The article, A Clinicians Encounter with the Auditory Steady-State Response (ASSR)1 published last year in HR, looked at the new auditory steady-state response (ASSR) for estimating frequency-specific behavioral hearing thresholds. The principles behind the ASSR, clinical experiences encountered when first attempting to administer the ASSR, as well as some case studies were described in that article. Essentially, it was concluded that ASSRs provide a good non-behavioral measure of hearing loss that is often unavailable, unreliable, or impractical via acoustic reflexes, otoacoustic emissions (OAE), and automated brainstem responses (ABR).
In an excellent related article,2 James Hall III, PhD, cautioned us not to overshadow the tone-burst ABR with the ASSR. He pointed out why the ABR, as evoked with click or frequency-specific (tone-burst) signals, retains great importance for clinical audiologists, and noted the advantages and disadvantages of the systems. Dr. Hall also presented an efficient frequency-specific ABR measurement protocol, citing resources on ABR and ASSR measurement techniques.3-5
The present article takes the clinical encounter with the ASSR one step further; here, we examine the responses recorded on the same subjects with two different ASSR stimuli.
Mixed Modulation and Amplitude Modulation
The ASSR has been known to be successful in estimating behavioral thresholds using mixed modulation (MM) stimuli; however, alternative stimuli with exponential amplitude modulation (AM) have recently entered the scene. Exponential AM stimuli have been shown to increase the visibility of two frequencies that are normally hardest to see in the ASSR: 500 Hz and 4,000 Hz.6
The ASSRs described in last years article1 were recorded with mixed modulation. The following describes MM and exponential AM stimuli in more detail. The purpose of this article is to describe one clinicians encounter with the use of MM and exponential AM stimuli, and to compare the estimation of behavioral hearing thresholds on the same subjects with both of these stimuli.
A Review of the New ASSR
The ASSR is a steady-state response, in contrast to the commonly used ABR, which is a transient response. The ASSR arises from continuous, steady-state stimulation of pure-tone carrier frequencies, such as 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz. These are each separately modulated in amplitude and frequency. If measured neural activity follows the modulated stimulus envelope, it is assumed that the carrier tones must also be audible.
FIGURE 1. ASSR of a normal-hearing subject. The FFTs on the top-right graph show the amplitude of the modulating frequencies relative to those of adjacent frequencies. On the top-right, eight yellow bars rise above the 8 individual modulation frequencies of the 8 carrier frequencies (4 for each ear). The green circles indicate whether the measured neural activity is actually following the modulations in the stimulus envelope. If not, these circles would instead by yellow (5-10% chance of not being true ASSRs) or red (>10% chance). The number of sweeps and epochs are indicated just below the top-left graph.
The ASSR is typically recorded with a vertical or diagonal electrode montage, similar to those used in ABR. An advantage of a single-channel vertical montage is that it can be used to test both ears individually, without moving any electrode location. The filters used to record the ASSR are lower than those used for the ABR, because the ASSR spectral energy of interest is at the eight modulating frequencies of the four carrier tones to each ear, roughly between 80-100 Hz. Whereas the ABR waveform is measured in microvolts (ie, millionths of a volt), the ASSR waveform is so tiny that it is measured in nanovolts (ie, billionths of a volt). As such, one would be hard-pressed to see the ASSR waveform. The ASSR is thus visualized by other means, such as vectors in polar plots (as with the Audera ASSR system produced by GSI), or a concentrated analysis on frequencies of interest (as with the Vivosonic VivoScan), or else in the frequency domain as a Fast Fourier Transform (FFT), where vertical amplitude is seen as a function of frequency (as with the Navigator-Pro system produced by Bio-logic, the system used in this article as an example). The FFTs display the amplitude of the modulating frequencies relative to those of adjacent frequencies (Figure 1).
A possible advantage of the ASSRas opposed to the tone-burst ABRis the speed at which frequency-specific estimates of behavioral thresholds can be obtained. The tone-burst ABR normally involves delivery of a single frequency of tone burst at a time. With required replication, at least two ABRs should be done at each frequency for each ear. Tone burst ABRs done on clinically conventional ABR equipment for both ears at 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000Hz would thus require at least eight ABRs for each ear, for a total of 16 ABRs. Hopefully, your client wont wake up in the meantime. Admittedly, as Hall2 states, precious time can be saved if one stops the averaging as soon as an ABR is detected, and when a response is evident at a high intensity, drop down in intensity by at least 40 dB. The new ASSR, as implemented on the Bio-logic system, presents the four frequencies all at once to both ears simultaneously. In clinical situations where sedation is not an option, and where testing is attempted while the client is simply sleeping, this might offer real time-savings.
