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J Am Acad Audiol 13 : 205-224 (2002) Estimating the Audiogram Using Multiple Auditory Steady-State Responses Andrew Dimitrijevic* M . Sasha John* Patricia Van Roon* David W Purcell* Julija Adamonis` Jodi Ostroff Julian M. Nedzelski' Terence W. Picton* Abstract Multiple auditory steady-state responses were evoked by eight tonal stimuli (four per ear), with each stimulus simultaneously modulated in both amplitude and frequency. The modulation frequencies varied from 80 to 95 Hz and the carrier frequencies were 500, 1000, 2000, and 4000 Hz . For air conduction, the differences between physiologic thresholds for these mixed-modulation (MM) stimuli and behavioral thresholds for pure tones in 31 adult subjects with a sensorineural hearing impairment and 14 adult subjects with normal hearing were 14 ± 11, 5 ± 9, 5 -!- 9, and 9 ± 10 dB (correlation coefficients .85, .94, .95, and .95) for the 500-, 1000-, 2000-, and 4000-Hz carrier frequencies, respectively . Similar results were obtained in subjects with simulated conductive hearing losses . Responses to stimuli presented through a forehead bone conductor showed physiologic-behavioral threshold differences of 22 ± 8, 14 ± 5, 5 ± 8, and 5 ± 10 dB for the 500-, 1000-, 2000-, and 4000-Hz carrier frequencies, respectively. These responses were attenuated by white noise presented concurrently through the bone conductor . Key Words: Auditory steady-state responses, objective audiometry, thresholds Abbreviations : AM = amplitude modulation ; FFT = fast Fourier transform ; FM = frequency modulation ; MASTER = multiple auditory steady-state response ; MM = mixed modulation ; OAE = otoacoustic emission ; PTA = pure-tone average (of threshold hearing levels at 500, 1000, and 2000 Hz) ; SAL = sensorineural acuity level Sumario Se evocaron multiples respuestas auditivas de tipo estado estable (steady-state) por medio de ocho estimulos tonales (cuatro en cada oido), con cada estimulo simultaneamente modulado tanto en amplitud como en frecuencia. Las frecuencias de modulaci6n variaron desde 80 a 95 Hz y las frecuencias portadoras fueron de 500, 1000, 2000, y 4000 Hz . Para la via aerea, las diferencias entre los umbrales fisiol6gicos para estos estimulos de modulacion mixta (mixed modulation : MM) y los umbrales conductuales para tonos puros, en 31 sujetos adultos con hipoacusias sensorineurales y 14 adultos con audici6n normal, fueron 14 ± 11, 5 ± 9, 5 ± 9, y 9 ± 10 dB (coeficientes de correlaci6n de .85, .94, .95, y .95) para las frecuencias portadoras de 500, 1000, 2000, y 4000 Hz, respectivamente . Se obtuvieron resultados similares en sujetos con hipoacusias conductivas simuladas . Las respuestas a estimulos presentados a traves de un vibrador 6seo colocado en la frente mostraron diferencias entre los umbrales fisiol6gicos y conductuales de 22 ± 8, 14 ± 5, 5 ± 8, y 5 ± 10 para las frecuencias portadores de 500, 1000, 2000, y 4000 Hz, respectivamente . Estas respuestas fueron atenuadas por un ruido blanco presentado a mismo tiempo a traves del vibrador 6seo . Palabras Clave : Respuestas auditivas de estado estable, audiometria objetiva, umbrales Abreviaturas : AM = modulacion de amplitud ; FFT = transformaci6n rapida de Fourier ; FM = modulacion de frecuencia ; MASTER = respuesta auditiva multiple de estado estable ; MM = modulacion mixta ; OAE = emisi6n otoacOstica ; PTA = promedio tonal puro (de los niveles de umbral en 500, 1000, y 2000 Hz) ; SAL = nivel de agudeza sensorineural *Rotman Research Institute, Baycrest Centre for Geriatric Care, University of Toronto; tDepartment of Otolaryngology, Sunnybrook and Women's College Health Sciences Centre and University of Toronto, Toronto, Ontario Reprint requests : Andrew Dimitrijevic, Rotman Research Institute, Baycrest Centre for Geriatric Care, 3560 Bathurst Street, Toronto, ON M6A 2E1 205 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 major goal of objective audiometry is to obtain a pure-tone audiogram without A requiring any behavioral response on the part of the subject. Objective methods to evaluate hearing include the auditory brainstem response (ABR) (Galambos et al, 1994 ; Sininger and Abdala, 1996 ; Sininger et al, 2000 ; Stevens, 2001) and otoacoustic emissions (OAEs) (White and Behrens, 1993 ; Norton et al, 2000). Both methods have limitations . The ABR is commonly evoked by clicks that stimulate the cochlea along the whole basilar membrane . The major drawback of the click-evoked ABR is poor frequency specificity. More frequency-specific ABR responses can be obtained using tone-pips (alone or in notched noise) or clicks presented in masking noise at different high-pass cutoff frequencies so that "derived band responses" can be recorded (Don et al, 1979 ; Picton et al, 1979 ; Stapells et al, 1994 ; Oates and Stapells, 1997). These techniques are slow since multiple recordings must be obtained to estimate thresholds at the different audiometric frequencies . Transient or distortion-product OAEs can identify hearing losses when thresholds are above 40 dB HL and can suggest the audiometric profile of residual hearing at lower levels (Harrison and Norton, 1999 ; Norton et al, 2000). The major drawback when using OAEs to evaluate hearing loss occurs when the responses are absent since neither the severity of the hearing loss nor the audiometric configuration can then be determined (Wagner and Plinkert, 1999) Owing to the limitations ofABRs and OAEs, auditory steady-state evoked potentials have emerged as an attractive means of objectively estimating the audiogram. Auditory steadystate evoked potentials were first suggested as an objective means to assess hearing by Galambos and colleagues (1981), who demonstrated that the 40-Hz steady-state response was easy to identify at intensity levels just above behavioral thresholds . However, some limitations of using the 40-Hz evoked potential for objective audiometry are as follows : (1) the response diminishes with decreased levels of arousal owing to sleep or anesthesia (Linden et al, 1985 ; Jerger et al, 1986 ; Plourde and Picton, 1990 ; Cohen et al, 1991 ; Dobie and Wilson, 1998), (2) the response cannot be reliably recorded in infants (Stapells et al, 1988 ; Maurizi et al, 1990 ; Aoyagi et al, 1994a), and (3) response amplitude diminishes when several stimuli are presented simultaneously (John et al, 1998). Recent work has therefore concentrated on the steady-state responses at higher rates of 206 stimulus presentation . Cohen and colleagues (1991) showed that in adults, responses could be evoked at stimulus rates greater than 70 Hz and that these responses were little affected by sleep. Furthermore, these rapid responses can be easily recorded in infants and young children (Rickards et al, 1994 ; Lins et al, 1996 ; Savio et al, 2001 ; Cone-Wesson et al, 2002a, 2002b) . Lins and Picton (1995) demonstrated that multiple responses can be recorded simultaneously without loss of amplitude at these rapid stimulus rates, thereby allowing for rapid assessment of thresholds at different audiometric frequencies. The 80-Hz auditory steadystate evoked response has been extensively evaluated as an objective audiometric tool in hearing-impaired patients (Aoyagi et al, 1994b; Rance et al, 1995 ; Lins et al, 1996 ; Picton et al, 1998 ; Perez-Abalo et al, 2001 ; Cone-Wesson et al, 2002a, 2002b, 2002c) . Amplitude modulation (AM) has typically been used to elicit the auditory steady-state evoked response . Cohen and colleagues (1991), using single stimuli, showed that mixedmodulation (MM) stimuli that consisted of 100 percent AM and 20 percent frequency modulation (FM) evoked larger responses than 100 percent AM alone . In that study, Cohen and colleagues (1991) used a 0-degree phase difference between the AM and FM (i .e ., the maximum amplitude occurred at the same time as the maximum frequency) . John and colleagues (2001b), using multiple stimuli in each ear, showed that the response enhancement with MM stimuli was present at both threshold and suprathreshold intensities . They further showed that the phase difference between AM and FM that produced the maximum response varied with the carrier frequency. Setting the relative phase to 0 degrees will usually be beneficial but may not always align the AM and FM responses optimally. For most carrier frequencies, maximum responses could be elicited if the maximum amplitude occurred slightly earlier than the maximum frequency. The current study used the multiple auditory steady-state response (MASTER) technique (John and Picton, 2000b) . This technique uses an automatic statistical evaluation of the responses to multiple sinusoidally modulated stimuli. Since each carrier has a unique modulation rate, separate responses to each carrier can be distinguished in the frequency transform of the recorded activity by measuring the amplitude spectra at the frequencies of modulation . The major advantage of this technique is that by simultaneously presenting multiple stimuli (e.g ., Audiometry Using MASTER1Dimitrijevic et al four stimuli in each ear for a total of eight), multiple responses can be recorded during the time normally required to record one . This does not necessarily mean that audiometry can be performed in one-eighth of the time . If the patient has a sloping hearing loss and if the eight stimuli in MASTER are all presented at the same sound pressure levels, recordings at several intensity levels are required to bracket the thresholds at the different carrier frequencies . Nevertheless, audiometry should be able to be performed in one-third to one half of the time . The current study investigated the use of MM stimuli in evaluating hearing thresholds in 2. 