Advantages and Caveats When Recording Steady-State Responses to Multiple Simultaneous Stimuli

Transcription

1 J Am Acad Audiol 13 : (2002) Advantages and Caveats When Recording Steady-State Responses to Multiple Simultaneous Stimuli M. Sasha John* David W. Purcell* Andrew Dimitrijevic* Terence W. Picton* Abstract This article considers the efficiency of evoked potential audiometry using steady-state responses evoked by multiple simultaneous stimuli with carrier frequencies at 500, 1000, 2000, and 4000 Hz. The general principles of signal-to-noise enhancement through averaging provide a basis for determining the time required to estimate thresholds. The advantage of the multiple-stimulus technique over a single-stimulus approach is less than the ratio of the number of stimuli presented. When testing two ears simultaneously, the advantage is typically that the multiple-stimulus technique is two to three times faster. One factor that increases the time of the multiple-response recording is the relatively small size of responses at 500 and 4000 Hz. Increasing the intensities of the 500- and 4000-Hz stimuli by 10 or 20 db can enhance their responses without significantly changing the other responses. Using multiple simultaneous stimuli causes small changes in the responses compared with when the responses are evoked by single stimuli. The clearest of these interactions is the attenuation of the responses to low-frequency stimuli in the presence of higher-frequency stimuli. Although these interactions are interesting physiologically, their small size means that they do not lessen the advantages of the multiple-stimulus approach. Key Words : Auditory evoked potentials, masking, MASTER, objective audiometry, steady-state responses Abbreviations : MASTER = multiple auditory steady-state response ; MINT = multiple-intensity (technique) Sumario Este art(culo evalua la eficiencia de la audiometria de potenciales evocados utilizando respuestas de estado-estable evocadas por estimulos multiples simultaneos, con frecuencias portadoras de 500, 1000, 2000, y 4000 Hz. Los principios generales de reforzamiento de la relacion sepal/ruido por medio de la promediacion aportan la base para determinar el tiempo requerido para estimar umbrales. La ventaja de la tecnica de estimulos multiples sobre el enfoque de estimulo 6nico es menor que la tasa del numero de estimulos presentados. Cuando se evaluan dos oidos simultaneamente, la ventaja es tfpicamente que la tecnica de estrmulos multiples es dos o tres veces mas rapida. Un factor que incrementa el tiempo de registro de las respuestas multiples es el relativamente pequeno tamano de las respuestas a 500 y 4000 Hz. Incrementando en db las intensidades de los estimulos a 500 y 4000 Hz puede aumentar esas respuestas, sin modificar significativamente las otras. El use de estimulos multiples simultaneos causa pequenos cambios en las respuestas, comparado con las respuestas evocadas por estimulo 6nico. La mas clara de estas interacciones es la atenuacion de las respuestas ante estrmulos de baja frecuencia, en presencia de estrmulos de alta frecuencia. Aunque estas interacciones son fisiologicamente interesantes, su pequeno tamano implica que ellas no reducen las ventajas del enfoque de estrmulos multiples. Palabras Clave : Potenciales evocados auditivos, enmascaramiento, MASTER, audiometria objetiva, respuestas de estado-estable Abreviaturas : MASTER = respuesta auditiva multiple de estado-estable ; intensidad multiple MINT = tecnica de *Rotman Research Institute, Baycrest Centre for Geriatric Care, University of Toronto, Toronto, Ontario Reprint requests : M. Sasha John, Rotman Research Institute, Baycrest Centre for Geriatric Care, 3560 Bathurst Street, Toronto, ON M6A 2E1 ; sasha@psych.utoronto.ca 246

2 Advantages and Caveats of MASTER/John et al W hen multiple auditory stimuli are presented simultaneously at suprathreshold intensities, separate steady-state evoked potentials can be recognized for each stimulus, provided that each stimulus is modulated at a frequency different from the others. If the recordings are evaluated in the frequency domain, each response shows up at its "signature" modulation frequency (Lins and Picton, 1995). Recording auditory steady-state responses to multiple simultaneous stimuli is an attractive approach to objective audiometry because multiple responses can be detected in the time normally required to identify a single response. Provided that the modulation frequencies are more than 70 Hz, that the intensity is 60 db SPL or less, and that stimuli are presented separately to the two ears or separated by at least an octave in frequency within the same ear, the responses recorded with the multiple-stimulus technique are not significantly smaller than those recorded when the stimuli are presented singly (John et al, 1998). Presenting four stimuli with carrier frequencies of 500, 1000, 2000, and 4000 Hz to each ear (for a total of eight stimuli) should therefore bring about an eightfold increase in the speed of a hearing test. The goal of this article is to discuss why this may not occur in actual practice and to consider several methods of decreasing the time required for objective audiometry. Several factors must be considered when determining whether a testing session using multiple simultaneous stimuli is more efficient than multiple single-stimulus sessions. This article will discuss these various issues in relation to data reported previously in the literature and will describe the results of some simple experiments conducted to address some of these issues and increase the efficiency of the multiplestimulus technique. BACKGROUND ELECTRICAL NOISE IN THE RECORDING he electrical noise in the recording derives T from the brain ; from the muscles of the scalp, face, and neck ; and from the amplifier. The background electrical noise can be quantified in many ways. One can measure either its power or its amplitude, and one can make these measurements in the time domain or the frequency domain. Our approach is to transform averaged time domain data into the frequency domain and then measure the root mean square ampli- tude of the activity in the spectrum at frequencies adjacent to the frequencies at which the steady-state responses are measured. Since our typical data windows or "sweeps" last seconds, the resulting amplitude spectrum has a frequency resolution of Hz. We compare the amplitude of each steady-state response to the amplitudes in 120 adjacent frequencies (60 above and 60 below the modulation frequency of the stimulus, equivalent to ± 3.7 Hz). In relation to the steady-state responses, this background activity (quantified in the 120-bin noise estimate) can be