Another advantage of the ASSR is that artificial intelligence is used to determine the presence or absence of a response at any one frequency, and this takes some of the subjective guesswork out of interpreting the response. For each modulating frequency seen on the FFT, an analysis of variance F ratio with a 95% confidence is conducted in order to determine if an ASSR is truly present. Artificial intelligence is thus used to determine presence or absence of the ASSR. Contrast this to the tone-burst ABR, where the response is gathered through objective means (such as differential amplification, signal-averaging, common-mode rejection, etc) and yet, the selection of ABR peaks or determining the presence/absence of an ABR remains quite subjective. However, as pointed out by Hall,2 ABRs retain their unique and important standing in clinical audiology.
Several other things are worth noting concerning the new ASSR and how it relates or compares to the tone-burst ABR.1
- Subjects who elicit textbook ABRs also tend to exhibit the best ASSRs. Their ASSRs are measured fastest and at the lowest intensity levels.
- There is a tendency for normal-hearing subjects not to produce ASSRs at 500 Hz and 4,000 Hz, especially at 30 dBHL.
- The ASSR seems quite promising at estimating configuration or shape of the behavioral audiogram.
- Subjects who are sleeping tend to render the best ASSRs. This is more noticeable with the ASSR than with the ABR. It is also in direct opposition to the old steady-state response (also known as the 40 Hz response).
- The ASSR measurement, once having appeared, can disappear again with further signal averaging; in the authors experience, this seems to occur more with the new ASSR than with the tone-burst ABR.
- Unlike the ABR, there are various methods of ASSR stimulation, recording, and analysis offered on different available clinical equipment.
This article, like the earlier one,1 focuses on the ASSR as recorded with the multiple ASSR paradigm (known by the proprietary tradename MASTER). This system is implemented in the Bio-logic Navigator-Pro system. It should be noted that several other test systems to record ASSRs also exist and may perform comparably.
FIGURE 2. Mixed modulation (MM). Default carrier tones for both ears of 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz are modulated, and the resulting neural responses are analyzed by the system.
Description of ASSR Stimulus Construction
Stimulus construction for the ASSR is complex but remarkable. As mentioned previously,1 the MASTER system normally involves the presentation of four simultaneous carrier frequencies to both ears at the same time. This is what saves time recording the ASSR. The default carrier tones are 500 Hz, 1,000 Hz, 2,000 Hz, and 4,000 Hz; these carrier frequencies are modulated, and it is the neural response to the modulations that is of interest with respect to the ASSR (Figure 2). In general, these four combined default stimulus frequencies are about 5 dB more intense than the individual frequencies would be alone.6 According to the researchers, 500 Hz tends to be the most difficult behavioral frequency to estimate with the ASSR, while 2,000 Hz is the easiest.
FIGURE 3. Spectral splatter and comparisons of the various amplitude modulations.
Lets take a closer look at modulation, and compare some of the various types of modulation that can be given to pure-tone carrier frequencies. Pure-tone carrier frequencies can be modulated in amplitude, or in frequency, or in both of these modes. Amplitude modulation (AM) concerns how the amplitude of a pure-tone carrier frequency is made to fluctuate over time. Varying amounts of AM can be done; if the AM is 100%, the amplitude fluctuates from full maximum to zero (Figure 3). Also to be considered is the rate, or frequency, at which the AM causes the amplitude of the carrier tone to fluctuate (ie, 80/second (80Hz) or 100/second (100Hz)).
Tones can also be modulated in terms of frequency modulation (FM), and this too can be done in varying amounts. If the FM is set at 20%, this means that the frequency fluctuates between ±10% of the carrier tone frequency. Carrier tones modulated with FM alone result in ASSR with about half the amplitude of those rendered with AM alone.6
In the previous article,1 mixed modulation (MM) stimuli were used. These are formed from the combined modulations of AM and FM. Here, the phase of AM relative to FM can also vary, so this aspect also has to be considered with stimulus construction for the ASSR. When using MM stimuli on the MASTER system, the default combination has the following properties: 1) The AM is fixed at 100%; 2) Each of the 8 pure-tone carrier frequencies is assigned its own unique AM frequency between 80-100Hz, for a total of 8 modulation frequencies; 3) The modulation frequencies increase with carrier-tone frequenciesdone mainly for the sake of simplicity; 4) The FM is fixed at 20%. As for the phase of AM relative to that of FM, research has found that the largest ASSRs in adults and infants result when the highest frequency of FM is aligned with the largest amplitude of AM.6 Larger (by 15-17%) ASSRs tend to result when carrier tones are modulated with MM than when they are modulated with either AM or FM alone. Larger response amplitudes with MM are due to the addition of the independent effects of AM and FM.6 Larger ASSR amplitudes can shorten the test time required to visualize the ASSR.
