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Measurement Procedures For both theoretical and practical reasons, it is impor- tant to obtain characterizations of tinnitus--spectral location, degree of spectral complexity, magnitude, etc.-- that are as accurate as possible. Unfortunately, little basic research has gone into the important psychophysical questions of: (1) comparability of the various existing measurement procedures, (2) their test-retest reliabili- ties, (3) their relative efficiencies with different types of patients, etc. Such research may be unexciting, but it is necessary as a basis for establishing a standardized test procedure. The reader should keep in mind throughout this section on tinnitus measurement that for many people tinnitus is not constant in character, either within a day or across days. This raises two problems: (1) the psychophysical procedures, which are already unusual and troublesome for many naive subjects, can be made additionally difficult and frustrating by short-term fluctuations in the tinni- tus; and (2) these short-term and long-term fluctuations necessarily create an uncertainty as to whether what is measured on a given day is truly representative of the tinnitus experienced by the sufferer. Basically, there are three questions one might ask about a tinnitus: What is its quality? What is its spectral location? What is its magnitude? We consider these questions in turn. QUALITY OF THE TINtIITUS What is the perceived quality of the tinnitus? This ques- tion pertains to both its temporal and its spectral char- acteristics. Does it sound continuous, fluctuating, 32
33 interrupted, etc.? Is it spectrally simple--tonal or reasonably narrowband--or is it spectrally complex-- wideband, multiple but relatively discrete frequency regions, etc.? From where does it seem to emanate--one ear, both ears, somewhere inside the head, etc.? It is intuitive that different tinnitus qualities would be associated with different sites of origin and, thus, that information about quality would be important in diagnosis. Unfortunately, tinnitus quality is rarely an accurate guide to site of origin. Some exceptions are the pulsa- tile tinnitus of objective vascular origin and the low- frequency roaring tinnitus of Meniere's Disease and otosclerosis. Most of what we know about the quality of tinnitus comes from patients' self-reports, a procedure that is fraught with problems. For example, we cannot be sure that when two patients use the same word to describe their tinnitus--(say) "chirping" or "chuggingn--they mean the same thing. Different people have had different acoustic experiences and, as a consequence, may use words differ- ently. What a "high-pitched squeal" is to a piccolo player and to a nonmusician who has considerable high- frequency hearing loss obviously could be very differ- ent. Further, the words chosen by a tinnitus sufferer as best describing his or her experience may or may not touch upon all of the dimensions of interest to the scientist, and, unfortunately, the questioner may not always ask the follow-up question(s) necessary to discover the omitted information. Standardized procedures for gathering infor- mation on tinnitus quality would be welcome and valuable. With the various problems of self-report procedures in mind, let us consider some results. Heller and Bergman (1953) supplied a list of 39 words used by 80 normal-hearing and 100 hearing-impaired sub- jects to describe their tinnitus. For those with normal hearing, "hum, "buzz, n nring, n and "pulse" were used by 18 percent, 15 percent, 12 percent, and 8 percent, respec- tively. For the hearing-impaired, "ring," nbuzz,n "hum," and "whistle" were used by 30 percent, 11 percent, 10 per- cent, and 9 percent, respectively. Reed (1960) reported that about 70 percent of his 200 patients used "steam," "ring, n or "buzz"; he also noted that the patients' de- scriptions were not predictive of site of lesion--an un- fortunate but common finding. Some attempts have been made to acquire more objective information about the quality of tinnitus. Goodhill (1952) supplied patients with recordings of 27 different
34 sounds created to mimic previous descriptions of tinnitus. Patients were asked to indicate which sound was the most like their tinnitus, but only a few cases were reported. Hazell (1979b, 1981c) has developed a procedure that has much appeal. He uses a commercially available music syn- thesizer to create imitations of the patient's tinnitus. The procedure is said to be time-consuming--sometimes tak- ing as long as 2 hours--but rewarding to the patient and informative to the experimenter. The patient is pleased to finally have a nonverbal way of communicating his ex- perience to family, friends, and the clinician, and the experimenter discovers the wide range and richness of tinnitus experiences. One finding particularly worthy of note here is that tinnitus described as tonal by many patients is much more complex than a tone--a common product was a single frequency embedded in a less-intense narrow band of noise. To date about 200 patients have been studied in this way. Only preliminary findings are yet available, but these indicate that about 83 percent of the synthesized waveforms are nonpulsatile, about 52 percent involve a narrowband noise, and about 39 percent a tone, singly or in combination. Correlations of tin- nitus quality with presumed site of origin have yet to be reported. Related to this question of quality is the matter of the characteristics of the tinnitus in the two ears. Logically, there are multiple possibilities. The tin- nitus might be strictly monaural. It might be binaural and very similar in the two ears. It might be binaural and of similar spectral and temporal characteristics but not of equal magnitudes in the two ears (see "Is the Tin- nitus Monaural or Binaural? n in this chapter). It might be binaural and of quite different spectral and/or tem- poral characteristics in the two ears--(say) tonal in one ear at one frequency and rather broadband in the other ear at a different frequency. m is issue of possible binaural differences in tinnitus has not been extensively studied, but clearly it is important (and may explain some of the failures of (monaural) devices like tinnitus maskers/instruments to alleviate tinnitus). Until more is known, it should be clear that the pitch-matching and masking procedures described below should be done with headphones, not free-field presentations, so that there is at least the opportunity for the patient and the examiner to detect different origins for different aspects of the tinnitus.
