Auditory Brainstem Response to Clicks in Quiet, Notch ...Auditory brainstem responses to clicks in...

J Am Acad Audiol 2 : 76-90 (1991)

Auditory Brainstem Response to Clicks in Quiet, Notch Noise, and Highpass Noise Randall C. Beattie Jayme Spence

Abstract

Auditory brainstem responses to clicks in quiet, notch noise, and highpass noise were recorded from 10 normal-hearing adults . Contrary to some reports, the latency of wave V increased as the center frequency of the notch decreased. Response identifiability and wave V amplitudes were similar for the notch and highpass noises . Thus, the additional frequen-cies below the notch did not contribute sufficiently to the response to alter identifiability or amplitude. Notch noise, as compared to the highpass noise, is advantageous because of the increased frequency-specificity provided by the low-frequency masking noise. Presenting clicks in highpass or notch noise centered at 500 Hz is of limited value for assessing auditory sensitivity . This is because the range of testable hearing levels is restricted by (1) a high normal ABR threshold (-65 dB nHL) and (2) the high noise levels required to mask the click. This limited dynamic range for assessing hearing loss is a major limitation of click in noise testing, particularly for 500 Hz . Generating a two-point audiogram by presenting clicks in noise with notches centered around 1000 Hz and 4000 Hz may prove promising for as-sessing auditory sensitivity .

Key Words: Auditory brainstem response, clicks in notch noise, clicks in highpass noise, wave V latency-intensity functions, wave V amplitude-intensity functions

A

uditory brainstem responses may be useful to assess auditory sensitivity with individuals who are unable to

respond to behavioral audiometry (Jerger and Maudlin, 1978 ; Picton and Smith, 1978 ; Berlin and Dobie, 1979 ; Davis and Hirsh, 1979 ; Don et al, 1979 ; Stein et al, 1981 ; Hayes and Jerger, 1982 ; Hall, 1984 ; Fria, 1985 ; Jacobson, 1985; Hood and Berlin, 1986). A click stimulus typi-cally is used in auditory brainstem response (ABR) audiometry because it optimizes res-ponse identifiability (van Zanten and Brocaar, 1984 ; Hood and Berlin, 1986). Compared to tone pips, clicks evoke larger ABRs and lower thresholds. This has been attributed to the syn-chronous activation of neurons within the auditory system (Buchwald, 1983; Jacobson, 1985 ; Hood and Berlin, 1986). A major limita-

Department of Communicative Disorders, California State University, Long Beach

Reprint requests : Randall C . Beattie, Department of Communicative Disorders, California State University, Long Beach, Long Beach, CA 90840

tion of clicks is that they are characterized by a broad frequency spectrum that conforms closely to the frequency response of the transducer . This lack of frequency specificity limits the usefulness of clicks for estimating the presence and degree of hearing loss . A given ABR threshold to clicks can be obtained from a wide range of hearing losses and audiometric configurations (Jerger and Maudlin, 1978 ; Hood and Berlin, 1986). The sole use of un-masked clicks may not identify individuals with hearing loss restricted to the lower frequencies (Don et al, 1979 ; Kileny, 1981 ; Hood and Berlin, 1986). Thus, several authors suggest that an ABR evaluation for estimating auditory sen-sitivity should include at least one high-fre-quency and one low-frequency stimulus (Davis and Hirsh, 1979 ; Kileny, 1981 ; Eggermont, 1982 ; Jerger et al, 1985; Hood and Berlin, 1986 ; Kileny and Magathan, 1987; Fjermedal and Laukli, 1989). However, there is no consensus as to which stimuli are preferable for estimating low-frequency and high-frequency auditory sensitivity (Don and Eggermont, 1978 ; Davis

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and Hirsh, 1979 ; Kileny, 1981 ; Pratt and Bleich, 1982 ; Beattie and Boyd, 1985; Jerger et al, 1985 ; Stapells et al, 1985 ; Hood and Berlin, 1986 ; Gorga et al, 1988). Some suggested high-frequency stimuli include clicks, high-frequen-cy tone pips, and clicks or high-frequency tone pips in notch (also called band-reject or stop-band) noise. Suggested low-frequency stimuli include clicks or low-frequency tone pips in either highpass noise or notch noise.

As noted above, mixing the test stimulus with notch noise has been suggested to improve frequency specificity (Picton et al, 1979 ; Sta-pells and Picton, 1981 ; Pratt and Bleich, 1982 ; Laukli, 1983 ; Pratt et al, 1984; van Zanten and Brocaar, 1984). Notch noise is designed to enable reception of the stimulus at an accept-able level, but mask those higher and lower frequencies outside the region of interest. This is because the notch noise isolates the response to a particular region of the cochlea. Without the noise, clicks activate wide areas of the cochlea and the response is derived in part from regions on the basilar membrane outside the frequency of interest . The use of clicks in notch noise combines the most effective ABR stimulus with frequency-specificity . However, limited re-search has been conducted using this proce-dure, and the results have been inconsistent (Pratt and Bleich, 1982 ; van Zanten and Brocaar, 1984 ; Stapells et al, 1985). Whereas Pratt and Bleich (1982) found that ABR laten-cies did not change with notch frequency (250-8000 Hz), van Zanten and Brocaar (1984) found that wave V was about 3 msec longer for a notch centered at 500 Hz than for a 4000-Hz notch. Van Zanten and Brocaar (1984) noted that Pratt and Bleich (1982) used one-third octave notches as contrasted to their notch-width, which was one and two-thirds octaves wide . Van Zanten and Brocaar (1984) also suggested that perhaps Pratt and Bleich (1982) did not find latency differences because inadequate masking was used to mask those frequencies outside the notch. On the other hand, Pratt et al (1984) confirmed their findings and con-cluded that clicks in notch noise are not effective for estimating frequency-specific hearing thresholds . Finally, because of the low-frequen-cy spread of masking into the notch, Stapells et al (1985) stated that the amplitude of the ABR to clicks in notched noise may be too small to provide an effective stimulus . It is evident from the above discussion that there is no agreement on the value of presenting clicks in notch noise.

