skills. The listener must attend to, compare and integrate temporal cues across multiple frequency channels; and, if listening in noise, must do so while attending to a target signal in a temporally and spectrally varying background. Perhaps only complex psychoacoustic tasks that require more effortful processing will show a relationship to speech recognition (Pichora-Fuller and Souza 2003). Only a few studies have collected complex-task psychoacoustic data and speech recognition from the same older subjects. In the Boike (2004) study, subjects who performed best on the modulation detection and modulation masking tasks also had the best speech recognition. Snell, Mapes, Hickman and Frisina (2002) found a correlation between gap detection and recognition of speech in babble when the gap detection task was conducted under similar conditions as the speech task; that is, with a fluctuating background masker. In both of those studies, performance was worse with increasing age. Percent correct (+/- one standard deviation) Age group (by decade) Figure 3. Results of consonant recognition for speech processed to restrict spectral cues. Five test conditions are shown, listed here in order of increasing spectral cues: single-channel speech in which no spectral cues are available (filled triangles); speech with two- (x), four- (shaded diamonds), and eight- (open triangles) channels of spectral information; and unprocessed speech (filled circles). What about tasks that fall in the border area between speech and psychoacoustics: speech tests, but under highly controlled conditions? Souza and Boike (2006) measured older listeners use of temporal cues using highly processed speech that eliminated or restricted spectral information. When spectral cues were unrestricted, older listeners performed as well as younger listeners with similar amounts of hearing loss. When spectral cues were restricted, forcing listeners to depend on temporal cues, including duration, gaps, and modulation, to extract speech information, older listeners performed worse than young listeners (figure 3). Another example of using highly processed speech to create psychoacoustic-like conditions is work by Haubert and Pichora-Fuller (1999), who created a speech-gap task by varying the duration of the silent interval which codes fricative/affricate (e.g., cash/catch) differences. Older adults have more problems identifying these contrasts, especially for rapidly spoken speech; and results for this speech-gap task correlate with those for a nonspeech-gap task. Gordon-Salant, Yeni- Komshian, Fitzgibbons and Barrett (2006) took a similar approach by manipulating temporal cues in natural speech: vowel duration (wheat/weed); voice-onset time (buy/pie); glide duration (beat/wheat) and silent interval duration (dish/ditch). Older listeners perform more poorly than younger listeners when silent interval duration or glide duration is manipulated. Specifically, older listeners require a larger change in the acoustic cue than younger listeners do. There is no effect of age for manipulation of vowel duration or voice-onset time. Similarly, Lister and Tarver (2004) find that older listeners have more difficulty detecting temporal gaps in dynamic stimuli that simulated the frequency characteristics of speech. Taken together, these studies provide convincing evidence that deteriorating temporal processing in older listeners affects their ability to recognize speech. Making Decisions About Amplification Parameters What can we say about hearing aid fitting based on this research? If there is little research on how psychoacoustic results relate to speech recognition in older listeners, there is even less on how that relationship is affected by amplification. We can speculate, however, based on what we know about the acoustic effects of amplification. For example, consider the speech gaps illustrated in figure 2. Although a hearing aid would not literally shorten the gap duration, the variable gain of a fast-acting WDRC amplifier will

6 52 Hearing Care for Adults CL WDR % Correct Presentation Level db SPL CL WDR % Correct Presentation Level db SPL Figure 4. Right panel: consonant recognition scores for four-channel, wide-dynamic range compression (WDR, filled circles) and linear output compression limiting (CL, open triangles) amplification. Left panel shows the listener s audiogram. reduce the contrast between low and high-amplitude sounds (Jenstad and Souza 2005). If there is background noise, fast-acting WDRC amplification can increase noise during silent periods in the speech, effectively filling the gaps (Souza, Jenstad and Boike 2006). Both effects can reduce the perception of gaps, and thus perception of sounds cued by those gaps. This is supported by consonant error patterns for speech amplified by fast-acting WDRC (see Chapter 5 in this proceeding). There is also evidence that some older listeners need longer release times than younger listeners, possibly related to their temporal processing abilities but also more globally related to cognitive capacity (Gatehouse, Naylor and Elberling 2003). However, such work is in early stages. An eventual goal for clinical practice is to identify characteristics of the listener that might dictate signal processing choices. As any clinical audiologist knows, the audiogram alone is not sufficient to make all