3. hearing-impaired and normal-hearing subjects using the MASTER technique . First, we wanted to examine the accuracy of MASTER in predicting hearing thresholds . Second, we wanted to see how well MASTER could estimate both sensorineural and conductive hearing losses . Third, we wanted to validate the MASTER technique for obtaining bone-conduction thresholds . METHOD Subjects Four groups of subjects participated in the experiments: 1. Hearing-impaired adults (n = 31, 15 male and 16 female) were volunteers from the audiology clinic at Sunnybrook and Woman's College Health Science Centre . These subjects varied in age from 32 to 86 (mean 69) years . Three of the subjects had a profound hearing loss in one ear and only the better ear was tested, resulting in a total of 59 ears being examined . The mean three-tone pure-tone average (PTA) threshold (500, 1000, and 2000 Hz) was 46 . 20 dB HL, with a range of 15 to 87 dB HL . Mean PTAs indicated normal hearing in 7 ears (<-25 dB HL, most of these subjects having a hearing loss at 4000 Hz), mild hearing loss in 17 ears (26-40 dB HL), moderate hearing loss in 19 ears (41-60 dB HL), and severe hearing loss in 16 ears (61-90 dB HL) . Of the 59 ears examined, 44 had sensorineural hearing loss, 12 had mixed hearing loss (sensorineural predominant), and 3 had normal hearing (subjects with a unilateral hearing loss) . Only 1 subject had interaural threshold differences of 40 dB or greater at any frequency. This subject's ears were tested separately, and the better ear was masked 4. with white noise when the poorer ear was evaluated . The audiometric configurations for the ears were flat (n = 25), sloping to a high-frequency loss (n = 31), and sloping from a low-frequency loss (n = 3) . Normal-hearing adults (n = 14 ; 4 male and 10 female) were volunteers obtained from a departmental subject database . These subjects varied in age from 23 to 63 (mean 36) years. All had pure-tone thresholds below 25 dB HL across frequencies 500 to 4000 Hz . Normal-hearing adults (n = 10 ; 5 male and 5 female) participated in the experiment to evaluate a simulated conductive hearing loss . These subjects varied in age from 23 to 35 (mean 28) years . Three of these subjects were also in group 2 . Simulated conductive hearing loss was achieved by plugging the insert earphones with plasticine . All subjects had pure-tone thresholds below 25 dB HL across all audiometric frequencies used (500-4000 Hz) prior to the simulated hearing loss . After the insert earphones were plugged, the audiograms showed a moderate flat hearing loss, with mean PTAs of 52 -} 4 dB HL ranging between 47 and 57 dB HL . In this group, behavioral and MASTER thresholds were evaluated in the plugged earphone condition only. Normal-hearing adults (n = 16 ; 5 male and 11 female) were used to study bone conduction. These subjects varied in age from 23 to 49 (mean 28) years . Five of these subjects were also in group 2. All subjects had air- and bone-conduction thresholds below 25 dB HL between 500 and 4000 Hz . All subjects were evaluated to obtain behavioral thresholds for forehead placement of the bone-conduction vibrator. Eleven of the subjects were also evaluated using MASTER to assess bone-conduction thresholds, and 10 of the subjects participated in a separate bone-conduction masking study. Auditory Stimuli Each stimulus consisted of a sinusoidal tone, with a carrier frequency of f.. MM stimuli have both AM and FM components . Both AM and FM occur at the same modulation rate (f,,) . The depth ofAM (ma) was defined as the ratio of the difference between the maximum and minimum amplitudes of the signal to the sum of the maximum and minimum amplitudes. The FM component of the stimulus was formed by modulating the phase of the carrier frequency (p). The depth 207 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 of FM (mf) was defined as the ratio of the difference between the maximum and minimum frequencies to the carrier frequency. The formula used for generating the stimuli (s) was: s(i) = a(l + m¢sin(2aTfnti))sin(2iTf ti + 0(i))/M (formula 1) where cp(i) _ (mJ,l(2Q)sin(2arfmti + 8) (formula 2) and M = (1 + mazl2)'12 (formula 3) where a is the amplitude, i is the address in the output buffer, t is the time per address at which digital-to-analog occurs, 6 is the difference in phase between AM and FM (expressed in radians), and fm is the modulation frequency. The term mff 1(2ffm) represents the modulation index for FM (usually denoted by (3). The final divisor M is used to maintain a constant root-meansquare amplitude across the different amounts of AM (Viemeister, 1979). In all of the experiments in this study, the auditory stimuli were MM, with an AM depth of 100 percent (m ) and an FM depth of 25 percent (mf) . In the above formula, the phase of the modulations as determined by the formula is such that when O is -90 degrees, the maximum frequency occurs at the same time as the maximum amplitude. For simplicity's sake, and because the relative phase is arbitrary, we shall henceforth state that relative phase is 0 degrees when the maximum amplitude coincides with the maximum frequency since this fits better with the previous literature . The sign of e is such that if this term increases, the maximum FM moves ahead relative to the maximum AM. Table 1 illustrates Table 1 Relative Phase between FM and AM (degrees) MASTER Setting (degrees) 0 -90 90 180 0 90 270 180 these effects. Figure 1 illustrates one of the stimuli. One of the properties of MM stimuli is that varying the relative phases between the maximum AM and maximum FM will alter the frequency spectra . For example, when the maximum amplitude of the AM and maximum FM frequency occur at the same time (i .e ., 6 = 0 degrees), the peak of the spectra skews toward higher frequencies . Conversely, when maximum AM occurs at the minimum FM (i .e ., maximally out of phase or 0 = 180 degrees), the peak of the spectra skews toward lower frequencies. For these experiments, the relative phases between AM and FM were chosen to produce the largest combined (AM and FM) response (John et al, 2001b) . Relative phase values for the 500-, 915-, 1850-, and 3810-Hz carriers were 45, 315, 315, and 315 degrees, respectively. Because of the asymmetry of the spectra, the carrier frequencies were adjusted so that the maximum energy of the spectra was at 500, 1000, 2000, and 4000 Hz for the left ear, and the same carrier frequencies were used in the right ear. Although the spectra of the 500-Hz carrier were asymmetric, the maximum energy still occurred at 500 Hz, and the carrier frequency was thus not adjusted . Each carrier was modulated by a unique modulation frequency. Table 2 provides the stimulus parameters . The highly specific modulation frequencies were attributable to the requirement for an integer number of cycles of a stimulus within each recording epoch of 1.024 seconds (John and Picton, 2000b) . In the remainder of this article, the modulation frequencies will be reported with only single-digit precision. The digitally generated auditory stimuli were converted to analog form at a rate of 32 kHz using 12-bit precision. The analog waveforms were routed to a Grason Stadler Model 16 audiometer for presentation at the desired root-mean- Stimulus Characteristics Time Waveforms Maximum frequency occurs at the same time as maximum amplitude Maximum frequency occurs 1/4 cycle before maximum amplitude Maximum frequency occurs at the same time as minimum amplitude Maximum frequency occurs 1/4 cycle after maximum amplitude FM = frequency modulation ; AM = amplitude modulation ; MASTER = multiple auditory steady-state response . 