considered as random noise. If several sweeps are averaged together, the amplitude of the background noise attenuates with averaging according to the square root of the number of sweeps averaged. The time taken to reach a required signal-tonoise ratio is T = S(BR/A)2 (Equation 1) where S is the sweep duration, A is the amplitude of the response, B is the amplitude of the background activity in a single sweep, and R is the signal-to-noise ratio (in terms of amplitude) required to conclude that a response is significantly different from the background noise. This equation assumes that, during the averaging procedure, the amplitude of the response and the amplitude distribution of the noise both remain constant. We have provided empirical evidence that this is generally true (John and Picton, 2000a). The equation demonstrates that doubling the amplitude of the background noise (B) will increase the required averaging time by 4 ; doubling the amplitude of the response (A) will decrease the time by 4. The amplitude of the background electrical activity decreases with increasing frequency. Since most multiple-stimulus paradigms use modulation frequencies that are close together, the background noise levels for these paradigms can be considered as relatively uniform across the range of modulation frequencies. Figure 1 illustrates some noise measurements from the frequency domain. These data are taken from experiments reported in John and Picton (2000a). The left graph of the figure shows a small decrease in noise amplitude with increasing frequency. The right graph shows that the noise amplitudes decrease over time (as sweeps are averaged together) according to a square root rule. The noise should be the same regardless if one stimulus or more than one stimulus is being 247

3 Journal of the American Academy of Audiology/Volume 13, Number 5, May Modulation Frequency (Hz) Sweep Number (N) m N=1 A N=4 v N=12 O actual data - " 1/-~_N_ Model Figure 1 Residual background noise. This figure presents a re-evaluation of some data already published (John and Picton, 2000a). On the left, the root mean square noise levels are plotted as a function of frequency over the range at which the steady-state responses were recorded. Each data point combines noise data over ± 3.7 Hz from the plotted frequency (the modulation frequency of the stimulus). There is a small decrease in the amplitude of the noise with increasing frequency. In addition, the noise decreases as the number (N) of sweeps averaged increases. This is further evaluated in the right graph, which combines data across all of the frequencies measured in the left graph and then follows the averaged noise level as N increases. The large circles represent the actual data (averaged over 10 subjects) and the small dots represent the theoretical decrease according to a VVYrule. tested during an experiment. In a recent experiment, we evaluated the evoked potentials to multiple tone pairs. The amplitude spectra of these stimuli contain only two frequencies rather than having a carrier with two sidebands, as in the case of a sinusoidally amplitude-modulated carrier. However, the evoked responses to tone pairs are similar to those produced by amplitude-modulated tones (Dolphin et al, 1994), and, for the issues addressed here, these two types of stimuli can be considered identical. Each tone pair produced a beating stimulus with a repetition rate that was equal to the difference between the tones. We examined responses to these beating stimuli at different beat frequencies (82, 84, and 88 Hz or 178, 180, and 183 Hz), at different response ranges (near 85 Hz or near 180 Hz), and under singlestimulus or multiple-stimulus conditions. We shall discuss the responses data from this experiment in a subsequent section of this article and only consider the noise levels here. The noise levels were significantly greater in the 85-Hz range compared with the 180-Hz range (F = 410.3, df = 1, 9, p <.001). Within these ranges, there was a small effect of the beat frequency with smaller noise levels at higher beat frequencies (F = 6.1, df = 2, 18, p <.05). There were no significant differences in the noise levels between the single and multiple conditions (F =.2, df = 1, 9, p =.63). The results reviewed in this section indicate several characteristics of the background electrical noise in which the auditory steady-state responses are recorded. The noise decreases regularly according to the square root of the number of trials averaged, decreases slightly with increasing frequency, and does not change significantly when the number of simultaneous stimuli is increased. AMPLITUDE OF THE RESPONSE A nother factor that affects the testing time for multiple-stimulus technique concerns any changes in the amplitude of the responses when using multiple rather than single stimuli. When there is no change in the background noise, the multiple-stimulus technique remains more efficient if the decrease in the size of the 248

4 Advantages and Caveats of MASTER/John et al response in the multiple-stimulus condition is less than where M is the number of stimuli (John et al, 1998). As a specific example, when presenting four stimuli together in a single ear, the multiple-stimulus technique is more efficient as long as the amplitudes of the responses do not decrease to less than half of their amplitude when the stimuli are presented singly. John and colleagues (1998) used this principle to evaluate the use of multiple simultaneous stimuli at different modulation frequencies, at different intensities, and at different separations between the carrier frequencies of the stimuli. In no case was it less efficient to record responses to multiple stimuli simultaneously. The amplitudes could be sufficiently reduced so that there was no significant difference between the multiple- and single-stimulus techniques at modulation rates of 30 to 50 Hz, at an intensity of 75 db SPL or when the carrier frequencies were separated by less than an octave. For example (John et al, 1998, Table 3), at 75 db SPL, the amplitudes recorded when four stimuli were presented were reduced to 56 percent and 49 percent of what they were in the singlestimulus condition. This approximately equals the 50 percent obtained from 1/when M = 4. These findings indicate that, under certain circumstances, the amplitude of the responses may decrease when the number of simultaneous stimuli is increased. However, even when this occurs, this decrease does not overcome the increased efficiency of the multiple-stimulus technique. As long as certain caveats are respected, in many conditions there is little change in the response amplitude in the multiplestimulus condition compared with