ABR to ClicksBeattie and Spence

Moreover, the effectiveness of this procedure is dependent on several stimulus variables, in-cluding notch-width, notch-depth, center fre-quency, and the intensity relationship between the click and the notch noise. Therefore, the present study was designed to further inves-tigate the effects of clicks in notch noise on the amplitude and latency of the ABR.

Another procedure for achieving more fre-quency-specific responses is to present the test stimulus with a highpass filtered noise (Don and Eggermont, 1978; Kileny, 1981; Egger-mont, 1982 ; Stapells et al, 1985 ; Fjermedal and Laukli, 1989). Highpass noise has the ad-vantage of requiring less complex instrumenta-tion than the notch noise. Moreover, in contrast to notch noise, highpass noise does not contain low-frequency energy that may spread into the notch and reduce the effectiveness of the click stimulus (Picton et al, 1979; Stapells et al, 1985). This low frequency spread of masking may reduce the signal-to-noise ratio sufficiently so that the amplitude of the ABR to clicks in notch noise may be too small to provide an ef-fective stimulus (Stapells et al, 1985). On the other hand, when click stimuli are used with highpass noise, less frequency-specificity may be achieved than with notch noise because the stimulus includes all frequencies below the highpass cut-off frequency. Nonetheless, Eg-germont (1982) and Fjermedal and Laukli (1990) state that it is feasible to obtain a two-point ABR audiogram consisting of (1) a high-frequency (3000 Hz) unmasked click threshold and (2) a low-frequency (500 Hz or 1000 Hz) point obtained by presenting a click with high-pass noise having a 1000-Hz or 2000-Hz cut-off. Because the relative effects on the ABR of high-pass noise versus notch noise using click stimuli have not been well documented, the second pur-pose of the present study was to obtain wave V latencies and amplitudes as a function of click intensity to both notch noise and highpass noise. A comparison of ABRs using notch noise and highpass noise will enable an assessment of whether the increased frequency specificity associated with notch noise, as compared to highpass noise, is offset by a reduction in the ABR amplitude.

In conclusion, many authors suggest that the assessment of auditory sensitivity with ABR audiometry should include at least one low-frequency and one high-frequency stim-ulus . Although clicks are optimal stimuli for eliciting the ABR, they are characterized by a

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Journal of the American Academy of Audiology/Volume 2, Number 2, April 1991

broad frequency spectrum that does,not allow a frequency-specific assessment of auditory sensitivity . One solution to this problem is to present clicks simultaneously with highpass or notch noises that are designed to mask those portions of the cochlea that are not of interest . Highpass noise may be advantageous because less complex instrumentation is required, and it may yield larger ABR amplitudes than notch noise. On the other hand, notch noise allows greater frequency-specificity because both the higher and lower frequencies around the notch frequency are masked . Sufficient data do not exist to assess the relative value of these two procedures as compared to the presentation of clicks alone. Consequently, this study was designed to compare latency-intensity and amplitude-intensity functions for wave V using (1) clicks in quiet, (2) clicks in notch noise with center frequencies of 500, 1000, and 2000 Hz, and (3) clicks in highpass noise. More specifi-cally, the following research question was ad-dressed: Are there differences in (a) detect-ability, (b) latency, or (c) amplitude of the ABR (Wave V) using clicks presented to normal-hearing subjects in quiet, highpass noise, and notch noise centered around 500, 1000, and 2000 Hz at 65, 50, 35, 20, and 10 dB nHL?

METHOD

Subjects

Ten normal-hearing subjects participated in the study. This group was composed of five females and five males ranging in age from 21 to 26 years (mean = 24 years) . All subjects reported no history of otoneurologic path-ologies. They passed a 15 dB HL (ANSI, 1970) screening test at the octave frequencies from 250 Hz to 8000 Hz, had tympanometric peaks within ± 50 daPa, and exhibited ipsilateral acoustic reflexes at 100 dB SPL for 1000 Hz and 2000 Hz tones. The right ear was selected for testing.

Instrumentation and Calibration

Testing was conducted with the subjects in a supine position in a darkened sound-at-tenuated suite meeting ANSI (1977) specifica-tions for air-conduction testing. The subjects were asked to remain as still as possible during testing.

Responses were recorded with three elec-trodes using a vertex-neck-neck montage (Beat-tie et al, 1986). The electrode connected to the noninverting preamplifier input (G1) was placed on the vertex, the electrode connected to the inverting input of the amplifier (G2) was positioned on the neck ipsilateral to the test ear, and the electrode connected to the common input (C) was situated on the neck contralateral to the test ear. The neck electrodes were placed 7 cm below the lower mastoid. The electrode sites were cleaned with isopropyl alcohol and an abrasive . Impedances were less than 5000 ohms and approximately equal.

Responses were obtained with gold-plated electrodes (Grass, E5GH), which were directed to a physiologic amplifier (Grass, Model P511K). The responses were shaped by a filter with a bandpass of 30 to 3000 Hz and having rejection rates of 6 dB/octave . The dwell time was 30 microseconds (psec), and the analysis time was 21 msec . The artifact reject was adjusted so that approximately 10 to 20 percent of the evoked potentials were discarded (Hyde, 1985).

The test stimulus consisted of clicks that were produced by directing a 100-psec rectan-gular pulse from a click/noise generator (Grass, Model S10) to a custom mixer/amplifier and then to an earphone (Telex, Model 1470) encased in a cushion (Telephonics, Model P/N 51). The clicks alternated in polarity and were presented at a repetition rate of 25.2 per second . The Power Spectrum Analysis Program (RC Electronics Inc., 5386-D Hollister Ave., Santa Barbara, CA 93111) was used to perform a Fast Fourier Transform (FFT) on the click (resolution = 24

0 1 2 3 4 5 6 7 8 9 10

Frequency in kHz

Figure 1 Acoustic spectral analysis of a click presented from a Telex, Model 1470 earphone .