208 Spectra Skewed toward high frequencies Symmetric Skewed toward low frequencies Symmetric Audiometry Using MASTER/Dimitrijevic et al +1 .0 100% AM and 25% FM f , = 84 .96 Hz Instantaneous Frequency (Hz) 1 .0 ak Amplitude Spectrum X 0.0 1000 2000 Hz Figure 1 Mixed-modulation stimuli. The upper part of the figure shows the time waveforms for a stimulus with a carrier frequency of 915 Hz that is 100 percent amplitude modulation (AM) and 25 percent frequency modulation (FM) . Both the AM and FM occur at 84 .96 Hz . The middle plot shows the instantaneous frequency of the sound. The phase difference (O) between the AM and FM components is +45 degrees in this case . The maximum FM occurs a little later than the maximum AM . The bottom plot shows the amplitude spectrum of the stimulus . A 915-Hz carrier was chosen to estimate 1000-Hz puretone thresholds since the spectrum is skewed toward a higher frequency and the maximum energy in the stimulus occurs at 1000 Hz (the carrier frequency of 915 Hz plus the modulation frequency of 85 Hz). square sound pressure level intensity levels through Eartone 3A insert earphones calibrated with a DB 0138 coupler. Stimuli were calibrated in hearing level in the MASTER setup using the reference values of Wilber and colleagues (1988) . According to Wilber and colleagues (1988), using Eartone 3A inserts with a DBO138 (HA-2) 2-cc coupler, the reference equivalent thresholds for 500, 1000, 2000, and 4000 Hz are 8 .0, 3 .5, 6 .5, and 7 .0 dB SPL . The amplitude of the 1000-Hz stimulus relative to loudest stimulus (500 Hz) should be -4 .5 dB (3 .5-8 dB) . The amplitude (a in formula 1) of the 500-Hz stimulus was arbitrarily set at 20 (a number based on the range of the digital-analog converter, which has a maximum of 100), and amplitudes of the other stimuli were adjusted according to the formula : Table 2 ax = a500 *10d500-/20 (formula 4) where ax is the amplitude at x Hz and d500 -x is the decibel difference between the threshold at x and the threshold at 500 Hz . In the 1000-Hz case, this difference in decibel sound pressure level is -4 .5 and the amplitude is 11 .91 . The more recent standard reference equivalent thresholds for insert earphones (ANSI, 1996) are lower than those of Wilber and colleagues (1988) :5.5,0 .0,3 .0, and 5 .5 dB SPL for 500, 1000, 2000, and 4000 Hz . However, since both the pure tones and the MASTER stimuli were similarly calibrated, this does not affect our comparisons . For the bone-conduction studies, a Radioear model B-71 oscillator was placed on the middle of the forehead and held in place with an adjustable elastic strap exerting an average force of 7 .5 (range 5-9) Newtons or 765 g. Four stimuli were simultaneously presented using the parameters listed for the left ear in Table 2. Both ear canals were occluded with inserts throughout both the behavioral threshold estimations and the MASTER recordings . Normal hearing levels for both pure tones and the four individual MASTER stimuli were determined in 16 normal-hearing subjects (Table 3) . The pure-tone stimuli were calibrated in hearing level. For the masking studies, the boneconducted tones were presented in white noise with an effective masking level (as determined by the audiometer for speech) of 50 dB HL (root-mean-square dynamic force measured using an artificial mastoid as 0.27 Newtons, or 109 dB relative to 1 p,N) . This was significantly higher than the average level needed to mask the perception of the tones (see Results) since we wished to ensure that there was no undermasking (see Discussion). The air-conducted stimuli were calibrated using a Briiel & Kjaer model 2230 sound level meter with a DBO138 2-cc coupler. On repeated testing, the accuracy of calibration for airconducted tones was ± 2 .5 dB . The bone- Stimulus Parameters Right Ear Left Ear f (Hz) fn (Hz) Phase (degrees) Amplitude f (Hz) f (Hz) Phase (degrees) Amplitude 500 915 1850 80 .08 84 .96 89 .84 45 315 315 20 .00 11 .91 15 .00 500 915 1850 78 .12 83 .01 86 .91 45 315 315 20 .00 11 .91 15 .00 3810 94 .73 315 15 .89 3810 91 .80 315 15 .89 209 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 conducted stimuli were calibrated using a Bruel & Kjaer Artificial Mastoid Type 4930 (ANSI 1992, 1997) with a static application force equivalent to that used on subjects during the experiment. The levels were calibrated in decibel hearing level for bone-conduction stimuli presented to the mastoid with unoccluded ears . The accuracy of calibration for the bone-conducted stimuli was ± 5 dB . At the levels used for our recordings (20 and 30 dB HL), there was no measurable mechanical distortion artifact above the noise floor (about -10 dB HL) at the MASTER modulation frequencies. Recordings Each of the four experiments was carried out in one session that lasted about 2 hours. Electrophysiologic responses were collected from a gold-plated Grass electrode located at Cz and referenced to the midline posterior neck (7-8 cm below the inion) . All electrode impedances were under 5 kOhm at 10 Hz . Recordings occurred in a sound-attenuated testing booth with the subjects sleeping in a reclining chair. The subjects slept through most of the experiments . The responses were amplified using a Grass P50 battery-powered amplifier with a filter handpass of 3 to 300 Hz . The MASTER data acquisition system (John and Picton, 2000b; see also <w.hearing.cjb .net>) collected the data using an analog-digital conversion rate of 1 kHz with 12-bit precision. Consecutive data epochs of 1 .024 seconds were linked together to form sweeps of 16 .384 seconds, which were averaged and then submitted to a fast Fourier transform (FFT) to produce an amplitude spectrum with a resolution of 0.061 Hz . When an epoch contained electrophysiologic activity exceeding ±90 nV it was rejected, and the next acceptable epoch was used to build the sweep . Table 3 Subjects Sensorineural hearing impaired Normal hearing Combined Simulated conductive hearing loss Normal-hearing bone conduction Weighted averaging (John et al, 2001a) was used to combine sweeps . Briefly, weighted averaging involves multiplying the data in each recorded epoch (1 .024 sec of data) by a factor that is inversely scaled by the amount of variance in that particular epoch. The more noise in a recording, the less it contributes to the overall average. To determine if the FFT components at the stimulus modulation frequencies were different than background electroencephalographic activity, the value at each of these frequencies was compared, using an F ratio, to the 120 adjacent frequencies (60 bins above and 60 below the stimulus frequency, or -3 .7 Hz), excluding those frequencies at which other stimuli were modulated. Comparing this ratio against the critical values for F at 2 and 240 degrees of freedom gives the probability of a response being within the distribution of the background noise (John and Picton, 2000b) . Responses were considered significantly different from background noise when p < .05 . Threshold Estimations Behavioral thresholds were established for the pure tones using a standard 10-dB down 5-dB up searching protocol (Carhart and Jerger, 1959). For the physiologic thresholds, stimuli were initially presented at 20 dB above the behavioral PTA. If the initial stimulus level did not yield eight significant responses (four if hearing was unilateral), the intensity was increased by 10 dB until all responses reached significance . Stimulus presentation level never exceeded 90 dB HL and was never set at levels that caused discomfort to the subject. The recording was stopped if any of three criteria were met . First, the recording was stopped when the responses to all stimuli presented at an intensity above behav- Difference between Physiologic and Behavioral Thresholds Carrier Frequency (Hz) Number of Subjects 500 1000 2000 31 13 - 11 (-10-+40) 5 ± 8 (-15-+30) 5 ± 9 (-15-+25) 14 45 10 17 ± 10(-10-+35) 14 ± 11 (-1-+40) 20 - 10(0-+35) 4 - 11 (-15-+25) 5 - 9 (-15-+30) 15 ± 8 (0-+30) 4 - 8 (-5-+25) 11 22 - 8(10-+40) 14 - 5 (5-+20) Differences are in decibels . Results are given as mean - SD, with range in brackets . 210 4000 8 ± 11 (-20-+40) 11 - 7 (-5-+25) 5 ± 9 (-15-+25) 11 ± 7 (-5-+25) 9 - 10 (-20-+40) 13 ± 9 (-5-+30) 5 ± 8 (-5-+20) 5 ± 10 (-10-+20) Audiometry Using MASTER/Dimitrijevic et al ioral threshold reached significance . In cases in which there was a sloping hearing loss, stimuli were below behavioral thresholds for some carriers and suprathreshold for other carriers . Second, the recording was stopped when the mean noise level was below 10 nV Third, the recording was stopped after reaching a maximum allotted recording time of 17 minutes (equivalent to 64 sweeps, each lasting 16 .384 sec) . Typically, a noise level of 10 nV was reached after about 15 minutes of recording . In some cases (n = 26 ears), less than four stimuli per ear were used to estimate hearing thresholds . In many of these ears, this occurred when there was a steeply sloping high-frequency hearing loss . Even when some stimuli were still below threshold, the other suprathreshold stimuli could be uncomfortably loud for a subject, and we continued with single stimuli. In these subjects, a comparison could not be made between the thresholds estimated with four stimuli and with one stimulus . In 17 ears, a formal comparison could be made between threshold estimation using multiple stimuli and using one or two stimuli. In 11 ears, this was done because the physiologic thresholds were more than 20 dB greater than the behavioral thresholds . In the other ears, the extra testing was performed since the subject was willing to donate some extra time . Physiologic thresholds were primarily defined as the lowest intensity level at which there was a significant response . In 13 cases (of a total of 348 threshold estimations), nonsignificant values were recorded for highintensity stimuli even though significant responses were recorded at lower intensities . In these cases, the absent response at high intensity might have been a "miss" or the present response at lower intensity might have been a "false alarm ." We therefore used a secondary rule