the singlestimulus condition. EFFECTS OF CARRIER FREQUENCY ur discussion so far has assumed that all 0 of the multiple simultaneous stimuli evoke responses of similar size. However, in practice, this is not true. Responses to carrier frequencies between 1000 and 3000 Hz are generally larger than those outside this range (John et al, 2001, 2002). Figure 2 illustrates results from several studies. This difference in amplitude across carrier frequency causes some responses to become significant before others. This increases the time needed when using the multiple-stimulus technique to assess thresholds. Although it is always certain that more data will be collected using the multiple- rather than the single-stimulus technique, the testing time will be prolonged if the recording period has to be extended so that the " MM, 30 db HL, BC (Dimitrijevic et at, 2002) O AM, 60 db SPL, D (Herdman and Stapells, 2001) m MM, 20 db SL (Dimitrijevic et al, 2002) 13 MM, 40 db SPL (John et al, 2001) A AM, 40 db SPL (John et a1, 2001) A AM 2, 30 db pspl (John et al, 2002) Carrier Frequency (Hz) Figure 2 Carrier frequency. This figure plots the amplitudes of responses recorded using the multiple-stimulus technique at each of the audiometric frequencies 500, 1000, 2000, and 4000 Hz in several studies. All responses are for airconducted sounds except the Dimitrijevic and colleagues bone-conduction (BC) responses. The B responses are relatively large because both cochleae are activated by each stimulus. The Herdman and Stapells data are from their dichotic (D) condition (four stimuli in each ear). The mixed-modulation (MM) stimuli from the Dimitrijevic and colleagues' and John and colleagues' studies combined 100 percent amplitude modulation (AM) with 25 percent frequency modulation. The AM2 stimuli used in the John and colleagues (2002) study were modulated using an exponential envelope. 249

5 Journal of the American Academy of Audiology/Volume 13, Number 5, May 2002 smallest of the multiple responses becomes significantly larger than noise. For example, when stimuli with carrier frequencies of 500, 1000, 2000, and 4000 Hz are presented singly at 20 db SL, the time required to detect each response will vary. To obtain typical times, we can substitute values in equation 1 from our studies of subjects with sensorineural hearing loss (Dimitrijevic et al, 2002). The amplitudes (A) of the responses at 20 db SL are, on average, 29, 46, 45, and 37 nv (plotted in Fig. 2). The background noise level of a single unaveraged sweep (B) in these studies is approximately 80 nv (data not shown). This is almost twice as high as that obtained in the sleeping normal subjects studied in the experiments that provide the data for Figure 1. Many of the hearing-impaired subjects were older volunteers who tended to be less able to sleep for the entire recording period, and, accordingly, their muscle activity was higher. The minimum signal-to-noise ratio (S) that allows us to state that the response is statistically different from noise is 1.7, which is the square root of F at p =.05 with degrees of freedom 2 and 240. For the purposes of this illustration, we shall make S equal to 2. Given a sweep of 0.27 minutes ( seconds), the times required to detect the responses are 8.4, 3.4, 3.5, and 5.1 minutes. The testing time for the single-stimulus technique is the sum of these times minutes. The testing time for the multiple-stimulus technique is the maximum of these times minutes. Rather than being 4 times as fast as the single-stimulus technique, the multiple-stimulus technique is only 2.43 times as fast. If one stimulus evokes a response that is much smaller than all of the others, the recording time for the multiple-stimulus technique will approach (but never reach) that for the single-stimulus technique These results demonstrate that the efficiency of the multiple-stimulus technique can be decreased if the amplitudes of the different responses are not equal. This attenuation cannot render the multiple-stimulus technique less efficient than the single-stimulus technique. SLOPING AUDIOGRAMS n issue that arises when performing threshold evaluations is that it takes longer to A determine that a response is absent than it does to recognize that a response is present (significantly different than the background noise levels). The decision that a response is not present usually requires that the response is not recognizably different from the background noise after this noise has been reduced to a criterion level. The time (T) required to reach this criterion noise level is T = S(B/N)z (Equation 2) where S is the sweep time, B is the single-sweep noise level, and N is the noise level at criterion. Using the values from the Dimitrijevic and colleagues' (2002) study (B = 80 nv and S = 0.27 minutes) and a criterion (N) of 10-nV noise would predict a total test time of 17.5 minutes. Using a testing period of 17.5 minutes would allow the detection of a 17-nV response using the F test at the p =.05 level of significance. If the subject has a sloping audiogram, there are several intensities at which an investigator must decide that at least one response is absent. Recordings at these intensities need to be continued until the criterion noise level is reached (rather than until the other responses are recognized). Let us work through an example in which a subject has hearing thresholds of 30, 40, 50, and 60 db HL at frequencies of 500, 1000, 2000, and 4000 Hz, respectively. We shall begin the recording at 70 db HL and work down in 10-dB steps for both multiple- and singlestimulus recordings. We shall arbitrarily set the threshold levels for the steady-state responses at 10 db above the hearing thresholds. Table 1 shows the times that would be required to measure the thresholds for the steady-state responses. These were predicted on the basis of typical response amplitudes and background noise levels from the study of Dimitrijevic and colleagues (2002). The total time to measure thresholds using the single-stimulus technique was minutes compared with 83.5 minutes for the multiple-stimulus technique. The multiple-stimulus technique is only 1.65 times as fast. If both ears were evaluated simultaneously and if the thresholds in the two ears were similar, then the multiple-stimulus technique would be 3.3 times as fast. If there were an asymmetry in the ranges of the thresholds between the two ears, the improvement in speed would be less than 3.3. Deciding that a response is absent requires a longer time than deciding that a response is present. This may attenuate but not completely remove the advantage of the multiple-stimulus technique, particularly when the subject has a sloping audiogram.