78


Hz). Figure 1 shows that the spectrum exhibited a resonant energy peak around 3000 Hz, al-though acoustic energy was prominent through 7000 Hz .

The highpass masking noise and notch noise were produced by directing white noise from the noise generator (Grass, Model S10) to highpass and lowpass filters (Krohn-Hite, Models 30 and 31). The highpass and lowpass cutoff frequencies were selected with the as-sociated programmable mainframe (Krohn-Hite, Model 3905). The outputs from the filters were mixed, and the resultant notch noise was then directed back into the generator (Grass, Model S10) so that the associated attenuator could be used for varying the intensity of the noise. The click and noise stimuli were selected from different channels of the generator (Grass, Model S10) and directed to a custom mixer. Finally, the click, click/highpass noise, or click /notch noise stimuli were directed to the ear-phone.

Behavioral click thresholds were obtained on a group of 10 normal-hearing subjects . A fixed 20-dB attenuator was inserted before the earphone to ensure sufficient attenuation. As recommended by ANSI (1978), thresholds were obtained using an ascending method, although 2-dB rather than 5-dB increments were used. The mean behavioral click threshold was desig-nated as 0 dB nHL. The corresponding levels for the peak SPL and peak equivalent SPL were 37 dB and 28 dB, respectively . All stimulus levels (dB nHL) are specified with reference to the full spectrum of the click in quiet. Psycho-acoustic thresholds were not obtained for the filtered clicks .

Frequency responses of the highpass and notch filters were obtained by presenting pure tones (Bruel & Kjaer, Model 1049) into the fil-ters, through the mixer/amplifier, and then to a sound level meter (Bruel & Kjaer, Model 1023). Figure 2 shows that the bandwidths centered around 2000 Hz (1140-3250 Hz), 1000 Hz (625-1625 Hz), and 500 Hz (310-800 Hz) were ap-proximately one and one-half octaves wide (-3 dB down-points) with notch depths of about 70 dB . The same highpass filter frequency settings that were used for notch noise also were used for the highpass noise. The lowpass rejection rates ranged from 84 dB/octave to 93 dB/octave, and the highpass rejection rates ranged from 92 dB/octave to 105 dB/octave. The minimum stop-band attenuation ranged from 79 dB to 83 dB .

Previous authors have noted that the rela-

tive level of the click with respect to the noise is critical (Pratt and Bleich, 1982 ; van Zanten and Brocaar, 1984 ; Stapells et al, 1985 ; Fjer-medal and Laukli, 1989 ; Hecox et al, 1989). That is, if the notch noise is set too low it will not mask those frequencies above and below the center frequency of interest . On the other hand, if the notch noise is adjusted too high, the click stimulus may be inadequate to elicit a response . We reasoned that setting the click stimulus 8 dB below the masked threshold for broadband noise (i .e ., 5 dB below the half-power points) would ensure adequate masking yet ap-proximate an optimum ABR. This level was determined as follows. The click was presented at 65 dB nHL and white noise was adjusted initially in 5 to 10-dB steps until the vicinity of threshold was identified . Next, the search for the masked threshold began by setting the noise 10 dB below this point and then increas-ing the noise in 2-dB increments until the click was masked by the noise. This process was repeated five times and the masked threshold was defined as the level at which the noise just masked the click on at least 4 out of 5 trials . The median white noise sound pressure level was 88 dB SPL, and the range was from 85 to 94 dB SPL. The noise level was set 8 dB above the median threshold of 10 normal-hearing sub-jects (96 dB SPL) . No subject was able to per-ceive the click in the presence of white noise at this level. Moreover, as discussed below, the adequacy of the masking noise was verified with absent ABRs in broadband noise.

0

-10

-20

-30

-a0

-50

-e0

-70

0.3 0.5 07 1 .0 1 .5 2.0 2.5 3.0

frequency in KHz

Figure 2 Frequency responses of the highpass and notch filters. The notches are centered around 500, 1000, and 2000 Hz and have notch-widths about one and one-half octaves wide at the half-power points .


Procedure

Preliminary testing included two tracings in response to a 65-dB nHL click in quiet to ensure a normal ABR. Additionally, to ensure that the noise was sufficiently intense to mask the ABR, tracings to 5000 stimulus repetitions were obtained with the click (35 dB nHL and 65 dB nHL) mixed with unfiltered noise. The signal-to-noise ratio was adjusted to the same level that was used for the notch noise and high-pass noise conditions . We reasoned that if the ABR was absent when the white noise was mixed with clicks at both low (35 dB) and moderate (65 dB) levels, then it was unlikely that the click would stimulate those regions outside the notch during the filtered conditions . Also recall that the level of the noise was ad-justed so that no subject perceived the click. That is, the unfiltered noise prevented the click from being detected psychoacoustically and electrophysiologically .