that if the difference between the louder (nonsignificant) and softer (significant) stimulus was 20 dB or greater, the threshold was taken to be the intensity of the louder stimulus . If the difference was 10 dB, then the threshold was taken to be the intensity of the less intense stimulus . In some cases (five at 500 Hz, two at 1000 Hz, five at 2000 Hz, and eight at 4000 Hz), no significant responses were recorded, regardless of intensity. In these cases, the threshold was arbitrarily set to 10 dB above the highest intensity presented. Physiologic thresholds for bone-conducted stimuli were determined in 11 normal subjects using the four stimuli . Additional recordings were performed with white noise added to the bone-conducted stimuli in 10 subjects . In four separate recordings, MASTER stimuli were presented at two intensity levels, 20 and 30 dB HL, both with and without white noise, at an effective masking level (on the audiometer) of 50 dB HL . The actual behavioral masking level was checked by presenting the MASTER stimuli at 30 dB HL and increasing the noise intensity until the MASTER stimulus could not be recognized . The mean level was 37 dB (range 35-40) . The white noise was therefore approximately 13 dB higher than the intensity needed for perceptual masking. Statistical Analyses The main variable for analysis was the difference between the threshold estimated using MASTER (the physiologic threshold) and the threshold estimated using pure-tone audiometry (the behavioral threshold) . Changes in this variable between different subject groups were evaluated using a two-way group by carrier frequency analysis of variance (ANOVA) with repeated measures across carrier frequency. Relations between variables were evaluated using linear regression, and the significance of these relations was assessed using Pearson product-moment correlation coefficients . Incidence data were assessed using the cumulative binomial distribution function . The amplitudes and phases of the responses were quite variable . Recordings were stopped when responses were recognized rather than when a good signal-to-noise ratio was obtained . Many of the measurements were therefore contaminated with more residual noise than usual . Furthermore, comparing measurements across groups at equivalent sensation levels led to unequal numbers at different intensities . Rather than dispensing with these data entirely, we decided to present grand mean data and to evaluate these in terms of general trends . RESULTS Subjects with Sensorineural Hearing Impairment or Normal Hearing The amplitude of the responses increased with increasing intensity above threshold in both the subjects with hearing impairment and the normal subjects . Figure 2 shows the amplitudes of the responses plotted relative to behavioral threshold in the two groups of subjects 211 Journal of the American Academy of Audiology/ Volume 13, Number 4, April 2002 80 60 40 20 0 80 60 40 20 0 9 Hearing impaired o Normal hearing 0 10 20 30 40 0 10 20 30 40 Intensity (dB SL) Figure 2 Amplitudes of steady-state responses. Amplitudes for the responses to air-conducted MASTER stimuli are plotted for normal-hearing and hearing-impaired subjects . The intensity is given in decibel sensaton level, or decibel across all subjects for whom above each subject's behavioral threshold for pure tones. Data have been collapsed intensity increases for all amplitude increases as the stimulus each intensity. Response responses were available at circarrier frequencies: 500 Hz (top left), 1000 Hz (top right), 2000 Hz (bottom left), and 4000 Hz (bottom right) . Filled subjects . represent the normal-hearing subjects and open circles cles represent hearing-impaired using only those responses that were considered significantly different from noise. Average amplitudes across the carrier frequencies are shown in the left graph of Figure 3 (together with other results) . Since behavioral thresholds were accurate to within 5 dB and the physiologic measurements were obtained only in 10-dB steps and only for certain intensities, the data are based on different groups of subjects at the different intensities and could not be analyzed using an ANOVA. Nevertheless, the amplitudes are clearly larger for the patients with sensorineural hearing loss compared to normal subjects. Only 2 amplitudes of 28 failed to show a higher amplitude for the normal subjects (p < .001 using the binomial distribution to assess the probability that this number or lower 212 occurs in a sample of this size at a probability of .5). The onset phase was more variable across subjects than the amplitude. To evaluate phase, we therefore collapsed data across frequencies. Phases were averaged geometrically because of their circularity. In normal subjects, the onset phase of the responses increased significantly with increasing intensity (slope of 2.6 degrees/dB), but this did not occur in the patients with sensorineural hearing loss (slope of 0.2 degrees/dB). The right graph of Figure 3 plots these effects of intensity on onset phase. Figure 4 shows the steady-state responses and plots the behavioral and physiologic thresholds in an 80-year-old patient with a moderate bilateral hearing loss caused by Meniere's Audiometry Using MASTER1Dimitrijevic et al 80 Z 60 b 40 o. C O 20 -135 0 0 10 20 30 -180 40 0 dB SL 10 20 30 40 dB SL " Bone conduction (normal hearing) V Air conduction (normal hearing) A Air conduction (hearing impaired) E Air conduction (simulated conductive hearing loss) Figure 3 Amplitudes and phases . Average response amplitudes (left graph) and onset phases (right graph) for airand bone-conducted MASTER stimuli. Data have been collapsed across carrier frequency. Measurements for responses to air-conducted stimuli (normal hearing, hearing impaired, and simulated conductive hearing loss) are averages of the left and right ears . For both graphs, circles represent bone-conducted stimuli, triangles represent air-conduction stimuli in hearing-impaired subjects, in uerted triangles represent normal-hearing subjects, and squares represent air-conducted stimuli in subjects with simulated conductive hearing loss . disease. The amplitude spectra of the electroencephalographic recordings at various presentation levels are shown on the left . Stimuli presented at 80 dB HL resulted in eight significant responses (four per ear) . The number of sig- nificant responses decreased with decreasing intensity. The figure demonstrates how physiologic thresholds were determined . For example, when evaluating the physiologic threshold at 500 Hz in the right ear, significant responses MASTER Recordings Thresholds 0.5 40 v Right 0 v v 30 50 v 60 60 v vV 1 4 tH H VV Left 70 70 80 90 100 Modulation Frequency (Hz) 110 Hz portion of the response spectra is plotted. The triangles indicate when the responses were significantly different from the background noise (right ear, open triangles; left ear, closed triangles) . For each carrier frequency, threshold is 80 dB HL 2 Figure 4 Threshold estimation using MASTER. This figure demonstrates how kHz physiologic thresholds are determined . The subject is a patient with a sensorinerual hearing loss due to Meniere's disease. The left side of the figure shows the auditory steady-state responses and the right side shows the behavioral and MASTER audiograms . Steady-state stimuli were presented at 40, 50, 60, 70, and 80 dB HL . Only the 70- to 110- 120 dB HL tt 0 O Behavioral MASTER defined as the lowest intensity that produced a significant response. For example, the 500-Hz left ear response was significant at 80, 70, 60, and 50 dB HL but not significant at 40 dB HL . Therefore, the threshold is 50 dB HL . MASTER thresholds are shown by the black diamond in the audiograms . 213 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 (open triangles) were recorded at 80, 70, 60, and 50 dB HL and no response was found at 40 dB HL. The physiologic threshold was therefore determined as 50 dB HL, as is shown on the audiogram on the right . Of the 348 physiologic thresholds obtained (59 ears with sensorineural hearing loss and 28 normal ears, each at four frequencies), the physiologic-behavioral threshold difference was less than or equal to 15 dB in 83 percent of the responses. The mean results are presented in Figure 5 and compared between groups in the upper part of Table 3. The data plotted in Figure 5 also show the range of values since the upper and lower T bars show the 10th and 90th percentiles of the distribution . Combining the threshold estimations across the carrier frequencies showed a median value (50th percentile) of 5 dB with the 10th and 90th percentiles at -5 and 25 dB . The p < .5 confidence limits (2 .5th to 97 .5th percentile) for the threshold estimations were -10 and 30 dB . The normal and hearing-impaired groups were compared using a two-way (group by carrier frequency) ANOVA. The mean differences between behavioral and physiologic thresholds were not significantly different in hearingimpaired subjects compared to those with normal hearing (main effect of group: F = 0.7, df = 1, 85, p > .05) . In both normal-hearing and hearingimpaired subjects, the greatest difference between the physiologic and behavioral thresholds was seen for the 500-Hz carrier frequency T T 500 1000 'J' 2000 i 4000 Carrier Frequency (Hz) Figure 5 Physiologic-behavioral differences . This figure shows the differences in physiologic and behavioral differences for each carrier. The box boundaries represent the 25th and 75th percentile limits, and the T bars show the 10th and 90th percentile limits . The median (50th percentile) is represented by the horizontal line in the box, and the asterisks show the mean . The number of ears for which data were obtained (same for each frequency) was 87 . 