6 Advantages and Caveats of MASTER/John et al Table l T hreshold Testi ng in a Subject with a Sloping Audiogr am Sin gle-stimulus Tech nique Multiple- Stimuli Technique 500 Hz 1000 Hz 2000 Hz Intensity 4000 Hz Simult aneous Stimuli (db HL) R T R T R T R T R R R R T R = whether a response was recognized (+), not recognized (-), or not recorded (0) ; T = time taken in minutes to detect response or decide that it was absent The time taken for the multiple-stimuli recording (rightmost column) is equal to the longest time for any individual stimulus. MULTIPLE-INTENSITY (MINT) TECHNIQUE ne way to compensate for problems incurred 0 when multiple stimuli evoke responses of different sizes is to present stimuli with different intensities at different carrier frequencies. The intensities of the stimuli evoking the smaller responses can be increased so that the magnitudes of all of the individual responses being recorded are more or less the same. For simplicity, we shall refer to this approach as the MINT or multiple-intensity technique. An obvious concern about this technique is that the tones presented at higher intensities might mask those presented at lower intensities. We therefore examined the advantages of adjusting individual stimuli, within the multiple stimuli, to different intensities and investigated if any masking effects occurred for the stimuli presented at lower intensities. We used four stimuli in one ear (at carrier frequencies of 500, 1000, 2000, and 4000 Hz) in conditions wherein all stimuli were of equal intensity or wherein the 500-Hz and the 4000-Hz stimuli were either 10 or 20 db more intense than the other stimuli (Table 2). The stimuli were sinusoidally amplitude-modulated tones with the depth of modulation at 100 percent and modulation frequencies as described in Table 2. The conditions of this study contained two experiments : one examined a 10-dB enhancement at several intensities (conditions 1-6) and another investigated a 20-dB enhancement at the highest intensity (conditions 1 and 7-9). Steady-state responses were recorded in 10 subjects with normal hearing using the multiple auditory steady-state response (MASTER) data collection system (John and Picton, 2000a ; The system performed digital-to-analog conversion of the stimulus waveforms at 32 khz and routed them to a Grason Stadler Model 16 audiometer, where they were amplified to a calibration intensity, attenuated to achieve the desired intensity levels, and presented using Etymotic-2A insert earphones. Mixing stimuli of different intensities is more easily performed using analog circuity than within the digital-to-analog converter since the resolution of the converter can cause the representation of the stimuli of lower intensity to be less accurate. Stimuli of different intensi- Table 2 Experimental Conditions to Evaluate MINT Protocols ondition Recording Duration (min) f = 500 f = 80 Hz Stimulus Intensity (db SPL) f = 1000 f = 85 Hz f = 2000 f = 90 Hz f = 4000 f = 95 Hz

7 Journal of the American Academy of Audiology/Volume 13, Number 5, May 2002 ties were therefore created in separate digitalto-analog channels. For example, in condition 2, the 500- and 4000-Hz stimuli were created and routed to the tape A input of the audiometer, where they were amplified to 60 db SPL, and the and 2000-Hz stimuli were sent to the tape B input, where they were amplified to 50 db SPL. Both tape A and tape B were then routed to the left ear insert, which caused an analog addition of the signals prior to acoustic transduction. The electroencephalographic (EEG) data were obtained from an electrode placed at Cz, using a posterior midline neck electrode as reference and an electrode on the clavicle as ground. The EEG was obtained using a Grass P55 preamplifier with a gain of 10,000, a low-pass setting of 300 Hz, and a high-pass filter setting of 0.3 Hz. The recorded EEG data were again amplified with a gain of 5 by the analog/digital board and then analog-digital converted at 1000 Hz. During data collection, the data were submitted to an online weighted averaging procedure to reduce the effects of spurious bursts of unwanted noise, which occurred in our data (John et al, 2001). Because the signal-to-noise ratio is better at higher stimulus intensities, less time is needed for responses to reach significance. At 50 db SPL or higher, several of the eight stimuli will reach significance within the first minute of testing, whereas, in most subjects, about 6 minutes may be required for all eight stimuli to reach significance. At near-threshold intensities, up to 20 minutes may be required to obtain significant responses for all eight stimuli. Accordingly, we used increased durations for conditions for which stimuli were presented at lower intensities (see Table 2). The order in which the conditions were recorded was randomized across subjects. For the responses collected when the stimuli had equal intensity at 50, 40, or 30 db SPL (conditions 1, 3, and 5), the number of significant responses was 38, 37, and 33 out of 40, indicating that the test durations were adequate. Figure 3 shows representative data from a single subject and Figure 4 shows the average amplitudes of the responses across all 10 subjects. We decided to evaluate the 500- and 4000-Hz responses separately from the and Hz responses since the experimental manipulation affected them differently (a change in intensity of the stimulus or the effect of that on other stimuli). For the 500- and 4000-Hz responses, a repeated measures analysis of variance (ANOVA) (3 intensities x 2 stimulus types x 2 carrier frequencies) indicated significant main effects for intensity (F = 44.5, df = 2, 18, p <.001) since amplitude increased as intensity increased from 30 to 50 db SPL, for stimulus type (F = 34.4, df = 1, 9, p <.001) since the MINT stimuli produced larger responses than equal intensity stimuli, and for carrier frequency (F = 10.6, df = 1, 9, p <.01) since the 500-Hz response was significantly bigger than the 4000-Hz response. There was a significant interaction