Latency-intensity functions were obtained on each subject under three conditions : (1) clicks in quiet, (2) clicks in notch noise centered around 500 Hz, 1000 Hz, and 2000 Hz, and (3) clicks in highpass noise. The latency-intensity functions were obtained with clicks presented at 65, 50, 35, 20, and 10 dB nHL. Sixty-five dB was the upper intensity chosen because the SPL of the unfiltered masking noise (96 dB) approximates the maximum level that many normal-hearing subjects will tolerate . When the clicks were presented at 50 dB nHL and 65 dB nHL, a min-imum of 2000 stimulus repetitions were employed . The minimum number of stimuli in-creased to 3000 at 35 dB nHL, 4000 at 20 dB nHL, and 5000 at 10 dB nHL. If a response was not judged present by the examiner after 2000-4000 repetitions, the number of stimuli was in-creased to a maximum of 5000 repetitions . A minimum of two tracings and a maximum of three tracings were obtained for each condition to assess response replicability . Thus, a total of at least 74 tracings were obtained for each sub-ject. All of the conditions were randomized to guard against order effects. Responses were con-sidered identifiable if the waves were judged present by two trained examiners (the authors) in at least two of three trials . These judgments were made by pairing both test and retest trac-ings and placing them in descending order based on intensity. Although making judgments as to the presence or absence of a wave without knowledge of stimulus intensity and/or respon-

ses at other intensities would reduce bias, Weber (1985) notes that the examiner would be deprived of one of the most sensitive indicators of a response (i .e ., the increasing latency with decreasing intensity) . Our decision to require agreement between two examiners was designed to minimize false positives. Evidence indicates that trained examiners have false-positive rates of about 2 percent (Hayes and Jerger, 1982) and false-negative rates of about 1 percent (Pool and Finitzo, 1989). Nonetheless, we recognize that different judges may yield somewhat different results and that these questions require sys-tematic study.

Latency and amplitude measurements were made using the cursors associated with the ISC-16 . Wave V latencies were measured at the point just before the rapid negative in-flection, or at the midpoint of the shoulder when a clear inflection point was not identified . Amplitude measurements were made from the point corresponding to the latency of the wave to the succeeding trough (Chiappa, 1983).

RESULTS

Response Detectability

One purpose of this investigation was to examine the frequency of occurrence of wave V in response to clicks presented from 65 to 10 dB nHL in quiet, notch noise, and highpass noise. Table 1 shows that response detectability, par-ticularly for the lower intensities, was greater for the quiet condition than for the notch or highpass noises . For the quiet condition, re-sponses were observed for all subjects (N = 10) at 20-65 dB nHL and eight subjects yielded responses at 10 dB nHL. The table also reveals that identifiability was similar for the notch noise and highpass noise conditions . For 2000 Hz, nearly all subjects yielded responses at the 35, 50, and 65 dB nHL conditions . Iden-tifiability decreased as intensity was lowered from 35 to 10 dB nHL, where responses were observed in three and four of the 10 subjects . At 1000 Hz, identifiability decreased for both the notch and highpass noises as intensity decreased from 65 to 20 dB nHL. No responses were observed at 10 dB nHL for the notch or highpass noises. Table 1 shows that fewer responses were obtained at 500 Hz than at 1000 Hz and 2000 Hz. Approximately half of the sub-jects responded to 500 Hz at 65 dB nHL, and

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Table l Number of 10 Subjects Who Exhibited an Identifiable Wave V for Clicks Presented at 65, 50, 35, 20, and 10 dB nHL Under Conditions of Quiet (0), Notch Noise (NN), and Highpass Noise (HP)

Centered Around 2000 Hz, 1000 Hz, and 500 Hz

0 NN 2000 Hz

HP 1000Hz

NN HP 500 Hz

NN HP

10 10 9 10 7 4 6

10 9 8 8 8 0 4

10 10 10 6 5 0 0

10 7 6 0 2 0 1

8 3 4 0 0 0 0

Intensity (d8 nHL)

65 dB

50 dB

35 dB

20 dB

10 dB

few or no responses were observed at the lower intensities .

Overview and Illustrative Waveforms

Wave V latencies and amplitudes are sum-marized in Figures 3 and 4, respectively . Data are shown if responses were obtained from five or more of the 10 subjects . For example, at 500 Hz only the 65-dB nHL data are shown for the highpass noise because fewer than five respon-

Quiet 2 RHz NN -*- 2 SHz HP

-~ I SHz NN -x- t EHz HP - 0.5 SHz HP

m s e C

Intensity in dB nHL

Figure 3 Latency in milliseconds (msec) displayed as a function of intensity in dB nHL for the quiet, notch (NN), and highpass (HP) noises .

ses were observed for the other conditions . Rep-resentative waveforms for the quiet condition, and for the notch and highpass noises, centered around 2000 Hz, 1000 Hz, and 500 Hz, are il-lustrated in Figures 5 to 8, respectively . Each panel displays two tracings of each waveform from 65 to 10 dB nHL.

Figure 3 presents wave V latency-intensity functions. It is evident that mean latencies were shortest for the quiet condition and that latency increased as frequency decreased from 2000 Hz

-$'- Quiet 2 [HZ NN 2 LHZ HP

-Q- 1 [HZ NN I LHZ HP 0.5 [Hz HP

Figure 4 Amplitude in nanovolts (nV) displayed as a function of intensity in dB nHL for the quiet, notch (NN), and highpass (HP) noises .

81


Quiet

65

J S c m v

50

.N c F C

35

20

10

0 3 6 9 12 Latency in msec

Figure 5 An illustration of representative waveforms obtained for the quiet condition.

to 500 Hz . Compared to notch noise, Figure 3 shows longer latencies (-0.5 msec) for highpass noise centered around 2000 Hz and 1000 Hz. The figure also shows that latencies decreased as intensity increased from 10 to 65 dB nHL. These trends can be observed in Figures 5 to 8.

Figure 4 shows that wave V amplitudes were largest for the quiet condition and that amplitudes tended to decrease as frequency

Figure 6 Illustrative ex-amples of the responses ob-tained for the 2000 Hz notch (NN) and highpass (HP) noises at 10 dB to 65 dB nHL.

c m v c

N C N

decreased from 2000 to 500 Hz. Moreover, no consistent wave V amplitude differences were observed between the notch noise and highpass noise. Figure 4 shows only one datum point for 500 Hz (65 dB nHL, highpass noise) because fewer than five responses were observed in the other conditions . Finally, the figure shows that amplitudes tended to increase as intensity in-creased from 10 to 65 dB nHL.