214 (main effect of carrier frequency: F = 23 .0, df = 3, 285, p < .001). There were no significant interaction effects . The physiologic thresholds were significantly correlated with behavioral thresholds . The results of the regression analyses are plotted in Figure 6 and the regression parameters are given in Table 4. The probabilities of the correlation coefficients were all less than .001 . We compared the level of noise in the recordings to the magnitude of the physiologic-behavioral threshold differences to determine whether noisier recordings produced higher threshold differences. Because of the different stopping conditions, it was not simple to measure a comparable level of the residual background noise across the recordings . To estimate the amount of noise that was present in the recordings, we used the recording obtained at the lowest sound level since this went for the full 17 minutes, or 64 sweeps (since no responses were significant). We used simple rather than weighted averaging. The correlation coefficients with the thresholds at 500, 1000, 2000, and 4000 Hz were 0.06, 0.17, 0.07, and 0.01, none of which were significant . In the 17 ears for which the thresholds could be compared between single and multiple stimuli, 12 ears were presented with only a 500-Hz stimulus, 1 ear was presented with a 2000-Hz stimulus, and 4 ears were presented with a 4000-Hz stimulus . In the 500-Hz alone stimulus condition, the mean difference between MASTER and behavioral thresholds was 17 - 8 compared to 19 ± 10 dB in the four stimulus conditions (not significant) . For the 2000- and 4000-Hz stimuli combined, the mean differences were 9 ± 7 and 21 - 11 dB for the singlestimulus and four-stimulus conditions, respectively (t = 6.0, df = 4, p < .01) . Simulated Conductive Hearing Loss The simulated conductive hearing loss resulted in flat audiograms for all subjects . The physiologic-behavioral differences are presented in Table 3 . An ANOVA comparing these differences with those of the patients with sensorineural hearing loss showed a main effect of carrier frequency (F = 9.4, df = 3, 228, p < .01), with the difference being larger at 500 Hz than at other carrier frequencies . There was also a significant main effect of group (F = 21 .8, df = 1, 76, p < .001), with the differences being larger in the subjects with a simulated conductive hearing loss . There was no significant interaction between group and frequency. The average Audiometry Using MASTER/Dimitrijevic et al Figure 6 Regression analyses . This figure shows the regression lines between behavioral and physiologic thresholds for 500 Hz (top left), 1000 Hz (top right), 2000 Hz (bottom left), and 4000 Hz (bottom right) and all of the carriers combined (middle plot) . The data used for these regressions come from the Thresholds Behavioral (dB HL) 31 hearing-impaired subjects and the 14 normal-hearing adults (subject groups 1 and 2) . The size of the points varies with the number of overlapping points . Physiologic (dB HL) amplitudes and phases are plotted in Figure 3 (together with those for the normal subjects and those with sensorineural hearing loss). Bone-Conduction Responses The behavioral thresholds for the various bone-conduction stimuli are given in Table 5. The mean amplitudes and phases are shown in Figure 3, and sample bone-conduction results are shown in Figure 7 . Figure 7 uses polar plots to show both the amplitudes and the phases of the responses. Using the amplitude spectrum to show the responses, as was done in Figure 4, does Regression Analyses Table 4 Carrier Frequency 500 1000 2000 4000 All carriers Number of Points Slope (m) Intercept (b) r 87 0.88 -8 .87 85 87 348 0.99 0.92 -8 .32 -4 .47 95 92 87 87 0 .92 0 .89 -1 .46 -0 .21 94 95 The regression equation is of the form y = mx + b, where y is the behavioral threshold for pure tones, x is the physiologic threshold for the MASTER responses . m is the slope, and b is the y intercept . not provide any information about the phase of the responses. A polar plot is constructed such that the amplitude of the response (at only one bin in the spectrum where the frequency equals the modulation frequency) is represented by the length of the line (or vector) departing from the origin (the cross) and the phase is represented by the angle between the response line and the horizontal axis . The onset phase normally decreases (the vector moves clockwise) with decreasing intensity (discussed more fully in John and Picton, 2000a) . Responses are recognized in the spectrum by comparing the amplitude at the modulation frequency to the amplitudes in the adjacent bins . In the polar plot, the confidence limits of the amplitudes in these adjacent bins can be plotted as a circle, and this circle can be positioned over the furthest extent of the amplitude vector . If the circle does not include the origin (the shaded circles in the figure), then the response is significantly different from the noise levels recorded in adjacent frequencies. The amplitudes of the responses were larger than those found in normal subjects for air conduction . Although in some subjects, the phases changed quite regularly (see Fig . 7), the mean onset phases were quite variable . Although overall they tended to be a little larger than those for 215 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 Table 5 Behavioral Bone-Conduction Thresholds Frequency (Hz) Threshold 500 1000 2000 4000 5 .0 ± 3 .6 3 .8--2 .9 2 .7±6 .3 0 .9±6 .9 3 .6±8 .2 0 .9±8 .0 -2 .7±4 .6 -4 .1 +4 .6 7 .2 ± 5 .4 5 .9--4 .6 4.8±7 .0 2 .8±6 .8 5 .0-8 .0 2 .8!-8 .4 2 .7±5 .2 0 .3±4 .3 -5 .6--5,4 0 .3--6 .2 -5 .0-8 .4 1 .3±9 .2 5 .0±7 .7 6 .3±7 .4 5 .9-!-5 .5 5 .0-!-6 .8 Air conduction Pure tone Mean Best of ears Modulated Mean Best of ears Bone conduction Pure tone Modulated tone Mean = SD in dB HL (for mastoid placement and for unoccluded ears) . As discussed in the text, the forehead placement and the occlusion of the ears will change these thresholds . Data from 16 subjects . air conduction (on average 40 degrees between 5 and 30 dB SL), the mean bone-conduction phases did not show any clear relationship with dB HL 500 1 kHz 2 kHz 4 kHz 20 10 50 nV -10 0 Nonsignificant 0 Significant Figure 7 Response to bone-conducted stimuli. The bone vibrator was located on the forehead. This figure presents the data from a single subject, whose behavioral bone-conducted pure-tone thresholds were -10, -10, 0, and 10 dB HL at 500, 1000, 2000, and 4000 Hz, respectively. The data are presented in polar plots such that the onset phase of the response is represented as the angle from the x-axis measured counterclockwise . As intensity decreases, the onset phase decreases (the vectors move clockwise) . Since phase delay is equal to 360 minus the onset phase, this means that the phase delay increases with decreasing intensity. The p < .05 confidence limits of the residual noise in the recording are plotted as a circle centered at the point of the amplitude vector. If the circle does not include the origin (shaded circles), the response is significantly different from zero at p < .05. Since the recording at 20 dB HL was based on averaging 15 sweeps, the noise level is higher in this recording than in those at the lower intensities, where the recordings were based on 64 sweeps . Physiologic thresholds were determined as 20, 0, 10, and 10 dB HL . 