between intensity level and carrier frequency (F = 10.4, df = 2, 18, p <.01) because the 500-Hz responses grew more rapidly with increasing intensity than the responses at other frequencies. A second ANOVA performed using the amplitudes of the and 2000-Hz stimuli showed a main effect of intensity (F = 49.5, df = 2, 18, p <.001) but no effect of stimulus type (F = 0.6, df = 1, 9, p =.45). There was also a significant interaction between stimulus type and carrier frequency, to which we shall return later. The main findings clearly indicated that the responses to the 500- and 4000-Hz stimuli can be increased in amplitude by raising the intensity of these stimuli. This increase in amplitude occurs without any general effect on the responses to the other stimuli. The second experiment using stimuli that were 20 db rather than 10 db more intense showed similar results, with the more intense stimuli evoking larger responses, also without significantly affecting the responses to the other stimuli (Fig. 5). In general, the use of the MASTER technique provides a rapid method for collecting auditory steady-state response data because multiple stimuli are tested in the time normally required to test a single stimulus. The technique becomes less efficient if one of the responses being evaluated is considerably smaller than the other stimuli and is characterized by an amplitude that is near noise levels. In the results presented here, the 4000-Hz response shows a smaller response at all three intensity levels in the equal-intensity conditions. In actual practice, this unequal amplitude would cause the MASTER technique to become considerably less efficient. The MINT condition more than compensated for this possibility by using +10-dB relative intensity, suggesting that as little as +5 db could be used to make the size of the 4000-Hz response equivalent to the others. The 500-Hz responses in the equal-intensity 50 db SPL and 40 db SPL conditions were generally larger, rather than smaller, than the responses to the and 2000-Hz carriers (see Fig. 4). This is unlike previously reported data 252

8 Advantages and Caveats of MASTER/John et al f~ fm Equal Intensity MINT with 500 and 4000 Hz 10 db more intense V V 20 nv 40 db SPL 0 30 db SPL Modulation Frequency (Hz) Figure 3 Multiple-intensity stimuli : single subject. This figure plots in the frequency domain responses recorded from a single subject for the multiple-intensity technique. The carrier frequencies (f) and modulation frequencies (f) for the stimuli are shown in the upper left of the figure. The responses show up at the stimulus modulation frequencies as lines that are higher than the background noise levels at adjacent frequencies. Filled triangles indicate responses that are significantly different from the noise at p <.05 and open triangles indicate responses that cannot be distinguished from the background noise. The graphs on the left show response amplitudes in the equal-intensity condition. The responses are smaller for the 500- and 4000-Hz responses than for the 2000-Hz responses. The graphs on the right show how the response amplitudes increase when the 500- and 4000-Hz stimuli are increased in amplitude by 10 db (MINT condition). The noise levels at 30 db SPL are lower than at 40 db SPL since more trials were averaged (see Table 2). from the literature (Fig. 2) and might be related to interindividual variability in the response amplitudes. The single-subject data shown in Figure 3 contain amplitudes that more closely follow the expected distribution of response amplitudes for the four frequencies examined. In general, the 500-Hz response is often difficult to detect at lower intensity levels (Aoyagi et al, 1994 ; Rance et al, 1995 ; Lins et al, 1996 ; Perez-Abalo et al, 2001 ; Dimitrijevic et al, 2002). Several factors might contribute to the small size of this response. The activation pattern of the 500-Hz stimulus on the basilar membrane spans a broader area than the activation patterns of stimuli with higher frequency, and this area is in a region where the traveling wave is slowing down. Latency jitter between responses generated through different parts of the activated basilar membrane may therefore decrease the size of the compound response. Lins and colleagues (1996) also suggested that recording the 500-Hz response could be difficult owing to the masking effect of ambient background noise at the lower frequencies. Alternatively, higher frequencies within the set of simultaneous stimulus might mask the response to the 500-Hz carrier. However, as pointed out by Perez-Abalo and coleagues (2001), similar difficulties in the estimation of low-frequency thresholds have been reported for single-stimulus techniques (e.g., Aoyagi et al, 1994), suggesting that masking is unlikely. The 4000-Hz response was the smallest at all three intensity levels in the equal-intensity condition. The relatively small size of these responses is likely not owing to masking since both this study and previous studies (Dolphin, 1997) have shown that lower frequencies tend to enhance, rather than suppress, responses to higher-frequency stimuli. Part of the decreased amplitude is owing to the higher hearing level threshold at this frequency compared with 1000 Hz, but the small size persists when the 253