35 SPECTRAL LOCATION OF THE TINNITU S If the tinnitus is reasonably narrowband or tonal, where is it located spectrally? Two basic procedures--with numerous major and minor variations on each--have been used to obtain measures of the spectral location of a tinnitus. Pitch Matching As the name implies, in pitch-matching procedures the objective is to obtain from the subject a match in pitch between the tinnitus and a sound supplied by the experi- menter. Fowler (1928) and Josephson (1931) were among the first to use pitch-matching procedures. In the most common procedure, the matching sound is tonal; it is pre- sented to the ear contralateral to the one the patient believes contains the tinnitus (see "Is the Tinnitus Monaural or Binaural?" in this chapter); it is interrupted at a regular rate; and it is not adjusted by the patient, but by the experimenter on the basis of responses made by the patient. While pitch matching is perhaps the most intuitive pro- cedure for ascertaining the spectral locus of a tinnitus, it is, unfortunately, subject to major problems. For one thing, tinnitus is often not strictly tonal in quality (see "Quality of the Tinnitus" in this chapter), which makes matching to tonal stimuli both very difficult and of questionable value once accomplished. Further, in the best of circumstances, pitch matching is a difficult task even for highly practiced subjects, let alone for rela- tively inexperienced patients. In both populations, it is common to see so-called octave errors--settings for a match that are actually twice or one-half the frequency of the sound being matched (see Vernon, 1977). Pitch- matching procedures have their place in tinnitus diag- nosis, but the results must be interpreted with special care. The problem of octave errors is particularly bother- some when interpreting pitch-matching data obtained from tinnitus patients. One reason is that tinnitus patients are often given only the sounds available from a standard audiometer for use in matching their tinnitus. That is, frequencies above about 6-8 kHz are often not available to the patient. Among the problems raised by this equip- ment limitation is the possibility that the tinnitus
36 frequency is being greatly underestimated in many cases of high-frequency tinnitus. As noted above, about 64 percent of tinnitus sufferers match their tinnitus to frequencies in the range 3-8 kHz. But, since the fre- quencies available on audiometers are rarely higher than about 8 kHz, it would be impossible for a patient with tinnitus at (say) 12 kHz to reveal that fact; his "best match might be a 6-kHz tone or noise band. The use of standard audio oscillators or of the recently introduced tinnitus-measuring devices (e.g., Voroba, 1979a) would reduce this problem of underestimating tinnitus frequency. Furthermore, all tinnitus examination protocols should include a procedure to verify that any pitch match pro- duced by a patient is not in error by one or more octaves in one direction or the other (Vernon and Meikle, 1981). Penner (personal communication) has recently completed an extensive study using a pitch-matching procedure. Three sensorineural subjects with a history of noise ex- posure made approximately 80 matches over the course of 4 weeks. m e surprising finding was that for all subjects the frequencies chosen as matches to the tinnitus had a tremendous range across sessions--from 2 to 5 kHz for A similar effect was briefly noted different subjects. by Voroba (1979b). If this finding is confirmed, it will have great theoretical and practical importance. Nearly all tinnitus sufferers comment on the fluctuating nature of their perception, but it seems safe to assert that few investigators realized that it varied so much in fre- quency. If it does, it has obvious implications for the design of tinnitus maskers/instruments (see "Tinnitus Maskers/Instruments" in Chapter 4). Masking m e second basic procedure for estimating the spectral locus of tinnitus, masking, has the appearance of being free from some of the problems of the pitch-matching pro- cedure. In masking, a relatively narrowband (octave or one-third octave) stimulus is swept across the spectrum in successive steps and at each is adjusted in intensity until the tinnitus is just masked. me spectral location at which the least masker intensity is needed is taken as the locus of the tinnitus. This procedure is attractive because it appears to be more accurate than, and to lack some of the problems of, the pitch-matching procedure-- the task seems to require less sophistication and prac
37 Lice than pitch matching, and octave errors are less likely to occur. One problem noted with pitch matching applies to masking as well: if the tinnitus lies beyond the range of the equipment, such as a standard audiometer, it may be maskable, since lower frequencies can mask higher ones, but inferences drawn about the spectral locus (and the magnitude) of the tinnitus will be in error. There is another problem that is not so much pro- cedural as it is evidence of the great variety of maladies that can underlie tinnitus: in some patients the tinnitus cannot be masked, and in some others it can be masked by nearly any weak tone or noise band. Feldmann (1971) re- ported that the tinnitus in about 11 percent of his 200 patients was completely refractory to masking; they tended to be the patients with the most severe sensorineural losses. At the other extreme, about 32 percent of Feld- mann's patients reported masking of their tinnitus when any weak sound was presented; these tended to be patients having flat hearing losses due to Meniere's Disease, sud- den deafness, or otosclerosis. One must assume that, for this latter group, tinnitus must not have been much of an everyday problem, since environmental noise would pre- sumably be adequate to mask the tinnitus. Vernon et al. (1980) confirm Feldmann's observations that masking of tinnitus is different from masking of real sounds in a number of ways. This issue is addressed in the section "Some Ways Tinnitus Is Not Like an External Sound" in this chapter. Related Masking Results Another interesting finding by Feldmann (1971, 1981) deserves note and further study. In some patients with monaural tinnitus, he claims to have been able to mask it with tones and noise bands delivered to the opposite ear (also noted by Josephson, 1931). Feldmann argues that the obvious interpretation of cross-conduction can be excluded; in fact, the intensity necessary to mask the tinnitus was often less when presented to the contra- lateral ear than to the ipsilateral ear. Apparently he saw this contralateral masking effect most regularly with Meniere's and sudden deafness patients, but it was also present in presbycusic and noise trauma patients. Note that a contralateral masking effect can be ex- plained in a least two ways:
38 1. The contralateral masker in some way produces a reduction in the tinnitus signal at its source. By way of an example. the efferent mechanism might be activated, , _ _ , thereby causing a change in the hair cells or primary fibers in the contralateral (tinnitus-producing) ear. This alternative is meant to encompass Feldmann's own suggestion of a "neural mechanism of contralateral inhibition." 2. m e contralateral masker accomplishes a n true" masking at some neural level where information from the two ears is combined (the tinnitus might be originating at this neural level or at a more peripheral level and Just passing through"). m is alternative shares some features with so-called central masking (Zwislocki et al., 1968), but the latter is typically smaller in mag- nitude than is contralateral masking of tinnitus. Cur- iously, the possibility of contralateral control of tin- nitus has not been extensively explored by the advocates of tinnitus maskers/instruments (see "Tinnitus Maskers/ Instruments" in Chapter 4; CIBA Foundation, 1981:174-175). ... . . An important finding recently rediscovered by Feldmann (1971) is that for some patients the tinnitus does not re- turn immediately upon termination of the masker. Rather, termination is followed by a silent period and then a period of gradually returning tinnitus. (This effect was · previously noted by Spaulding [1903] and by Josephson [1931].) In an example shown by Feldmann, a 500-msec masker produced anywhere from about 1.0 to about 2.5 seconds of posttermination silence, depending upon the masker's intensity and the ear to which the masker was delivered. He indicates that in other patients the silent period was much longer than this, and he raises the prospect of trying to n train" the mechanism ',nder- lying the effect. m is period of silence or diminished tinnitus magnitude has since been named residual inhi- bition (Vernon, 1977). It is a topic to which we return in "Residual Inhibition" in Chapter 4. Formby and Gjerdingen (1980) studied masking in a single tinnitus sufferer using a procedure similar to that widely used in experiments on the so-called psy- chophysical tuning curve. The tinnitus was viewed as a signal, and each of a set of pure-tone maskers was ad- justed in turn until it "just masked" the tinnitus. When the signal is an external tone, the pattern of masker intensities obtained with such a procedure has a charac- teristic shape (see Small, 1959); relatively little . _ . . _ ~_
39 intensity is needed when the masker frequency equals or is close to that of the signal, and increasingly greater masker intensity is necessary for masker frequencies increasingly distant from the signal, with the slope being steeper on the high side than on the low side of the signal. Formby and Gjerdingen obtained this same general pattern when the signal to be masked was a tin- nitus. m is masking procedure has the virtue of providing a relatively precise estimate of the spectral locus of the tinnitus--be it tonal or narrowband--but it has the drawback of being relatively time-consuming and thus may not see wide application clinically. Relevant to the come mon assertion that tinnitus magnitude fluctuates greatly is the observation by Formby and Gjerdingen that substan- tially different masker levels were necessary in differ- ent sessions to mask the tinnitus. The range was about 40 dB in the most extreme case, and 15-20 dB was not uncommon. One finding by FormbY and Gierdingen (1980) deserves _ _ _ further study, for it has important theoretical ana prac- tical implications. They found that when the tonal mask- ers were binaural, they had to be between 8 and 15 dB more intense in order to mask the tinnitus than when they were monaural. Such an outcome is reminiscent of a phenomenon obtained with external sounds, known as the masking-level difference or MLD (reviewed by McFadden, 1975). If veri- fied to exist in some forms of tinnitus, the MLD might eventually prove useful in diagnosing the site of origin of the tinnitus. Penner (personal communication) used both pitch match- ing and tonal masking (psychophysical tuning curves) across a number of sessions with the same subjects and found that the intersession variability for pitch match- ing was much higher than that for masking. There are a number of possible explanations for this outcome. One is that pitch matching is a less reliable procedure than masking for determining the spectral locus of tinnitus. Another is that the procedure is accurate but that tinni- tus can somehow fluctuate in pitch without a concomitant fluctuation in its maskability. m is matter deserves further attention. Another intriguing feature of the relationship between masking and tinnitus has been discovered by Penner et al. (1981). Twenty patients were studied, all having tonal tinnitus and all diagnosed as having sensorineural hearing loss as a consequence of noise trauma or exposure (recall that, by Feldmann's [1971] account, such patients consti
40 tute about 34 percent of tinnitus sufferers). m e task was to continuously adjust the intensity of a broadband noise so as to keep masked either the tinnitus, in some runs, or, in other runs, a real tone of about 10 dB sensa- tion level (SL)--decibels above the patient's own absolute threshold at a given frequency. As expected, for the real tone, 90 percent of the subjects needed essentially the same intensity throughout the 5-minute test period; that is, the signal-to-noise ratio remained the same. When the task was to continuously mask the tinnitus, however, all of the subjects showed a need for increasing intensity during the first few minutes of listening. The changes were considerable, averaging about 30 dB across subjects. The typical trend was for a rapid change in the necessary intensity over the first 10-15 minutes, followed by a flattening of the function. Penner et al. (1981) explain this curious outcome by noting that throughout the presen- tation of the masking noise and real tone, the firing rates of primary auditory neurons (and thus, perhaps, the rates of neurons throughout the auditory system) would de- cline with time, but because both masker and signal were being about equally affected, the noise intensity for equal masking would stay about the same. mat the inten- sity does not remain the same when masking the tinnitus implies either that external sounds and peripheral tinni- tus do not affect primary fibers in the same way or that in these subjects the tinnitus is originating at a site beyond the (adapting) primary fibers being activated by the masker. As Penner et al. (1981) note, their outcome has impor- tant implications for one of the most puzzling problems of tinnitus--why its annoyance seems so out of proportion to its loudness. It may be that, for some patients, real- world sounds "fade" under continuous presentation, but the tinnitus does not and thus continues to annoy even in relatively high-noise backgrounds. The Penner et al. finding may also prove to be of great practical signifi- cance. Clearly, those tinnitus sufferers for whom con- tinuous maskers lose effectiveness through time should not be treated with continuous maskers, but with inter- mittent maskers so that masker level--and, thereby, the risk of additional hearing loss--can be minimized. (Parenthetically, Penner [personal communication] finds that only about one-third of her (highly selected) tin- nitus patients experiences residual inhibition following presentation of her maskers.)