7 The Psychoacoustics of Aging: Considerations for Amplification 53 decisions about amplification parameters. This is illustrated in figure 4, which shows results of amplified speech recognition testing for the right ear of two listeners with similar thresholds. The plots on the right show a comparison of consonant recognition at a range of input levels for four-channel fast-acting WDRC and linear output compression limiting. Listener 1 (top panel) performs similarly with both amplification types, with a slight advantage for compression limiting. Listener 2 (lower panel) does better with WDRC for soft speech, and better with compression limiting for loud speech. Understanding the source of these differences may help clinical audiologists select specific compression parameters to maximize speech understanding. Controlling for Age-Related Hearing Loss Older listeners in general, and certainly those seen by clinical audiologists, are likely to have some high-frequency hearing loss (Gates, Cooper, Kannel and Miller 1990). Listeners with sloping loss hear the world through a low-pass filter. This reduced listening bandwidth lowers performance on temporal processing tasks (Bacon and Gleitman 1992). This issue is important to young-old comparisons, especially if the comparison group is younger listeners without hearing loss. To study temporal processing, older and younger listeners should be matched for audibility (audiogram). More recently, some investigators have used a shaped masking noise to equate signal audibility for younger and older listeners with slightly different audiograms (e.g., Dubno, Horwitz and Ahlstrom 2005; Gordon-Salant et al. 2006). Practical Considerations in Psychoacoustic Tests of Older Listeners Traditionally, psychoacoustic test setups consist of a two- to four-button response box and a set of supraaural headphones. The signal interval is indicated with LEDs on the response box and the response options are indicated with (usually small) labels. Older psychoacoustic studies also tested only a few listeners, sometimes with many hours of testing for each person. With today s emphasis on sufficient experimental power, and also the recognized diversity of older listeners, larger groups are needed. Subjects in their 80s may not have the stamina (or, more commonly with our subjects, the time out of their busy schedule) to sit in front of a response box for hours. Thus, we need a new approach to psychoacoustics for older listeners. With improvements in technology, many different presentation and response modes are available. For example, insert earphones have replaced supraaural phones for many experiments, and in addition to improved comfort and hygiene they also solve the problem of collapsing canals in older adults (Marshall 1981). Vision, manual dexterity and sensitivity to touch all decrease with age (Carmeli, Patish and Coleman 2003; Gohdes, Balamurugan, Larsen and Maylahn 2005). In our own work, we have found that a response box is not the best response mode: it is unfamiliar; requires more training; and older listeners are sometimes confused by the need to associate an interval light with a specific button. Similarly, using a computer mouse for subjects unfamiliar with computer use meant the risk of selection errors from mispositioning the mouse cursor. We now use a large touch screen. Selection choices on the screen can be indicated with a large font size, different color text, or graphics. We also have found that our older subjects benefit from longer pauses between intervals, warning signals before each trial, adequate lighting, good temperature control, and a footstool (joint problems are common in our subject population). Finally, linking psychoacoustic results with hearing aids requires a thorough understanding of clinical issues. Although the goal may be to present processed speech under controlled conditions, it should be done in such a way as to have consequences for clinical practice. For example, speech can be presented via headphones under simulated conditions, but with frequency shaping similar to that used in the clinic rather than a flat amplification that might compromise audibility for subjects with hearing loss. Other processing parameters, such as compression ratios or number of channels, can be chosen to suit the experiment goals but also to be clinically realistic. Finally, with increasing sophistication and proprietary techniques for digital aids, coupling hearing aid simulations with psychoacoustic testing allows the experimenter to quantify all aspects of processing. Someday, we would like to be able to offer a simple, quick, clinically realistic psychoacoustic test that could dictate how amplification parameters would be set. Such work needs a combination of psychoacoustic principles with an understanding of clinical amplification parameters. Acknowledgment Work supported by National Institutes of Health (DC006014, F31 DC05092 and F32 DC007004)

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10 56 Hearing Care for Adults Viemeister, N Temporal modulation transfer functions based upon modulation thresholds. Journal of the Acoustical Society of America 66: von Wedel H., von Wedel U.C., and Streppel M Monaural and binaural time resolution ability in the aged. A psychoacoustic and electrophysiological study. Acta Otolaryngologica Suppl. 476: Werner, L.A The development of auditory behavior (or what the anatomists and physiologists have to explain). Ear and Hearing 17: Willott, J.F Aging and the auditory system: Anatomy, physiology, and psychophysics. San Diego, CA: Singular Publishing Group.