9 Journal of the American Academy of Audiology/ Volume 13, Number 5, May , 50 db SPL 40 db SPL 30 db SPL Carrier Frequency (Hz) O 500- and 4000-Hz stimuli 10 db more intense 0 All carriers at same intensity Figure 4 Multiple-intensity stimuli : mean data. The figure shows averaged mean response amplitudes computed across 10 subjects for both the MINT (+10 db) and equal-intensity stimuli presented at 30, 40, and 50 db SPL. In the MINT conditions, there is a clear enhancement of the responses to the more intense stimuli. There is also a small enhancement of the 1000-Hz response and a small attenuation of the 2000-Hz response. Although this interaction was significant, only the 1000-Hz enhancement reached significance with post hoc testing. The response to the 500-Hz stimulus at 40 db SPL is larger when the stimulus is presented with other stimuli at 40 db (filled circle in the middle graph) than when it is presented with other stimuli at 30 db SPL (open circle in the right graph). This is likely due to the greater noise levels in the recordings at 40 db (owing to less averaging). stimuli are calibrated in hearing level or sound level (e.g., the Dimitrijevic et al, 2002, data plotted in Fig. 2). The range of activation on the basilar membrane is smaller, and fewer neurons respond at 4000 Hz than at lower carrier frequencies. The use of MINT stimuli with the 500- and 4000-Hz stimuli being presented at levels 5 to 10 db higher than the other stimuli may help to compensate for the decreased response amplitudes and lead to increased recording efficiency when using multiple simultaneous stimuli. The small changes in the responses to the other stimuli following these manipulations are too small to affect the recording efficiency. INTERACTIONS BETWEEN STIMULI n evaluating the multiple-stimulus effect, we have so far assumed that if the responses are decreased in the multiple-stimulus condition, this reduction occurs uniformly across the different responses. The evidence presented in the literature on the human steady-state responses has generally found no significant effects of using multiple stimuli compared with using sin- gle stimuli (Lins and Picton, 1995 ; John et al, 1998 ; Herdman and Stapells, 2001) provided that the stimuli are 60 db SPL or less, are separated by ear or by an octave in carrier frequency, and are modulated at rates above 70 Hz. However, the literature also reports evidence for small interactions, and these are shown in some of our recent data. If we look closely at the amplitudes of the responses to the and 2000-Hz stimuli in the MINT experiment (see Fig. 4), we note a significant interaction between stimulus type (MINT versus equal intensity) and carrier frequency (F = 13.9, df = 1, 9, p <.01). This interaction was owing to the 1000-Hz responses having enhanced amplitudes in the MINT condition (F = 6.7, df =1, 9, p <.05) and the 2000-Hz responses showing a small but not significantly decreased amplitude (F = 0.7, df = 1, 9, p >.42). These data clearly indicate that the presence of a low-frequency stimulus at a higher intensity than the other stimuli can enhance the amplitude of the response to a stimulus with a carrier frequency one octave higher. A similar effect was noted in an experiment reported by John and colleagues (1998). The response to a 1000-Hz stimulus (the "probe") was increased from 59 to 80 nv 254

10 Advantages and Caveats of MASTER/John et al 80, L 0 0 u Carrier Frequency (Hz) O 500 and 4000-Hz stimuli 20 db more intense stimulus 20 db more intense 0 All carriers at same intensity A 4000 stimulus 20 db more intense Figure 5 Multiple-intensity stimuli : +20 db. The left graph shows averaged mean response amplitudes computed across 10 subjects for both the MINT (+20 db) and equal-intensity stimuli presented at 50 db SPL. The right graph shows the amplitudes when the MINT stimuli have either a 500- or a 4000-Hz carrier at the greater intensity level. when the stimulus was presented in the presence of a 500-Hz stimulus (the "masker"). Our experiment using tone pairs indicated that high-frequency sounds may attenuate responses to low-frequency sounds. The multiple tone-pair recordings employed three tone pairs with the higher stimulus frequencies of each pair (f2) separated from one another by an octave (approximately 900, 1800, and 3600 Hz), as shown in Table 3. We employed two groups of beat frequencies (near 85 Hz and near 180 Hz) to determine the frequency of the lower tone in each pair (f1). Tone pairs were presented at 50 db SPL, one pair at a time (single condition), and three pairs together (multiple condition) using either the 85-Hz or 180-Hz separation between each f2 and fl. Figure 6 presents the mean response amplitudes (for 10 subjects) measured under singleand multiple-stimulus conditions. The data were analyzed using a three-factor (single versus multiple, f2 frequency, beat-frequency range) ANOVA. The responses to beats in the 180-Hz range were smaller than the responses to beats in the 80-Hz range (F = 10.3, df = 1, 9, p <.01). There was no significant effect of the f 2 fre- quency or single versus multiple for the threefactor ANOVA. However, there was an interaction between single versus multiple and the f 2 frequency (F = 7.1, df = 2, 18, p <.01). Post hoc testing showed a small but statistically significant decrease in the magnitude of the response for the multiple tone-pair condition only when f 2 was 900 Hz and the beats were in the 180-Hz range (F = 8.4, df = 1, 9, p <.05). There was also a small decrease when f 2 was 900 Hz and the beats were in the 80-Hz range ; however, it was not statistically significant (F = 2.0, df = 1, 9, Response Range (Hz) Table 3 Tone-Pair Parameters Stimulus Pair f2 (Hz) f, (Hz) Beat Frequency (Hz)

11 Journal of the American Academy of Audiology/Volume 13, Number 5, May Hz Range 180-Hz Range " Single O Multiple Frequency of f2 (Hz) Figure 6 Multiple stimuli versus single stimulus. This graph compares the responses to tone pairs (beats) presented singly (one tone pair) or multiply (three tone pairs). The amplitudes were vector averaged across 10 subjects. The responses were recorded using beating frequencies that were near 85 Hz (left) or near 180 Hz (right). The stimulus frequency is plotted as the second frequency of the pair (f2). For both ranges of beat frequency, the response amplitude for the lowest frequency is slightly decreased in the multiple-stimulus condition (open circles). p =19). The amount of attenuation was -15 percent at 85 Hz and -35 percent at 180 Hz. These results indicate that when using multiple stimuli, higher-frequency stimuli can attenuate the responses to the lower-frequency stimuli. Other studies have found similar decreases in the low-frequency responses, although these have not been significant on