Latencies

Table 2 shows means, medians, standard deviations, and ranges for wave V latencies. Recall that the mean latency data are also dis-played in Figure 3. The table demonstrates that wave V latencies were shortest for the quiet con-dition and that latency increased as frequency decreased from 2000 to 500 Hz . Seven or eight subjects yielded responses for the quiet, 1000-Hz, and 2000-Hz conditions at 50 dB and 65 dB nHL; thus, a one-way analysis-of-variance (ANOVA) for repeated measures was conducted on the data from this subset of subjects to deter-mine if there were significant latency differences among the quiet, 1000-Hz, and 2000-Hz stimuli. Latencies for the highpass and notch noises at 1000 Hz and 2000 Hz were averaged for each subject. The analyses revealed statistically sig-nificant differences among the quiet, 1000-Hz, and 2000-Hz stimuli at both 50 dB nHL [F(2, 14) = 126.3, p < 0 .01] and 65 dB nHL [F(2,14) = 52.1, p<0.011 . Compared to the quiet condition, laten-cies were about 1 msec longer for 2000 Hz and

65

50

35

5 20

10

82


1000 Hz Notch Noise

65

i 50 C m v

35

0 c

5 20

10

1000 Hz Highposs Noise Figure 7 Illustrative ex- ---\ - amples of the responses ob-

0 3 6 9 12 0

Latency in msec

2.5 msec longer for 1000 Hz. Using Tukey's means comparison test (Bruning and Kintz, 1987), statistically significant differences (p < 0.05) were observed among all means (quiet, 1000 Hz, 2000 Hz) at both 50 dB and 65 dB nHL.

Table 2 and Figure 3 show that wave V latencies tended to be longer for the highpass noise than for the notch noise . To evaluate the significance of this difference at 50 dB and 65 dB nHL, a two-way ANOVA was conducted on the subset of subjects described above. The two factors were noise (notch versus highpass) and frequency (1000 Hz versus 2000 Hz). These

Figure 8 Illustrative ex-amples of the response ob-tained for the 500 Hz notch (HN) and highpass (HP) noises at 10 dB to 65 dB nHL.

65

J 50 c m v

35 T t

C

20

10

3 6 9 12

Lames for ine ivvu rrz nown (NN) and highpass (HP) noises at 10 dB to 65 dB nHL.

analyses revealed no interaction between fre-quency and type of noise, but statistically sig-nificant differences were observed between frequencies (p < 0.01) and between noises (p < 0.05) at both 50 dB and 65 dB nHL. At both hearing levels, the highpass noise yielded wave V latencies that were approximately 0.5 msec longer than the notch noise.

Latency increased as intensity decreased from 65 to 10 dB nHL. For example, the mean wave V latency for the quiet condition increased from 5.91 msec at 65 dB nHL to 8.79 msec at 10 dB nHL. Similarly, for the 2000-Hz notch


noise, latency increased from 6.97 cosec at 65 dB nHL to 8.52 msec at 20 dB nHL. Latency for the 1000-Hz highpass noise increased from 8.86 msec at 65 dB nHL to 12.64 msec at 35 dB nHL. Trends were not observed at 500 Hz due to insufficient data .

Table 3 presents descriptive statistics for wave V amplitudes ; Figure 4 also displays the mean data . Recall that few data are shown for 500 Hz because of the limited number of sub-jects with identifiable waves. Three major ob-servations are evident from the table.

First, the largest wave V amplitudes were obtained for the quiet condition. For example, at 65 dB nHL the mean amplitude for the quiet

Table 2

condition was 325 nV; amplitudes at 2000 Hz for the notch and highpass noises were 242 nV and 187 nV, respectively . For 1000 Hz at 65 dB nHL, amplitudes were 178 nV and 217 nV for the respective notch and highpass noises . At 500 Hz, the amplitude was 165 nV for the high-pass noise condition. To compare wave V amplitudes among conditions (quiet, 1000 Hz, 2000 Hz), a one-way ANOVA was conducted at both 50 dB and 65 dB nHL on the afore-mentioned subset of subjects . Notch noise and highpass noise amplitudes were averaged for each subject for these analyses . The results revealed that wave V amplitudes at both 50 dB and 65 dB nHL were about 100 nV larger for the quiet condition than for the noise conditions (p < 0.01) . Nonsignificant wave V amplitude differences were found between 1000 Hz and 2000 Hz (p > 0.05) .

Means, Medians, Standard Deviations, and Ranges of Wave V Latencies

Intensity (dB nHL) Q NN

2000 Hz HP NN

1000Hz HP

500 Hz NN HP

65 dB M 5 .91 6 .97 7 .72 8 .53 8 .86 10 .05 Md 5 .90 6 .95 8.06 8.49 8.70 10.67 SD 0.25 0.11 0.38 0.14 0.38 0.80 Range 5.49- 6.50- 6.27- 7.98- 8.37- 6.21-

6.25 7.43 9.12 9.29 9.69 11 .52

50dB M 6.94 7.40 7.86 9.33 9.31 Md 6.60 7.29 7 .83 9.33 9.29 SD 0.82 0.31 0 .60 0 .40 0 .72 Range 6.24- 7 .04- 7 .11- 8 .91- 8 .43-

8 .39 7 .89 8.76 9 .87 10 .22

35 dB M 7.43 8.12 8.13 9.95 10 .63 Md 7.23 8.04 7.97 9.89 10 .75 SD 0.51 0.18 0.17 0.30 0.62 Range 7.08- 7.35- 7.61- 9.18- 9.57-

8.39 8.84 9.12 11 .09 11 .34

20 dB M 7.96 8.52 8.69 Md 7.88 8.42 8.77 SD 0.61 0.37 0.77 Range 7.07- 8.10- 7.95-

9.36 9.17 10 .17

10 dB M 8.79 Md 8.69 SD 0.67 Range 7.94-

10 .17

84


A second observation evident from Table 3 is that no systematic wave V amplitude dif-ferences were found between the notch and highpass noises . The two-way ANOVA con-ducted at 65 dB nHL revealed no statistically significant differences (p > 0.05) for the inter-action (frequency x noise type), frequency (1000 Hz versus 2000 Hz), or noise type (notch versus highpass). At 50 dB nHL, although no sig-nificant interaction or frequency differences were found, the notch noise yielded wave V amplitudes that were about 40 nV greater than the highpass noise [F(1, 7) = 9.43, p < 0.051 .