41 Penner (1980) has introduced a variant on the basic masking procedure that is attractive because it offers the twin prospects of being more accurate and more infor- mative about the tinnitus. The experimenter begins with a broadband noise, adjusts it to just mask the tinnitus, .. . . . , ~ .. . e and then begins to (say) low-pass the noise in successive steps until the patient again hears the tinnitus. mi s value is noted, the noise is again made broadband and is now high-passed in successive steps until the tinnitus is again heard. The resulting pair of cutoff frequencies-- the "masking interval"--is taken as the bandwidth of the tinnitus. As noted above, the possibility of binaural tinnitus that is not spectrally or Qualitatively similar in the two ears points out the need for masking measurements to be made using headphones, with monaural presentations made first to one ear and then the other, while the contralat- eral ear receives a broadband masker of reasonable inten- sity. This may, for the first time, allow the patient to identify the respective origins of a complex, binaural tinnitus. Finally, a comment is necessary about people suffering from tinnitus who hear about masking as a treatment and wish to learn whether their tinnitus is of the maskable type. People may have heard about the common use of interstation FM noise as a masking source for tinnitus and may attempt to use it in a self-conducted maskability test. If people choose a small, inexpensive pocket radio for this test, however, they run the risk of reaching the wrong conclusion about the maskability of their tinnitus, because the speaker system on such a radio will not ordi- narily be able to transduce the high frequencies at high levels. Emus, there may be failure to mask a high- frequency tinnitus, not because it is inherently unmask- able, but because sufficient high-frequency energy is not present. Tinnitus sufferers wishing to use ~nterstat~on FM noise to test themselves for masking effectiveness should be encouraged to use a speaker system of reasonably high fidelity. . . . · . . . , Either for the purposes of this test or for the purposes of relief, use of a standard home stereo system is to be further preferred over a cheap radio, because a stereo system will allow the sufferer to selec- tively emphasize the high- or low-frequency regions to some extent, and thus it offers the prospects of effective masking at lower overall sound levels.
42 MAGNITUDE OF THE TINNITU S What is the magnitude of the tinnitus? The masking procedures described above for estimating the spectral locus of the tinnitus might also be thought of as measures of tinnitus magnitude--the greater the intensity necessary for masking (at the frequency requiring the least mask- ing), the greater the tinnitus strength must have been. Without denying this view, it must be noted that measures of tinnitus magnitude based upon masking procedures should be regarded as indirect at best, for a number of relevant factors are necessarily uncontrolled across patients, and these make conclusions and comparisons difficult. For example, the intensity necessary for masking will be affected by the relative widths of the noise band used for masking and the patient's critical band at the tin- nitus frequency. Also, using a masking procedure to esti- mate tinnitus magnitude is questionable in those patients for whom maskers at any frequency are equally effective, and, of course, it is impossible in patients whose tinni- tus cannot be masked. Many auditory scientists believe that a more direct measure of tinnitus magnitude is obtained from loudness- matching procedures (see Scharf, 1983) than from masking procedures. Loudness matching is similar in concept to pitch matching, and, as shall be seen, they have some mutual problems. Minton (1923) and Fowler (1928) were among the first to use loudness-matching procedures. In its purest and simplest form, loudness matching would proceed as follows. The tinnitus would be strictly monaural and either tonal or relatively restricted spec- trally. A sound that is the best match possible to the quality and pitch of the tinnitus would be periodically presented to the ear opposite the tinnitus, and the pa- tient would adjust its intensity (directly or indirectly by responses to the tester) until it matched the loudness of the tinnitus. There are several obstacles to this ideal case: 1. Quality, pitch, and loudness judgments are clearly all mutually interdependent--the best pitch match cannot be achieved until the loudness is known, etc. The solu- tion would appear to be to first get rough measures of all three characteristics and then to use this informa- tion when getting more precise measures (see below). 2. Tinnitus is often binaural and not intermurally identical in magnitude or spectral locus, so the choice
43 of ear to receive the matching stimulus could be com- plicated. 3. During the contralateral presentation of the matching stimulus, two types of cues are available to the patient--loudness and localization cues--and it is not possible to know which is being used. Since it is not possible to interrupt the continuous (say) monaural tin- nitus when the matching sound is presented, the situa- tion is analogous to the task known as the simultaneous binaural loudness balance, which has been controversial over the years just because of this uncertainty about the stimulus basis for the subjects' responses (see Elliott and Fraser, 1970; Scharf, 1983). No obvious solution exists for this problem, since heterofrequency loudness matches--monaural and binaural--are generally regarded to be more difficult and thus more variable than homofre- quency loudness matches (e.g.' Goodwin and Johnson, 1980b). 4. According to Feldmann (1971), the contralateral presentation of the matching stimulus may produce a par- tial masking of the tinnitus (see above), the obvious con- sequence being an underestimate of the tinnitus magnitude. The problematic solution--a monaural, heterofrequency match--was mentioned above. 5. Recruitment or marked intermural differences in audibility may exist at the tinnitus frequency, either of which is capable of producing an incorrect estimate of tinnitus magnitude. Comparisons of estimates obtained by matching and masking procedures might reveal errors of this sort. A procedural point needs to be made here. The Oregon group (e.g., Vernon et al., 1980) argues that it is impor- tant for loudness matches to be made using only ascending intensity series. In these, the intensity of the sound being adjusted to match the loudness of the tinnitus is gradually increased starting from a just-detectable level. In standard psychophysical practice, ascending series are interleaved with descending series (where the intensity would initially be considerably greater than the tinnitus magnitude). m e problem with descending series is that the high intensities could produce partial or complete residual inhibition (see "Residual Inhibition" in Chapter 4), and, thus, a large underestimate of tinnitus magni- tude, or even a total inability to measure it. m e point is well taken and should be considered when developing standard procedures for measuring tinnitus magnitude.