216 sensation level. These data were more variable than those for the air-conduction studies since we were not able to combine data across right and left ears. The mean differences between MASTER and behavioral thresholds for the bone-conduction stimuli are presented in Table 3. An ANOVA comparing the physiologic-behavioral threshold differences with bone conduction to air conduction showed no main effect of air versus bone but a significant effect of carrier (F = 18 .8, df = 3, 37, p < .001), with the 500-Hz measurement being higher than the others, and a significant interaction (F = 5.5, df = 3, 37, p < .01), with the 500-Hz threshold difference being higher for bone conduction than for air conduction . The masking effects were examined using a three-way (masking, intensity, carrier frequency) ANOVA. As shown in Figure 8, white noise masking significantly decreased the amplitude of the MASTER responses (F = 36 .3, df = 1, 9, p < .001). There were also significant effects of intensity (F = 12 .5, df = 1, 9, p < .01) and a significant interaction between intensity and masking (F = 13 .4, df = 1, 9, p < .01) caused by the noise floor limiting the masking change so that it was less for the less intense sound. The other tests were not significant. Usually, the masked responses were no longer recognizable in the background noise. However, the incidence of significant responses in the masked conditions was higher than the 5 percent false alarm rate predicted from the F statistic used to determine whether a response was significant (Table 6) . At the 30-dB intensity, the incidence of seven false positives of a total of 40 evaluations (17.5%) was significantly different from the expected 5 percent at p < .001 using the binomial distribution to check the probability of obtaining this Audiometry Using MASTER/Dimitrijevic et al 80 60 Stimuli 40 30 dB 30 dB 20 dB 20 dB Masking Noise levels Noise levels 20 0 500 1000 2000 4000 Carrier Frequency (Hz) Figure 8 Masking of bone-conduction responses . The figure plots the amplitudes of the responses to multiple stimuli presented through bone conduction with the vibrator on the forehead and both ears occluded . The MASTER stimuli were presented at 20 and 30 dB HL with and without masking. In the masking condition, the stimuli were presented mixed with white noise at 50 dB HL equivalent masking. The squares show the mean level of electrical noise in the recorded data with and without masking . number of false alarms or more from a sample of this size . There were no differences in the background noise levels between the unmasked and the masked conditions . DISCUSSION T he results of this study are similar to others reported in the literature . In normal- hearing subjects, we found mean differences between MASTER and behavioral thresholds of 17, 4, 4, and 11 dB for the 500-, 1000-, 2000-, and 4000-Hz carriers, respectively. These differences are similar to other comparable studies (tins et al, 1996 ; Picton et al, 1998 ; Herdman and Stapells, 2001 ; Perez-Abalo et al, 2001). For example, using AM tonal stimuli in naturally sleeping adults, Herdman and Stapells (2001) found differences between MASTER and behavioral thresholds of 14, 8, 8, and 9 dB . In hearingimpaired subjects, using MM stimuli, we found that the difference between physiologic and Table 6 Steady-State Responses to Bone-Conducted Stimuli Stimulus Intensity (dB HL) 30 30 20 20 Effective Masking Level (dB) Number of Significant Responses (of 40) Percentage Significant 50 -10 50 7 36 2 17 .5 90 .0 5 .0 -10 30 75 .0 behavioral thresholds was 13, 5, 5, and 8 dB for the 500-, 1000-, 2000-, and 4000-Hz carriers, respectively. Previous studies (Rance et al, 1995 ; Lins et al, 1996 ; Picton et al, 1998 ; Perez-Abalo et al, 2001) have reported similar results . For example, Perez-Abalo and colleagues (2001) found differences of 13, 7, 5, and 5 dB . Our results with bone conduction (22, 14, 5, and 5 dB) are similar to those (11, 14, 9, and 10 dB) reported by Lins and colleagues (1996), except at 500 Hz . We shall discuss this difference later. Factors Affecting Threshold Estimation Several points must be considered when comparing studies using auditory steady-state responses to estimate thresholds . First, the method of determining threshold can vary. In this study (and in the majority of other studies), physiologic thresholds were determined using a bracketing technique. This technique involves initially presenting a stimulus at sufficient intensity to record a response significantly different from the background noise and then reducing the stimulus intensity until no response is recognizable . The threshold is defined as the lowest stimulus intensity at which a response can be distinguished from background noise. The physiologic threshold is usually higher than the behavioral threshold, and this difference may vary with the frequency and with the degree of hearing loss . Rance and colleagues (1995) used regression analyses to estimate behavioral thresholds from physiologic thresholds . If the slope of the regression differs from 1.0, this is more accurate than simply subtracting the mean difference between the two techniques from the physiologic threshold . For our particular data set, the slopes of the regressions were close to 1.0 (see Table 4) . Another method for determining threshold takes advantage of regular relationships between stimulus intensity and the response amplitude and/or phase. Generally, as the stimulus level decreases, the response amplitude decreases and the onset phase of the response decreases (changes in a clockwise direction when plotted on polar plots) . Based on either amplitude or phase, or both, one could extrapolate from two or more high-intensity responses to obtain an intensity at which the responses should have gone away. A major problem with this approach is that the functions used for extrapolation may not be valid in patients with hearing loss . Another difference among studies is the size of the intensity steps at which the responses are 217 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 recorded . In this study, 10-dB steps were used . Although using 5-dB increments will yield more accurate thresholds, significantly more time is then needed to determine the threshold. Also, the state of the subjects must be considered . Recordings can be made while the subjects are awake, in natural sleep or sedated. Many auditory steady-state threshold studies have used sedated children . This will decrease levels of physiologic noise and therefore make the responses easier to detect at near-threshold values. This will, in turn, cause smaller differences in physiologic and behavioral thresholds . A final factor is the noise level at which a response is judged to be absent . This will depend on both the state of the subject (which determines the amount of noise to be reduced) and also on the extent of noise reduction by the analysis procedure . Noise reduction by averaging varies with the time over which the recording is continued . Ifthe threshold estimation procedure considers that a response is absent (and therefore that the stimulus intensity is below threshold) at a noise level of 20 nV then the physiologic thresholds will be higher than if one uses a criterion level of 10 nV Since we averaged until the noise level was below 10 nV or for 17 minutes (in which case, the noise level was close to 10 nV), the noise level of the recording did not explain the variance in our threshold estimations. This variance was then likely attributable to the variance in the amplitude of the response among the different subjects . Several conditions must be met if one wishes to compare how well two techniques (e .g., tonepip ABR versus steady-state responses or multiple versus single steady-state responses) estimate audiometric thresholds . First, one must set up the optimal recording parameters for each technique . Second, one must evaluate each over the same period of time . Third, the evaluations must be done in the same subjects or types of subjects . Unfortunately, such comparisons are not presently available . Types of Hearing Impairment In this study, the subjects with sensorineural hearing impairment showed slightly smaller differences between physiologic and behavioral thresholds than subjects with normal hearing. This difference (between rows 1 and 2 of Table 3) was not significant . Nevertheless, similar results were found in other studies (Rance et al, 1995 ; Lins et al, 1996 ; Picton et al, 1998 ; PerezAbalo et al, 2001). Furthermore, we did find 218 that the physiologic-behavioral differences in the subjects with simulated conductive hearing impairment (row 4 of Table 3) were significantly larger than the differences in patients with sensorineural hearing loss . This phenomenon has been related to recruitment (Lins et al, 1996). The basic idea is that the response increases in amplitude more quickly with increasing intensity in subjects with sensorineural hearing loss . This would make it easier to recognize the response at intensities close to behavioral thresholds . Other reasons related to the absolute intensity of the stimuli may also play a role since we did not find any threshold difference between the subjects with sensorineural hearing loss and those with normal hearing. Response amplitudes in the subjects with sensorineural hearing loss were significantly higher than in the subjects with normal hearing (see Fig. 2) . This result is compatible with the idea that recruitment occurs in the subjects with sensorineural hearing loss . Once the stimulus was above threshold, the response amplitude was more determined by the absolute intensity of the stimulus than by the sensation level. This relationship may actually depend on the slope of intensity change rather than the intensity. The phase measurements were quite variable . This is likely attributable to the fact that we stopped recording once responses were recognized, and this would occur at low signal-tonoise levels for some of the responses. The measurement of phase provided by the MASTER system is the phase of the signal at the onset of the recording sweep . Onset phase can be converted to phase delay by subtracting it from 360 degrees . Phase delay can be related to latency, although there are ambiguities in this relation related to the circularity