Finally, Table 3 shows that amplitudes tended to decrease as the intensity of the stimulus decreased. For example, wave V for the 2000 Hz notch noise decreased from 242 nV at 65 dB nHL to 175 nV at 20 dB nHL.

DISCUSSION

Response Detectability

The identifiability data suggest that ap-proximately one-half of the subjects yielded responses to both highpass and notch noises at 15 dB nHL for 2000 Hz, 35 dB nHL for 1000 Hz, and 65 dB nHL for 500 Hz. These frequen-cy-dependent differences are consistent with Laukli (1983) who, using 95-dB nHL, 500-Hz tone pips in notch noise and highpass noise, recorded identifiable responses in only two of 12 recordings ; ABRs were found in 17 of 22 recordings to 1000-Hz tone pips and in 25 of 27 recordings to 2000-Hz tone pips . Don et al (1979), using a derived technique, reported lower ABR thresholds than we observed at both 500 Hz and 1000 Hz. They found that ABRs

Table 3 Means, Medians, Standard Deviations, and Ranges of Wave V Amplitudes

Intensity 2000 Hz

(dB nHL) Q NN HP 1000 Hz 500 Hz

NN HP NN HP

65 dB M 325 242 187 178 217 165

Md 316 238 191 178 210 181

SD 80 56 47 47 54 47

Range 239- 178- 110- 93- 127- 90-

466 354 239 256 288 212

50 dB M 312 226 217 221 197

Md 300 225 214 222 196

SD 74 55 91 56 66

Range 220- 142- 108- 125- 110- 469 322 374 301 298

35 dB M 301 213 170 173 168 Md 293 226 159 169 166

SD 79 90 48 31 27

Range 171- 83- 120- 140- 132-

420 320 240 223 205

20 dB M 232 175 151

Md 235 171 145

SD 61 45 50 Range 129- 122- 81-

310 232 229

10 dB M 187

Md 180 SD 50 Range 129-

274


predicted 1000 to 4000-Hz thresholds within 10-20 dB, but that a correction of about 30 dB was required at 500 Hz. Fjermedal and Laukli (1989) tested 10 normal-hearing subjects with clicks in highpass noise and stated that 1000 Hz gave slightly better responses than 500 Hz, and that a correction factor of 30-40 dB is ap-propriate for estimating auditory sensitivity at both of these frequencies. In contrast, van Zan-ten and Brocaar (1984) reported that half of their seven subjects yielded responses between 10 and 20 dB SL for a click in notch noise centered around 500 Hz. These authors did report, however, that one 50-year-old male failed to exhibit identifiable responses at 500 Hz or 1000 Hz at the tested limits (70 dB SL). Although there is agreement among studies that 500 Hz yields fewer responses than 2000 Hz, there is no consensus on the magnitude of these differences . Moreover, it is difficult to as-certain the reasons for these discrepancies be-cause of numerous procedural differences .

The present study suggests that presenting clicks in highpass or notch noise centered at 500 Hz is of limited value for assessing auditory sen-sitivity. This is because the useful range of in-tensities is limited by the normal ABR threshold (-65 dB nHL) and the high levels of noise re-quired to mask the click (a 96-dB SPL white noise was used to mask the 65-dB nHL click) . Stapells et al (1985) recommend that noise levels for test-ing hearing-impaired subjects should not exceed 108 dB SPL. Because 108 dB SPL of noise masks a 77-dB nHL click, the maximum hearing loss that could be evaluated using our procedures is about 12 dB at 500 Hz, 42 dB at 1000 Hz, and 62 dB at 2000 Hz. This limited dynamic range for assessing hearing loss is a major limitation of click in noise testing, particularly for 500 Hz. In contrast, a dynamic range of about 95 dB is typical with a click in quiet (i .e ., the electro-physiologic threshold is -5 dB nHL and the audiometer limits are -100 dB nHL) . Stimuli that elicit a low electrophysiologic threshold are preferred so that sensitivity can be measured with a minimum amount of intensity and patient discomfort, and so that responses can be ob-tained from persons having moderate-to-severe hearing losses .

Recall that Fjermedal and Laukli (1989) state that it is feasible to obtain a two-point audiogram in which an unmasked click is used to assess 2000-4000 Hz and a click in highpass noise is used to assess 500 Hz or 1000 Hz . Similarly, Eggermont (1982) suggests using an

unmasked click to evaluate the 2000 to 4000-Hz range and a click in a 1000 Hz highpass noise to assess the 500 to 1000-Hz region. Eggermont (1982) comments that this two-point audiogram can distinguish between rising, flat, and falling configurations . Because we found that the electrophysiologic threshold was about 30 dB lower for 1000 Hz than for 500 Hz, highpass noise or notch noise centered around 1000 Hz is preferred to 500 Hz for estimating low-frequency sensitivity.

At least three explanations have been sug-gested to explain the relatively poor ABRs at 500 Hz as compared to 1000 Hz and 2000 Hz (Davis, 1976 ; Don et al, 1979; Hood and Berlin, 1986). First, minimum audible pressure data suggest that the sensation level of the stimulus may have been lower for 500 Hz than for the higher fre-quencies (ANSI, 1989). Recall that our sound pressures (dB nHL) were specified with refer-ence to the softest level a group of normal lis-teners could detect a click in quiet. Filtered psychoacoustic thresholds were not obtained . Second, because the nerve-fiber density per unit distance is less at the apex than at the base, fewer neurons may discharge synchronously for low-frequency stimuli (Gorga et al, 1988). Third, the limited number of responses obtained at 500 Hz may reflect neural dysynchrony due to the slower travelling wave velocity for the more apical regions of the cochlea. Thus, this dysynchrony among low-frequency neural elements, may ex-plain the relatively attenuated whole nerve response at 500 Hz.