44 With these comments in mind, let us examine some of the available data on tinnitus magnitude. After finding the best-quality matches and pitch matches that he could, Reed (1960) used a binaural matching procedure to estimate the magnitude of tinnitus. He found that about 69 percent of his patients needed 10 dB SL (sensation level) or less to match the loudness of their tinnitus, and only 5 per- cent needed more than 30 dB SL. While this widely cited survey is informative, interpretation of the data is hampered by obstacle 5 noted above; namely, when there are intermural differences in audibility in the spectral region of the tinnitus and the match is reported in terms of SL in the contralateral ear, there is necessarily un- certainty about the actual intensity needed for the match. Graham's (1960) data are similar to Reed' se-about 75 per- cent of the patients needed 10 dB SL or less, and only about 4 percent needed more than 20 dB SL. Vernon et al. (1980) have asserted that patients are "inordinately reliable" in making loudness matches to their tinnitus; they claimed that test-retest comparisons rarely reveal differences of more than about 1 dB (also see Goodwin and Johnson, 1980b). This claim is remarkable and worthy of additional study, for highly practiced normal listeners show much greater variability than this in alternate binaural loudness balance (ABLB) tasks (e.g., McFadden and Plattsmier, 1982b). It is difficult to attribute this high reliability to the presence of recruitment, for even if small increment thresholds did accompany recruit- ment (see McFadden and Plattsmier, 1982a), Vernon and his colleagues see patients having tinnitus of various eti- ologies, and not all of these patients have recruitment. For the sake of completeness, it should be noted that psychophysical methods other than masking and loudness matching might be adapted for use in estimating tinnitus magnitude. For example, with cross-modality matching (Stevens, 1966; Scharf, 1983), the subject would adjust the intensity of a stimulus presented to another modal- ity--a light or a vibrator, say--until its magnitude equaled that of the tinnitus. To our knowledge, this has never been attempted with tinnitus. In other tasks reli- able data have been obtained with this procedure; however, those subjects were typically rather well educated and sophisticated, and it is unclear whether cross-modality data from average people suffering from tinnitus would accurately represent tinnitus magnitude. One virtue of the procedure is that data can be obtained rapidly, so a simple test of the usefulness of the method should be easy
45 to implement. As another example, Goodwin and Johnson (1980a) have suggested reaction time as a measure of tin- nitus magnitude. ANNOYANCE OF THE TINNITUS As we have seen, typical measures of tinnitus magnitude indicate that it is rarely matched to sounds greater than about 30 dB SL, and it is often difficult for the nonsuf- ferer to understand how such apparently weak sensations can cause such great annoyance and distress to some tinni tus sufferers. At the root of this misunderstanding are two errors--presuming that near-threshold intensities cannot be perceived as loud and equating loudness with annoyance. Let us first consider the latter issue. Numerous everyday examples testify to the lack of a simple relationship between loudness and annoyance. A buzzing fluorescent light can be extraordinarily irritat- ing even though its level is below 30 dB SPL, while an air conditioner or heater bringing relief from the weather goes unnoticed at 50-60 dB SPL. A passing motorcycle that masks a segment of the evening news broadcast is more annoying than is the kitchen appliance being used to prepare the evening meal, even though the latter masks the same news segment. A neighbor's stereo system is more annoying than one's own even when 40 dB less intense. The barely audible crinkling of a cellophane candy wrapper can be highly distracting and annoying even during the loudest segment of a concerto with full orchestra. Beyond everyday examples, the noncorrespondence between physical and psychophysical measures of sound and its capacity to annoy has long plagued scientists interested in quantifying annoyance (see, for example, Fidell, 1978; Schultz, 1978). The so-called "message of the noise~-- while frequently difficult to measure beforehand--is far more predictive of annoyance than are physical measures of the sound involved. For example, airline employees or military dependents living at the foot of a jet runway are far less likely to complain about the aircraft noise than are neighbors whose livelihoods derive from other activities. And the noise from delivery trucks is far more annoying to neighbors than it is to the recipient of the delivery. There is evidence that in some situations annoyance is nearly synonymous with speech interference (Fidel!, 1978), but this finding probably has little rele- vance to most tinnitus cases, where speech intelligibility is essentially unaffected by the tinnitus. Interference -
46 with sleep is a common source of annoyance, but, somewhat surprisingly, tinnitus produces sleep difficulties for only about one-half of tinnitus sufferers (CIBA Founda- tion, 1981:27). Various writers have revealed a misunderstanding about the loudness/annoyance relationship when discussing tin- nitus. Fowler (1942, 1943) was among the first to note the poor correlation between annoyance and psychophysical measures of tinnitus magnitude. He talked of the "illu- sion of loudness" of tinnitus and claimed that through conscientious effort the clinician could eventually di- minish or abolish the illusion. One suggested step was to have the patient perform a loudness match--which would invariably be achieved with a relatively weak sound--and then to present this sound while emphasizing to the pa- tient that it obviously did "not correspond to his state- ments as to the severity of the symptom" (p. 397). The clear implication is that Fowler did not accept the pos- sibility that a tinnitus could behave differently from a weak external sound in its ability to produce annoyance, an attitude that is not uncommon today. More recent discussions of the discrepancy between the apparent magnitude and the annoyance of tinnitus have tried to emphasize the possibility of a basis for the effect other than a "psychological" one. The reader should recall here that tinnitus magnitude is typically reported in units of SL--decibels above the subject's own absolute threshold at that frequency. So, if hearing loss is substantial, a small value of SL will correspond to a large SPL. Appreciating this fact, Vernon (1976) suggested that the small tinnitus magnitudes reported might somehow be under the influence of a mechanism like the one functioning in certain pathological conditions to produce recruitment of loudness. Recruitment is defined as an abnormally rapid rate of growth of loudness; its effect is to render sounds well above (a pathology- elevated) threshold to be essentially normal in loudness even though weak sounds only 20-30 dB above threshold are depressed in loudness. Vernon (1976) suggested a "super- recruitment" might be operating on some forms of tinnitus to make them more loud, and thus more annoying, than they might seem. This explanation is unlikely to be univer- sally applicable, for recruitment is present in only some of the pathological conditions that are routinely accom- panied by annoying tinnitus, but the proposal is worthy of study. Goodwin and Johnson (1980b) attempted a test of the Vernon proposal using a small sample of tinnitus suffer . . -