statistical testing. Lins and Picton (1995, Table 4) found a multiple-stimulus effect of -18 percent for the 500-Hz stimulus for four stimuli in one ear and -9 percent for four stimuli in each ear. Herdman and Stapells (2001, Fig. 2A) found effects of -9 percent for four stimuli in one ear and -24 percent for the dichotic condition (four stimuli in each ear). Dolphin (1997) found that the gerbil responses to tone pairs were attenuated when tone pairs of higher frequency were simultaneously presented. The attenuation was larger for the tone pairs of lower frequency (both in terms of the tones and in terms of the beat frequencies). The greatest attenuation occurred for the 299- to 337-Hz pair, which had a 38-Hz beat frequency. This attenuation was sufficient to make the efficiency of the multiple-stimulus technique equal to that of the single-stimulus technique. A modulation rate of 38 Hz is lower than normally used for audiometry with the auditory steady-state responses, and the response may be generated as much in the cortex as in the brain stem. All other interactions were smaller ; thus, the multiple-stimulus technique was generally more efficient. Indeed, the presence of the lower-frequency tone pair could enhance the amplitude of the responses to higher-frequency tone pairs. If the tone pairs are very close together, such that tones are all within the same critical band, Dolphin (1996) found that the responses in dolphins were attenuated to a level (-3 db) where it was not advantageous in terms of efficiency to record responses to simultaneous stimuli. This is similar to data reported by John and colleagues (1998, Fig. 4), which found that presenting two sinusoidally amplitude-modulated tones with carrier frequencies of 1000 and 1050 Hz reduced the responses to one half of what they were when the stimuli were presented alone. Dolphin and his colleagues (Dolphin and Mountain, 1993 ; Dolphin et al, 1994) looked at the interaction between an "interfering" pure tone on the gerbil response to a probe stimulus that was either a sinusoidally amplitudemodulated tone or a tone pair. The probe 256

12 Advantages and Caveats of MASTER/John et al response was attenuated when the frequency of the interfering tone was higher than the probe stimulus. This is the opposite of what is found for tone-on-tone masking, for which low-frequency tones attenuate the response to (or raise the threshold for) higher-frequency tones. Very similar findings were found for the human steady-state response (John et al, 1998). The mechanism of the effect is not known. Dolphin and Mountain (1993) suggested either some desynchronization of envelope-following neurons or some effects of suppression. Bernstein (1994) suggested that there may be some acoustic disruption of the envelopes when the interfering tone is too close to the probe tone. Other possible interactions between stimuli may occur, but these are smaller than those already described. Lins and Picton (1995) found that the responses to dichotic stimuli (two stimuli in each ear and 2000 Hz) were significantly larger than those to single stimuli, but this effect was not replicated when four stimuli were presented to each ear. Interactions occur when stimuli are presented simultaneously. The most reliable of these are an attenuation of the responses to lower-frequency stimuli when presented with stimuli of higher frequency and an enhancement of the responses to higher-frequency stimuli when presented with lower-frequency stimuli. These interactions are worth studying in their own right since they indicate physiologic interactions in the cochlea or auditory nervous system. However, these interactions are generally small and do not substantially affect the efficiency of the multiple-stimulus technique. AUTOMATICALLY TRACKING THRESHOLDS here are two main indications for using T stimuli with different intensities at differ- ent carrier frequencies. The first reason is to enhance small responses by making the stimuli that evoke these responses more intense. By making responses roughly the same size, the recording is not delayed by the requirement that smaller responses have to have more time to reach significance. A second reason why different intensities might be used is to evaluate patients with hearing losses that differ significantly across frequency. Individual stimuli could be independently increased or decreased in intensity to determine thresholds independently at the different frequencies as long as using stimuli of different intensity did not substantially attenuate the responses to other stimuli. Even with differences of 20 db between the simultaneously presented stimuli, this appears true. At present, the MASTER system records responses to a set of modulated tones that are all presented at the same intensity. The duration of the recording period at that intensity is determined by how long it takes to recognize the response with the smallest amplitude or how long it takes to determine that one or more of the responses is not present. A more dynamic version of the MASTER technique could adjust the intensity of the stimulus on the basis of whether a response has been recognized. For example, the system could begin with all of the stimuli at 50 db HL. As the larger responses to some stimuli become significant, the system could present these stimuli at 40 db HL, while maintaining at 50 db HL the stimuli that have not yet evoked significant responses. If one of the responses does not become significant after some criterion averaging time has passed or some criterion noise level has been reached, the intensity of that stimulus can be increased. The program will assess the responses in the frequency domain after each sweep. Rather than using one averaged sweep, the responses (and the appropriate noise values) will be allocated to separate averages on the basis of the modulation frequency and intensity of the stimulus. The computer will thus store a set of responses at different intensities, with each response averaged sufficiently either to recognize