Latencies

Using a notch-width of one and one-half octaves and a notch-depth of -70 dB, we found that the latency of wave V increased about 1 .5 msec as the center frequency of the notch decreased from 2000 to 1000 Hz. This finding is consistent with van Zanten and Brocaar (1984), but in contrast to Pratt and his as-sociates (1982, 1984) who found that latency was independent of the center frequency of the notch. These discrepancies may be attributed to differences in notch-width and/or notch-depth. Van Zanten and Brocaar (1984) used a notch-width of one and two-thirds octaves and a notch-depth of -45 dB, whereas Pratt et al (1984) selected a one-half octave width notch and a notch-depth of - 15 dB . Another explana-tion of the differences among studies was sug-

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gested by van Zanten and Brocaar (1984) . They speculated that Pratt and Bleich (1982) used insufficient masking. That is, perhaps the ABRs were in fact elicited by the full click spectrum rather than by the acoustic energy corresponding to the notch filter . Laukli (1983) also stated that perhaps the stimuli provided by the notches were ineffective and that the constant latency findings reflect the click spectral content at 2000 Hz. In order to better understand the differences among these studies, additional research is required of the effects on the ABR of notch-width, notch-depth, and the relative level of the click and noise (signal-to-noise ratio) .

Slightly longer latencies were observed for highpass noise than for the notch noise. This suggests that the additional frequencies below the notch contributed to the response to alter latency. These results are consistent with the expected increase in basilar membrane travel time for the additional low-frequency com-ponents of the noise. Future research should compare highpass noise to notch noise from a variety of hearing-impaired subjects to ascer-tain the extent to which the frequencies below the notch contribute to the latency of wave V.

The quiet condition yielded wave V laten-cies that were shorter than those that were obtained for the highpass and notch noises . This fording suggests that the response elicited by the click in quiet was, in part, mediated by frequencies above the 2000-Hz notch. That is, the shorter latencies for the quiet condition may reflect the contribution of the high-frequency components of the click spectrum (above the 2000 Hz notch) . The basal region of the cochlea may contribute to a more synchronized neural response for the quiet condition, as contrasted to the notch noise and highpass noise conditions that mask the more basal regions of the cochlea (Picton et al, 1979 ; Borg, 1981 ; Stapells and Picton, 1981) . Thus, the resultant response latencies are longer for the notch and highpass noises than for the quiet condition.

Using highpass noise, the latency of wave V increased 2.3 msec as frequency decreased from 2000 to 500 Hz. This finding is consistent with other investigators who have used clicks with highpass noise (Don and Eggermont,1978 ; van Zanten and Brocaar, 1984). For example, Don and Eggermont (1978) presented highpass noise with a 60 dB nHL click and reported laten-cies of -7.5 msec at 1000 Hz and -9 msec at 500 Hz. The increase in latency with decreasing fre-


quency probably reflects the longer travel time required by low-frequency stimuli to progress to more apical portions of the cochlea (Bekesy, 1949 ; Kiang, 1965 ; Picton et al, 1979 ; Borg, 1981 ; Stapells and Picton, 1981). Moreover, these latency differences support the assump-tion that the various notch noises isolate the response to different segments along the basilar membrane .

Latencies increased as intensity decreased from 65 to 10 dB nHL. This observation is con-sistent with previous investigations and may be due to changes in (1) cochlear hydrodynamics and/or (2) synaptic transmission time (Teas et al, 1962 ; Picton et al, 1977). Teas et al (1962) stated that latency is determined by the posi-tion at which the cochlear traveling wave first rises above threshold, and that as intensity in-creases this position shifts toward the base with a consequent decrease in travel time . Moreover, an increase in intensity is associated with a decrease in the time required for synaptic trans-mission . Davis (1976) stated that synaptic delays are due to the time required for the release of the transmitter substance, its dif-fusion across the synaptic cleft, and the development of a postsynaptic potential in the dendrites. He also commented that minimal synaptic delays are about 0.5 msec for high-in-tensity stimuli, but that synaptic delays are longer for low-intensity stimuli.

Amplitudes

The amplitude of wave V decreased as in-tensity decreased from 65 dB to 10 dB nHL. This finding is consistent with previous re-search and apparently reflects a decrease in the number of neurons that are activated at the lower intensities (Pratt and Sohmer, 1976; Huang and Buchwald, 1978 ; Picton et al, 1979 ; Buchwald, 1983).

The wave V amplitude for the quiet condi-tion was larger than for the notch and highpass noises, particularly at 500 Hz. This reduction in amplitude with the addition of noise was ex-pected (Picton et al, 1981 ; Stapells and Picton, 1981 ; Thummler et al, 1981 ; van Zanten and Brocaar, 1984) . Apparently masking noise eliminates some of those neural elements that contribute to the unmasked response, thereby attenuating. the whole nerve potential (Stapells and Picton, 1981). Furthermore, the high fre-quency regions of the cochlea contribute more to


the synchrony of the response than the more api-cal cochlear regions. The notch and highpass noises mask these more synchronous regions with a consequent decrease in amplitude (Sta-pells and Picton, 1981 ; Buchwald, 1983 ; Moore, 1983).