47 ers of different types. They compared loudness matches obtained with two procedures: a standard alternate binaural loudness balance (ABLB) and a monaural method that used a matching tone whose threshold was in the nor- mal range (and thus was presumably free from recruitment even if the tinnitus frequency was not). The result was that the estimated magnitudes of tinnitus were, in every case, greater with the monaural than with the binaural procedure, and the conclusion was that tinnitus magnitude is frequently underestimated due to the contribution of recruitment. (While this may be a correct conclusion, it is also possible that for some subjects the results are complicated by the fact that, in order to find a matching frequency having a normal threshold, frequencies below about 1000 Hz had to be used, and these frequencies are known to have steeper loudness functions than tones between 1000 and 4000 Hz [Scharf, 19781--that is, they have a Normal recruitment.n) The Goodwin and Johnson results imply that the apparent discrepancy between the magnitude and the annoyance of tinnitus may be due in part to underestimation of the magnitude. Tyler and Conrad-Armes (no date) arrived at a similar conclusion using similar procedures. Penner et al. (1981) have suggested that the high an- noyance of tinnitus may in some cases be due to its fail- ure to behave like an external sound once it gets into the central nervous system. In particular, Penner et al. point to their demonstration that a constant external sound can gradually lose its masking power over a tinni- tus through the course of about 30 minutes of continuous listening, while it does not over a second external sound. Whatever the eventual neurophysiological explana- tion of this effect it is clear that the tinnitus is be- having aberrantly. Expanding on the comment of Penner et al., it may be that this and other aberrant behaviors "bring the tinni- tus to the attention" of certain neural mechanisms that persist in unsuccessful attempts to force the tinnitus to conform to the behavior of external sounds and that their ongoing failure reaches consciousness as annoyance. It is difficult to make this idea any more concrete at this time, but an analogy to sound localization comes to mind. When dichotic sounds are presented to a person's ears over headphones instead of in the normal, free-field manner, the sounds are not perceived as originating from external sources, but are perceived as having an intracranial origin. Various explanations of this phenomenon exist, but a long-standing one points to the fact that with head
48 phones the waveforms are not altered as they ought to be by normal head movements. The "expectations" of certain neural networks are not met following certain motor ac- tions, and this gives rise to a unique perceptual experi- ence--not annoyance, to be sure, but the analogy neverthe- less appears relevant. Finally, we return to "psychological" explanations of the apparently disproportionate annoyance of some tinni- tus. Glass and Singer (1972) have studied the stress- inducing properties of noise exposure and have found evidence of much greater cognitive, emotional, and phy- siological effects when the schedule of noise presenta- tions was perceived as being beyond the control of the subject. Perception of control greatly reduced the aver- siveness and the aftereffects of the situation. This out come is reminiscent of the comments made by some success- ful users of tinnitus maskers/instruments when attempting to explain their apparently paradoxical preference for one continuous sound (the masker) over another (the tinnitus). These patients often comment on the control they have over the masker. m ey can turn it on and off, change its in- tensitY etc.. while their tinnitus is nearly totally out , , of their control--whatever changes it undergoes bear no immediately apparent relation to anything the patient has factor of tinnitus, it is necessary to consider several facts. First of all, since the loudness of real-world sounds is not a good predictor of their annoyance, it is not reasonable to expect that it would be for tinnitus either. Second, the low sensation level of most tinnitus does not mean it is not unpleasantly loud. Finally, tin- nitus does not behave like external sounds in a number of ways, and this aberrant behavior may have various neuro- physiological and psychological consequences capable of producing high annoyance, directly or indirectly. done. It is believable that the feeling of helplessness induced by this lack of control over their tinnitus is an important contributor to the annoyance reported by many tinnitus sufferers. In summary, when attempting to comprehend the annoyance I S THE TINNITUS MONAURAL OR BINAURAL ? As noted above, about 37 percent of all tinnitus is believed to be monaural--a statistic based primarily upon patients' self-reports (Vernon, 1978a). It is possible that such reliance on self-reports is producing an under- estimate of the incidence of binaural tinnitus, which may
49 in turn be leading to depressed success rates with treat- ments such as tinnitus maskers. The possibility of this error lies in the perceptual experience aroused by bin- aural stimulation. When external sounds that are similar spectrally and equal in intensity are presented to the two ears, the phenomenological experience is of a single fused image located in the center of the head. (In everyday listening, of course, sounds correctly appear to originate outside the head from locations in auditory space, but, with headphone listening and with self-generated sounds such as tinnitus, the sounds appear to originate from a location somewhere within the head; this difference be- tween localization and lateralization, respectively, has never been fully explained--but see Schroeder [1975].) If an intermural difference in intensity is now intro- duced, the fused image appears to move in the head toward the ear receiving the more intense sound. The larger the intermural intensity difference, the more lateralized the acoustic image appears to be. Once the interaural dif- ference gets to