of phase measurements (discussed more fully in John and Picton, 2000a) . Phase measurements vary with both the carrier frequency and the modulation frequency. However, it is justifiable to collapse across carrier frequency to view overall trends in the data (see Fig. 4) . The main findings were that the onset phases in the subjects with sensorineural hearing loss were larger (or the phase delays were shorter) than in subjects with normal hearing, and these did not appear to change with intensity. These trends might be related to recruitment (the sounds at equivalent sensation level being processed as louder), to absolute stimulus intensity (sounds at equivalent sensation level being more intense), or to a broadening of the tuning curve (the less effective filter being associated with a shorter filter build-up time). Resolving Audiometry Using MASTER/Dimitrijevic et al these possibilities would need more data collected with higher signal-to-noise ratios . Efficiency of Testing The efficiency of objective audiometry varies with the rate at which threshold information is collected . This rate can be altered by making the noise level in the recording lower, making the amplitude of the responses bigger, and testing at multiple frequencies simultaneously. Several techniques can decrease the noise level of the recording. Since the main noise source is the scalp and neck muscles and since sleep relaxes these muscles, testing the subject when asleep is important. Another approach of reducing noise is to use weighted averaging (Ldtkenhoner et al, 1985 ; John et al, 2001a). Finally, the likely phase of the response is known; the statistical testing can be biased by projecting both noise and signal onto this expected phase (Dobie and Wilson, 1994 ; Picton et al, 2001). Response amplitudes can be increased by using MM stimuli (Cohen et al, 1991), exponential envelopes (John et al, in press), or multiple carrier frequencies at the same modulation rate (Sturzebecher et al, 2001). MM stimuli have been used before in estimating behavioral thresholds (Rance et al, 1995, 1998) . In these studies, the maximum AM and maximum FM occurred at the same time (H = 0 degrees) . We have previously shown that the MM response amplitude varied with different phases of AM and FM and that these differences varied with different carrier frequencies (John et al, 2001) . In the current study, we used MM stimuli with phases that gave maximum response amplitudes in normal-hearing subjects . These "ideal" phases may not have been optimal for the hearing-impaired population . However, since the physiologic-behavioral differences for hearing-impaired individuals were the same or slightly smaller than that obtained for subjects with normal hearing, the phase values seemed to have worked successfully. Recording responses to multiple simultaneous stimuli can increase the speed of testing over recording response to individual stimuli. If the responses do not interact when the stimuli are presented together, if the response amplitudes are equal across the carrier frequencies, and if the thresholds are equal at the different audiometric frequencies, then recording responses to four stimuli instead of one stimulus would speed the time of testing by a factor of 4. However, as discussed more fully by John and colleagues (2002), these conditions are not always fulfilled. Nevertheless, testing with multiple stimuli should be faster (by perhaps a factor of 2 or 3) than testing each audiometric frequency and each ear separately. Multiple responses can be recorded in both the frequency and time domains . In the frequency domain, multiple steady-state responses are disentangled by selecting different modulation frequencies for each carrier frequency. Time domain analyses can also be designed to disentangle multiple responses to concomitantly presented stimuli, for example, using maximum length sequences (e .g., Hall and Rupp, 1997) . The frequency domain analyses have an added advantage in that the statistical tests for response recognition are simple and reliable . Effects of Carrier Frequency The biggest discrepancy between physiologic and behavioral thresholds was for the 500-Hz stimulus . Other studies have found similar elevated thresholds for this frequency (Aoyagi et al, 1994a ; Rance et al, 1995 ; Lins et al, 1996; Herdman and Stapells, 2001 ; Perez-Abalo et al, 2001) . This 500-Hz discrepancy may be related to issues of neural synchrony. There is likely more latency jitter in the neurons responding to the low-frequency sounds caused by both the slowly changing stimulus and the broader region of activation on the basilar membrane . The jitter would decrease the time-locked summation of responses . Interestingly, 500-Hz thresholds are quite accurate when modulation rates in the 40-Hz range are used to evoke steady-state responses (e .g., Dauman et al, 1984 ; Aoyagi et al, 1993) . The lower modulation rate may allow the neurons to be more precisely time-locked to the stimulus . Alternatively, the 40-Hz response is mainly generated in the cortex, and it is also possible that the cortex has compensatory mechanisms that rely on other information in the auditory code to make up for the decreased synchrony. Using stimulus envelopes different from a simple sinusoid might evoke larger responses and thereby compensate for the smaller response at 500 Hz . John and colleagues (2001, in press ; see Fig. 4) showed that changing the shape of the stimulus envelope to an exponential sinusoid with an exponent of 2 (as opposed to the firstorder sinusoidal envelopes typically used) can increase response amplitudes by 20 to 25 percent for the 500- and 4000-Hz carriers . In some subjects, the physiologic thresholds at higher carrier frequencies (2000 and 219 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 4000 Hz) showed significant elevations (around 20 dB) compared to the behavioral thresholds, and these differences were significantly reduced by presenting the stimuli singly rather than together with multiple stimuli. Similar findings were noted in a study of aided thresholds (Picton et al, 1998). These effects may have been caused by masking of high-frequency responses by the concomitant low-frequency sounds . However, the high-frequency region of the basilar membrane may have been unresponsive - a "dead region," as described by Moore and colleagues (2000) . In this case, the response to the single tone may have been mediated by spread of activation to adjacent low-frequency regions of the basilar membrane . This could have been prevented by the masking effect of the lowfrequency stimuli in the multiple-stimulus condition . These different possibilities can be disentangled only by more sophisticated masking experiments. The elevated thresholds in selected patients at 500 Hz were not affected by changing between the single- and multiple-stimulus protocol . These threshold elevations were likely attributable to the relatively small size of the response rather than to any unresponsiveness of the cochlea. Bone-Conduction Thresholds Determining bone-conduction thresholds is not an easy task even when thresholds are determined behaviorally. Thresholds vary with where the vibrator is located on the scalp, how much tension is used to hold it in place, and whether the external ear canals are occluded (Dirks, 1994). Even when all of these variables are held constant, bone-conduction thresholds are more variable from one subject to the next than airconduction thresholds . Two main factors affected our behavioral thresholds for the bone-conducted stimuli : the occluded ears and location of the vibrator on the forehead . The main explanation for the occlusion effect is that the unoccluded ear allows passage of bone-conducted sound in the cochlea through the middle ear and out of the external ear canal. The normal middle ear is exquisitely designed to convey sound from the external ear canal to the cochlea and works just as well in the reverse direction. The energy activating the cochlea is thus decreased by this loss of sound. Occluding the canals attenuates this loss. Other factors contributing to the occlusion effect are the decrease in the background noise from air220 conducted sound, the relative phases of the boneconducted energy in the middle and inner ears, and the acoustic filtering effects of the occluded canal (which will vary with the depth of the occlusion) (Tonndorf, 1972 ; Khanna et al, 1976). Occluding the ear canals changes the thresholds by -16, -8, 0, and 2 dB at frequencies of 500, 1000, 2000, and 4000 Hz (average of values from Goldstein and Hayes, 1965, and Dirks and Swindeman, 1967). When the vibrator is located on the forehead, the thresholds are higher than when it is located on the mastoid by 14, 9, 12, and 8 dB at frequencies of 500, 1000, 2000, and 4000 Hz (Frank, 1982 ; ANSI, 1992). Summing the effects of occlusion and forehead placement together gives an expected difference between our behavioral thresholds and the normal mastoid thresholds with unoccluded ears (i .e ., hearing level for bone conduction, to which our audiometer was calibrated) of -2, 1, 12, and 10 dB . This pattern (low