it as different from noise or to determine that it is not significantly larger than some criterion. The software will have to include algorithms to assess responses, decide on the next intensity, allocate responses to appropriate memory locations, and keep track of these different responses. The hardware will also have to be more sophisticated than that used presently. Producing stimuli at different intensities will require a high-precision digital-to-analog converter. Alternatively, a digital-to-analog system with eight channels of audio output would allow the intensity of the individual carrier frequencies to be independently adjusted prior to being mixed and sent to the left or right ear. A dynamic MASTER system could thus continually adjust the intensities of the stimuli on the basis of the significance of the recorded response until the intensities begin to hover around the thresholds for the response. The present system automatically determines when a response is present. In the envisioned system, even the decision about which intensity or intensities to use would be automatic. 257

13 Journal of the American Academy of Audiology/Volume 13, Number 5, May 2002 CONCLUSION his article has considered the efficiency of T evoked potential audiometry using steadystate responses evoked by multiple simultaneous stimuli. The general principles of signal-tonoise enhancement through averaging have allowed us to evaluate empirical results and estimate times using simple mathematical equations. Having obtained normative values for both signal and noise levels, we can estimate how long recordings must last for stimuli of different intensities. These calculations can also be used to assess the required testing time for an individual subject using the signal and noise values from the data collected early in the recording procedure. In the clinical setting, the advantage of the multiple-stimulus technique over a singlestimulus approach is generally less than the ratio of the number of stimuli presented. This occurs when the responses are of different sizes or when the thresholds are not equal for the different stimuli. Response amplitudes evoked by multiple stimuli can be made similar by increasing the intensities of those stimuli that are producing smaller or no responses. Within the limits that we have evaluated so far (20 db), this can be done without producing significant changes in the responses to the other stimuli. The issues raised here are important when using the MASTER technique in clinical audiometry, for which one needs to determine as quickly as possible if a response is present or absent. Within the laboratory setting, the concern may be more related to response parameters (amplitude and phase) rather than to response detectability. When investigating how an experimental manipulation affects responses evoked by stimuli of different frequencies, the MASTER technique generally decreases the time required by a factor of the number of stimuli. In addition, nonexperimental factors such as the arousal level of the subject or the placement of the stimulus transducer will be the same for the responses to all frequencies. In John and Picton (2000b), we examined the phase of responses evoked by carriers modulated at several different modulation rates and then converted the phase data into latency. The standard deviations of the latency data were very small because long recording durations were used to decrease background noise to very low levels and obtain stable estimates of phase. Rather than 2 hours, it would have taken closer to 16 hours to obtain these data with equivalent noise levels using the single-stimulus technique. Our general experience is that using multiple stimuli rather a than single stimulus increases the speed of measuring response parameters such as amplitude or latency by the number of stimuli presented simultaneously. Certain caveats concerning the stimulus intensity and the differences in carrier frequency between stimuli need to be followed for this to be true. In the context of evoked potential audiometry, for which decisions are based on the presence or absence of a response, the advantage of using multiple stimuli rather than a single stimulus is reduced. When using eight stimuli, we would estimate the multiple-stimulus technique as two to three times faster than the single-stimulus technique. Acknowledgment. This research was funded by the Canadian Institutes of Health Research. The authors would also like to thank James Knowles, the Catherall Foundation, and the Baycrest Foundation for their support. Patricia van Roon assisted with the data collection and preparation of this manuscript. REFERENCES Aoyagi M, Kiren T, Furuse H, et al. (1994). Pure-tone threshold prediction by 80 Hz amplitude modulation following response. Acta Otolaryngol Suppl (Stockh) 511 : Bernstein LR. (1994). Comments on "The envelope following response (EFR) in the Mongolian gerbil to sinusoidally amplitude-modulated signals in the presence of simultaneously gated pure tones" [JAcoust Soc Am 94 : , JAcoust Soc Am 96 : Dimitrijevic A, John MS, van Roon P, et al. (2002). Estimating the audiogram using multiple auditory steadystate responses. J Am Acad Audiol 13 : Dolphin WY (1996). Auditory evoked responses to amplitude modulated stimuli consisting of multiple envelope components. J Comp Physiol A 179: Dolphin WY (1997). The envelope following response to multiple tone pair stimuli. Hear Res 110:1-14. Dolphin WF, Mountain DC. (1993). The envelope following response (EFR) in the Mongolian gerbil to sinusoidally amplitude-modulated signals in the presence of simultaneously gated pure tones. JAcoust Soc Am 94 : Dolphin WF, Chertoff ME, Burkard R. (1994). Comparison of the envelope following response in the Mongolian gerbil using two-tone and sinusoidally amplitude-modulated tones. JAcoust Soc Am 96 : HerdmanAT, Stapells DR. (2001). Thresholds determined using the monotic and dichotic multiple auditory steadystate response technique in normal-hearing subjects. Scand Audiol 30 :41-49.