be about 15-20 dB, most listeners report the image to be so strongly lateralized toward the more - ~, ~ intense ear that it is unclear whether the stimulus is binaural or monaural, and sometimes the presentation of a truly monaural stimulus is required to convince the listener that the previous stimulus was binaural but highly asymmetric. The relevance of these facts to tinnitus should be clear. There is the danger that when a patient indicates that his tinnitus is present in one ear only, he may be in error: it may be originating at both ears, but more intensely at one of them, thereby producing a strongly lateralized image that misleads the patient into believing the problem is monaural. Note that the tinnitus need not be spectrally identical in the two ears for this to be a possible problem; real sounds that are several hundred Hertz apart can be fused and lateralized, and, in fact, the frequency limits for dichotic fusion grow with center . . . . frequency (Scharf, 1969, 1974; however, these experiments used brief stimuli and thus may not be relevant to the binaural tinnitus situation). If the spectral regions of tinnitus origin are widely different in the two ears, or if the quality or com- plexity of the tinnitus is different in the two ears, the monaural/binaural question will be less of a worry than in patients whose configuration of hearing loss is similar in the two ears and whose tinnitus might thereby be ex- pected to be spectrally similar in the two ears. There is also less concern when the matched intensity of the
50 tinnitus is about 10 dB SL or less (and the audibility is approximately equal in that frequency region in the two ears), for then the maximum possible intermural intensity difference would not be adequate to produce an acoustic image strongly lateralized to one side. For completeness, it should be noted here that an effect not known for external sounds does occur with tin- nitus. Some patients report their tinnitus to be similar or identical in the two ears but not fused--there is a (similar) image localized to each ear (Vernon, 1978a). In the laboratory a simple test is available to a listener trying to decide whether an input is monaural or binaural--pulling the plug to one of the two earphones and observing any change in locus of the acoustic image-- and while this option is obviously not available to a tinnitus sufferer, fortunately the means are available to test psychophysically for the possible binaural origin of tinnitus that is perceived to be monaural. Once a subject has matched his or her tinnitus for center frequency, com- plexity, and intensity, a similar waveform--but located in a spectral region adjacent to the tinnitus--could be presented binaurally and the subject asked to adjust the intensity in the ear opposite the perceived tinnitus until the image of this external sound appeared to originate from the same intracranial location as the tinnitus. If the intensity difference required for this match is small, it would be evidence for a binaural origin of the tinni- tus; if it is relatively large, or if the patient is sat- isfied with a match to a monaural stimulus, it would argue for a monaural origin. It is possible that some of the patients who have found tinnitus maskers to be of little long-term value might have been better satisfied had they been fitted binaur- ally. Similarly, at least some of the reports of tinni- tus persisting after sectioning of the auditory nerve may be traceable to unrecognized binaural origin of the tinni- tus prior to surgery. Finally, there is a misunderstanding about the locali- zation (lateralization is the correct term) of the tinni- tus that must be corrected. Some authorities seem to believe that the locus of the source of a tinnitus is revealed by where the patient hears it. Reed (1960) and Goodhill (1979), for example, seem to believe that a tin- nitus that sounds as if it originates at the ear(s) does so and that one that appears to be inside the head has a central rigin. This simply does not follow.
51 THE ISSUE OF BEATS WITH TINNITUS If some forms of tonal tinnitus were identical to an external tone in all ways but their origins, one might expect to be able to produce the experience of beats by introducing an external tone slightly different in fre- quency and similar in intensity to the tinnitus. In fact, reports of such beats are few. Wegel (1931) claimed to be able to hear an irregular, "mushy" beat when some single tones were introduced, but it is clear that Wegel appre- ciated how different his experience was from the normal beat heard when two external tones interact. For example, he found combinations of frequency and intensity of the external tone that should have produced beats but that instead produced "complete silence. n Over the years, Wegel's report has been widely cited, but it has gen- erally been viewed with skepticism and dismissed; how- ever, recent developments have brought new attention and credibility to it. Specifically, the spontaneous otoacoustic emissions (OAKS) discovered by Kemp (1979b) and studied by Wilson (1979, 1980) and Zurek (1981) have several features in common with Wegel's report, as well as with the reports on monaural diplacusis by Flottorp (1953) and Ward (1955). For example, in some subjects a heard OAK has been sup- pressed by an external tone of appropriate frequency and intensity; this is reminiscent of Wegel's "complete silence. n OAEs have been observed, both perceptually and acoustically, to fluctuate in level (beat) when appro- priate tones are introduced (Wilson, 1980; Zurek, 1981), and these fluctuations are often irregular, just like the "mushy" beats reported by Wegel (1931), Flottorp (1953), and Ward (1955). me fact that Flottorp and Ward did not have a tinnitus against which to beat, and Wegel did, appears to be a much-less-important difference since the discovery of heard and unheard OAEs. It is now possible to imagine that, in some of those rare instances in which beats against tinnitus were reported (Wegel, 1931; Wever, 1949; Vernon et al., 1980), what was being heard were interactions between the external tone and a heard OAK and that the beatlike effects observed by Flottorp and Ward were interactions with an unheard (perhaps narrow- band) OAK. (In this regard, it is sobering to see how close Flottorp was to discovering OAKS and how modern both his and Ward's discussions are.) Some other reports of beats against tinnitus are apparently not explainable by appeal to OAKS. Stanaway