The amplitude of wave V tended to increase with increases in frequency for both the notch and highpass noise conditions . Most previous in-vestigations did not report wave V amplitudes to clicks in notch noise or highpass noise (Egger-mont, 1982 ; Pratt and Bleich, 1982 ; Pratt et al, 1984 ; van Zanten and Brocaar, 1984). However, Stapells et al (1985) stated that a 70-dB nHL click yielded amplitudes of 180 nV across fre-quencies (500-4000 Hz) and Don and Eggermont (1978) reported larger wave V amplitudes to a 60-dB SL click in noise at 500 Hz (-700 nV) than at 2000 Hz (-450 nV). It is apparent that there is little agreement among the limited studies that have reported wave V amplitudes in re-sponse to clicks in notch noise or highpass noise. Meaningful comparisons among these studies are difficult because of the substantial differen-ces in experimental designs.

No consistent differences in amplitude were observed between the notch and highpass noises . Thus, the wider click spectrum associated with the highpass noise did not yield larger wave V amplitudes than the more restricted click spec-trum associated with the notch noise. These find-ings are in contrast to the expectation that (1) the low-frequency masking and (2) spread-of-masking associated with the notch noise would attenuate the ABR and, thus, render notch noise less effective than highpass noise (Pratt et al, 1984 ; Fjermedal and Laukli, 1989). The reader is cautioned, however, that different results may be expected with other populations (e.g ., subjects having steeply sloping audiograms) and with fil-ters having narrower notches and more gradual rejection rates. These variables deserve inves-tigation .

SUMMARY AND CONCLUSIONS

T he latency of wave V increased as the cen-ter frequency of the notch decreased. This

finding is consistent with van Zanten and Brocaar (1984), but in contrast to Pratt and his associates (1982, 1984) who found that latency was independent of the center frequency of the notch. These discrepancies may be attributed to differences in notch-width, notch-depth, and/or the relative level of the click and noise.

These areas deserve investigation using both normal-hearing subjects and hearing-impaired subjects having a wide range of audiometric configurations .

Latencies were about 0 .5 msec longer for the highpass noise than for the notch noise. This increase in latency may be explained by the longer basilar membrane travel time for the additional low frequencies in the highpass noise. Future research should compare high-pass noise to notch noise from a variety of hear-ing-impaired subjects to ascertain the extent to which the frequencies below the notch con-tribute to the latency of the wave V.

Response identifiability was similar for the notch and highpass noises . It is interesting to note that the additional stimulus energy as-sociated with the highpass noise did not yield lower thresholds. The use of the notch noise, as compared to the highpass noise, is advant-ageous because of the increased frequency specificity provided by the low-frequency mask-ing noise. However, these observations must be verified with hearing-impaired subjects having various audiometric configurations .

The present study suggests that presenting clicks in highpass or notch noise centered at 500 Hz is of limited value for assessing auditory sensitivity . This is because the range of hearing loss is restricted by (1) a high normal ABR threshold (-65 dB nHL) and (2) the high noise levels required to mask the click. If a noise level of 108 dB is chosen as the maximum recom-mended SPL, then the maximum hearing loss that can be evaluated using our procedures is about 12 dB at 500 Hz, 42 dB at 1000 Hz, and 62 dB at 2000 Hz. This limited dynamic range for assessing hearing loss is a major limitation of click in noise testing, particularly for 500 Hz. On the other hand, although a click in quiet has a dynamic range of about 95 dB, clicks are not frequency-specific .

Similar amplitudes were observed for both the notch and highpass noise conditions . In other words, the wider click spectrum associated with the highpass noise did not yield larger wave V amplitudes than the more restricted click spec-trum associated with the notch noise. The iden-tifiability data suggest that approximately one-half of the subjects yielded responses to both highpass and notch noises at 15 dB nHL for 2000 Hz, 35 dB nHL for 1000 Hz, and 65 dB nHL for 500 Hz. Although there is agreement among studies that 500 Hz yields fewer responses than 2000 Hz, there is no consensus on the magnitude

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of these differences . Moreover, it is difficult to ascertain the reasons for these discrepancies be-cause ofnumerous procedural differences among studies (e.g ., notch-width, notch-depth, signal-to-noise ratio, preamplifier bandwidth, notch versus derived procedures).

The amplitude of wave V tended to increase as frequency increased from 500 to 2000 Hz for both the notch and highpass noise conditions . Although these findings are not consistent with the limited studies that have reported wave V amplitudes in response to clicks in notch noise or highpass noise (Don and Eggermont, 1978 ; Stapells et al, 1985), meaningful comparisons among these studies are difficult because of the substantial differences in experimental design . Additional research is required to obtain a bet-ter understanding of these differences .

Some authors recommend using clicks in noise to obtain a two-point audiogram in order to distinguish among rising, flat, and falling audiometric configurations (Eggermont, 1982; Fjermedal and Laukli, 1989). An unmasked click is used to evaluate 2000-4000 Hz and a click in highpass noise is used to assess 500 Hz or 1000 Hz . The present data suggest that high-pass noise or notch noise centered around 1000 Hz is preferred to 500 Hz for estimating low-frequency sensitivity . Moreover, presenting a click in notch noise centered around 4000 Hz may be preferable to using an unmasked click. Notch noise provides improved frequency-specificity, and wave V amplitudes were similar for both highpass noise and notch noise. These suggestions require experimental verification .

The present investigation suggests that while notch or highpass noise may provide a useful means of evaluating sensitivity at fre-quencies >-1000 Hz, our procedures do not ap-pear promising for assessing 500 Hz. Moreover, even at 2000 Hz, the high noise levels necessary to mask the click serve to limit the upper range of testable hearing loss . Several other frequen-cy-specific procedures have been suggested for estimating sensitivity, including presenting tone pips in quiet, highpass noise, and notch noise (Davis and Hirsh, 1979 ; Picton et al, 1979 ; Kileny, 1981 ; Suzuki et al, 1981; Laukli, 1983 ; Davis et al, 1984; Beattie and Boyd, 1985 ; Jerger et al, 1985 ; Gorga et al, 1988). Research comparing these procedures is necessary to es-tablish their relative advantages .


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