frequency less than high frequency) fits with the behavioral thresholds in the bottom two lines of Table 5. The MASTER bone-conduction threshold was higher at 500 Hz than we expected based on the data obtained before by Lins et al (1996). The resonance characteristics of the individual human skull are unique, and there may be changes of 20 dB or more across different skulls (Khalil et al, 1979 ; Hakansson et al, 1994). The changes in the bone-conduction thresholds at 500 Hz between the two studies could have been owing to such effects. However, this is unlikely since there were no specific changes in the behavioral thresholds at 500 Hz (see Table 5) . We are left with some variability of the physiologic thresholds at 500 Hz . The Lins and colleagues' (1996) study used simple AM stimuli, whereas the present study used MM stimuli. One possibility is that the AM and FM do not combine in the same way with bone-conducted stimuli. However, we feel that this is unlikely. The various frequency components of a modulated stimulus will experience magnitude scaling and phase shifts during bone transmission that are different than they would receive during transmission through the ear canal and middle ear (Hakansson et al, 1996). However, given the relatively narrow band of the MM stimuli, one would not expect the signals to be significantly altered at the cochlea relative to the mechanical output of the bone conductor. The possible role of MM stimuli needs to be ruled out, but our present results may just be attributable to the intrinsic variability of the MASTER thresholds for 500-Hz carrier frequencies. Audiometry Using MASTER/Dimitrijevic et al Stimuli presented via bone conduction are transmitted to both ears . When the vibrator is placed on the mastoid, there is a slightly greater activation of the ipsilateral cochlea for higher frequencies (Dunlap et al, 1988 ; Vanniasegaram et al, 1994), although this difference is very small, and protocols for clinical masking consider the cochleae to be equally activated (Goldstein and Newman, 1994) . When the vibrator is placed on the forehead, the activation of the two ears should be more exactly equal . If there is an asymmetry in the thresholds of the cochleae or asymmetry in the transmission of sound from the forehead to the cochleae (Khalil et al, 1979), the bone-conducted sound will cause greater activation of the cochlea with the lower threshold, and the sound will be localized to that ear. Many of our subjects reported such perceptual asymmetries . Nevertheless, since the steady-state responses were effectively evoked by binaural stimuli, their amplitudes should have been larger than the responses evoked by monaural air-conducted stimuli . Lins et al (1995) showed that the responses to binaural air-conducted stimuli were equal to the sum of the responses to monaural stimuli . Lins et al (1996) found that the binaural bone-conduction response was actually larger than the sum of the monaural responses (obtained by masking one ear via air conduction) . This difference may have been caused by the masking used to isolate the monaural bone-conduction response . The data in Figure 3 show that the binaural bone-conduction responses were approximately twice the size of the monaural air-conduction responses in subjects with normal hearing for higher-intensity sounds, although these differences were not clear for sounds of lower intensity. The phases of the responses to boneconduction stimuli were highly variable . This was in part owing to the noise levels in the recording, since we stopped averaging as soon as responses were detected, and in part owing to the variability of skull transmission characteristics . The responses were earlier (had a larger onset phase) than the air-conduction responses . Part of this difference was caused by the air-conduction responses being evoked by stimuli delayed by the time taken to pass through the tube from earphone to ear canal -0 .9 msec or approximately one-twelfth of a cycle (30 degrees) at the modulation frequencies being used . Some subjects showed clear decreases in onset phase with decreasing intensity (see Fig. 7), although this was not apparent in the collapsed data (see Fig. 3) . There are two ways to determine boneconduction thresholds . One can simply present the stimuli directly through a bone-conduction vibrator and evaluate the thresholds . Air-conducted masking is usually necessary to isolate the responses of one ear from the other. In bilateral conductive hearing loss, this is not always possible . The sensorineural acuity level (SAL) test (Jerger and Jerger, 1965) measures the amount that air-conduction thresholds are increased by noise presented through bone conduction . Cone-Wesson and colleagues (2002c) have used the SAL technique to estimate boneconduction thresholds with steady-state responses and found the results promising. We have used the direct approach to estimating bone-conduction thresholds with the steady-state responses . In the acoustic spectra of air-conducted stimuli, energy exists only at the carrier frequency and at its sidebands, not at the modulation frequency. Accordingly, there is no acoustic energy at the rate of modulation in the stimulus itself. Any electrical artifact from the air-conduction transducer would have no energy at the modulation frequency. The boneconduction transducer may not reproduce the stimuli with the same fidelity and may generate electrical activity at the frequency of modulation . If this electrical activity is picked up as an artifact in the scalp recording, one may not be sure whether the recorded responses are coming from the brain or from the bone vibrator . We believe that our responses were not artifactual because the amplitude changed less with intensity than a stimulus artifact (which would generally triple with a 10-dB increase), because in some subjects the onset phase of our recorded responses decreased regularly with decreasing intensity (see Fig. 7), and because the thresholds were related to behavioral thresholds in a similar manner to air-conduction thresholds . However, artifacts are clearly possible since the bone-conduction vibrator and its interface with the skin may be nonlinear and thus generate activity at the modulation frequencies . White noise masking might be a helpful tool in determining whether the recorded responses are contaminated by artifact . In our experiment, white noise masking at levels sufficient to eliminate the hearing of the sounds significantly attenuated the response (see Fig. 8) . The premise of the masking experiment was that an electrical artifact would not have been affected by the masking noise, other than that the background electrical noise level of the recording might have been increased by the 221 Journal of the American Academy of Audiology/Volume 13, Number 4, April 2002 artifact from the random noise stimulus . The fact that the noise levels in our recording were not significantly affected by the presence of masking noise itself suggests that the bone-conduction vibrator caused very little artifact . Choosing the level of white noise to mask the response was not simple . We chose levels that were about 15 or 25 dB above the levels necessary to mask perception . The problem is that perceptual masking may occur at levels of the nervous system higher than the levels generating the steady-state responses . In this situation, a physiologic response might be recognized even when there is no perception . When deciding on masking levels for the derived response technique, one can ensure that the physiologic response is masked, but this approach is not helpful if one is worried that the physiologic response may be contaminated by artifact . It is possible that the higher number of false alarms for the 30 dB HL stimuli was caused by undermasking. However, we feel that it was more likely that small electrical artifacts from the bone conductor were recorded . In clinical situations, if a bone-conduction MASTER response is detected, one can then rule out an artifact by presenting the stimuli in masking noise and seeing whether this significantly attenuates the response . Another approach to estimating the incidence and size of bone-conductor artifacts would be to record the steady-state responses to subthreshold bone-conducted stimuli in patients with bilateral sensorineural hearing loss . CONCLUSIONS T his study showed that the thresholds for MASTER can predict behavioral thresholds . On average, across all carrier frequencies, physiologic thresholds were 8 dB higher than behavioral thresholds for air-conducted stimuli. The overall confidence limit (p < .05) for these differences is -10 to 30 dB . One in 20 estimations will equal or exceed these limits. Both air- and bone-conduction thresholds can be estimated. When using bone conduction, one must be careful to rule out electrical artifacts. White noise masking might be helpful in this regard . Acknowledgment . This research was supported by the Canadian Institutes of Health Research . The authors also thank James Knowles and the Baycrest Foundation for addititional support. 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