52 et al. (1970) reported on a single subject who could hear beats between an external tone and a tinnitus induced by exposure to an intense broadband noise, but no such interactions were reported by Loeb and Smith (1967) or Atherly et al. (1968). Lackner (1976) claims to have produced beats in patients whose tinnitus was of central rather than peripheral origin, but his arguments make no sense. For one thing, his procedure for producing beats was to introduce an external tone to the ear contralat- eral to the one in which the tinnitus was localized. Two external tones to different ears can produce beats--they are called binaural beats--but such beats are different from monaural beats in many ways. Of primary interest here is the fact that binaural beats cannot be produced above about 800-1000 Hz using a single pure tone to each ear (Licklider et al., 1950; Perrott and Nelson, 1969; but compare McFadden and Pasanen, 1975). Yet Lackner reports binaural beats at frequencies from about 2500 to 5000 Hz. It is conceivable that his patients were experi- encing some other form of binaural interaction with their tinnitus--contralateral masking, for example, or perhaps the tinnitus had components in both ears and only appeared to be monaural (see n Is the Tinnitus Monaural or Bin- aural?" above), and the beat heard was a monaural one-- but whatever the explanation, it is certain that these patients were not hearing what is ordinarily called a binaural beat with such high-frequency tones. m e weight of the evidence, then, favors the conclu- sion that tonelike tinnitus does not ordinarily behave like an external tone when it comes to the issue of beats--monaural or binaural. There is a possibility that some of the reports of beats against tinnitus could be traced to an OAK underlying the tinnitus. The rarity of the reports of beats against tinnitus is in accord with this interpretation, for the growing belief is that relatively few cases of tinnitus have an OAK as their basis. SOME WAYS TINNITUS I S NOT LIKE AN EXTERNAL SOUND Some of the facts of tinnitus masking discussed in this report naturally raise the question of whether tinnitus masking should really be regarded as masking or whether it involves mechanisms fundamentally different from those involved when one external sound obscures another. An easy decision on this question is prevented by our ignor
53 ance of the bases of the various forms of tinnitus. That is, in some instances the tinnitus and the masker may interact physiologically in much the same way as the physiological concomitants of two external sounds inter- act when masking occurs, whereas in other instances tin- nitus "masking" may be accomplished via quite different physiological interactions. At this point all that can be done is to review and emphasize those features of successful tinnitus "masking" that have led some to ques- tion whether it is better called by another name in order to promote its better understanding (e.g., Vernon, 1981). In what follows it is important for the reader to remember that not all of these features of tinnitus masking are present in all patients, and exactly which features clus- ter as sets is not yet known. Further, some of these features may be refuted or modified by future research. Tinnitus masking has shown itself to be unlike ordinary masking in several ways: 1. Sometimes tones of almost any frequency can ~mask" tinnitus (Feldmann, 1971, 1981). For external sounds, masking can only be accomplished by sounds within certain spectral regions surrounding the signal; those regions do increase with increasing masker intensity, but they never reach the point of being several octaves above or below the signal, as has sometimes been reported for tinnitus "masking" (Vernon and Meikle, 1981). 2. When tinnitus can be masked, the intensity neces- sary for masking is often abnormally great or small rela- tive to the signal-to-noise ratios required by external signals and maskers. 3. In order to keep some forms of tinnitus masked, it is necessary to gradually increase the intensity of the masker over the course of a half-hour listening session; this increase can amount to 30-45 dB (Penner et al., 1981). An external sound requires no such increase in the masker to remain at the same level of detectability. 4. It is claimed that sometimes tinnitus cannot be masked no matter how intense the masker is made (Vernon and Meikle, 1981). For external sounds this is never strictly true, although signals intense enough to be near the upper intensity limit of audibility--the so-called threshold of pain--may require maskers that are themselves dangerous to hearing. 5. In a reasonably large fraction of tinnitus cases, the tinnitus can be masked with sounds presented to the ear contralateral to the side of reported origin of the
54 tinnitus (Feldmann, 1971, 1981). In itself this is not unlike the behavior of external sounds--an external sound can mask a contralateral external signal via either cross- conduction or central masking. m e peculiarity is that the intensity necessary for masking tinnitus often appears to be the same ipsilaterally and contralaterally. 6. Following the termination of an effective masker, ipsilateral or contralateral, there is sometimes a period of residual inhibition during which the tinnitus is absent or reduced in magnitude. In contrast, a masked external signal returns to audibility essentially immediately upon termination of the masker. 7. If spectrally similar sounds are presented to the two ears over headphones, they are perceived as a single fused sound having an apparent intracranial position that depends upon the relative levels and timing of the wave- forms at the two ears. In contrast, tinnitus that is spectrally similar in the two ears will often not fuse, but will remain localized at the two ears (Vernon, 1978a). . . . · · . . SUMMARY OF MEASUREMENT PROCEDU~ S This section has been concerned with psychophysical pro- cedures and techniques for measuring and describing tin- nitus. For the foreseeable future, such procedures will continue to be the primary source of clinical and experi- mental information about tinnitus, but it is believable that the long-term future will see the development and wide application of new and modified objective measures of tinnitus (or its correlates) useful in diagnosis and in choice of treatment. m e recording of spontaneous oto- acoustic emissions (OAEs) from the canal of the outer ear (Kemp, 1979b; Wilson and Sutton, 1981; Zurek, 1981) has already been discussed (see " m e Objective/Subjective Issue" in Chapter 2), but other possibilities exist. For example, Shulman and Seitz (1981) have claimed that the brain-stem-evoked response (BSER) in patients having tin- nitus of central origin is different in specifiable ways from that of people with normal hearing. Some other pos- sibilities are that the response of tinnitus to manipula- tions of air pressure in the middle ear (see "Alteration in Air Pressure" in Chapter 4) may prove to have diag- nostic value, as may its response to electrical stimula- tion (see" Electrical Stimulation" in Chapter 4) or to certain drugs (see "Drug m erapy for Tinnitus" in Chapter 4). m e development and use of such diagnostic aids is clearly to be encouraged.