Even larger ASSRs are found when the carrier tones are modulated using exponential AM, also known as AM2. According to John et al,7 MM tends to increase ASSR amplitudes at 1,000 Hz and 2,000 Hz, while exponential AM tends to increase ASSR amplitudes at 500 Hz and 4,000 Hz. At these low and high frequencies, the ASSR amplitudes are normally smallest and most difficult to visualize. As a result, exponential AM using the MASTER system has recently been advocated as an alternative stimulus to MM. With exponential AM, the carrier tones are modulated only with respect to amplitude, and the AM is fixed at 100%. Furthermore, the phase of AM relative to FM no longer needs to be considered in stimulus construction. The rates of exponential AM remain between 80-100/second.
Exponential AM, however, provides different (exponential) rise and fall times for the amplitude modulations of the carrier tones. Compared to regular AM, exponential AM provides amplitude modulations with shorter rise and fall times, and steeper slopes (Figure 3). Holding the AM rate or frequency constant, the shorter, steeper rise/fall times with exponential AM means that the silent intervals between the modulation peaks are longer. John et al6 speculate that this might increase ASSR amplitudes due to increased synchrony of the neural response. They also suggest that ASSRs can be tested with a new type of MM, a combination of FM and exponential AM, and therefore, tap the best of all possibilities.
Modulation and Spectral Splatter. Whenever pure-tones are modulated by AM or FM, some additional frequencies are introduced along with the pure-tone frequency of the carrier tone (Figure 3). This is known as spectral splatter which can show up as additional frequencies on either side of the carrier-tone frequency. Frequency specificity of the pure-tone carrier frequencies is thus compromised due to the modulations.
According to John et al,6 with 100% AM, the width of the splatter around the pure-tone carrier frequency is equal to the frequency of modulation on either side of the carrier tone frequency. With 20% FM alone, the spectral splatter is relatively wider; on either side of the carrier tone frequency the splatter width is equal to the FM itself, plus additional low-intensity splatter at 2FM and 3FM. The width of MM spectral splatter (when the highest frequency of the FM is aligned with the highest intensity of the AM) is between those of the AM and FM described here. With this phase relationship between AM and FM, the spectral splatter width is shifted slightly towards the right of (or higher than) the carrier-tone frequency.
FIGURE 4. Exponential amplitude modulation (AM).
With exponential AM, this shift no longer occurs. The spectral splatter is once again centered around the carrier-tone frequency, just as with regular AM. The spectral splatter width, however, is slightly wider than it is with regular AM, but not quite as wide as the splatter associated with MM. The different rise/fall envelopes for MM versus exponential AM stimuli, as well as the different amount of spectral splatter for each of these stimuli, can also be seen in Figures 2 and 4.
Field Test Using AM and MM
Eight subjects (1 male, age 49, and 7 females, ages 7-48) were tested with the MASTER ASSR. The 1 male subject and 3 of the female subjects had normal hearing, and the remaining 4 had varying degrees and configurations of hearing loss. Electrodes were placed in the vertical (Fz-C7) montage, and stimuli were sent through insert (ER-3A) headphones. The subjects were instructed to lie down, close their eyes, and try to sleep or relax as much as possible while being tested.
All subjects were tested with two types of ASSR stimuli using the MASTER protocol8: mixed modulation (MM) and exponential amplitude modulation (AM). Each subject was thus tested twice, and both tests took place in the same time session. The order of stimulus presentation was randomly chosen. The one male subject was also tested with a third stimulus: a combination of MM and exponential AM.
Obtaining ASSR measurements. Figure 1 shows an example of the screen display on the system: the completed ASSR from one of the normal-hearing female subjects at 60 dBHL. The waveform on the top left contains the ASSR, but the amplitude of the response will always be far too small to see in the time waveform. (The single large bump towards the right side of the waveform is actually the subjects heartbeat) The waveform takes place over a time span of 1024 milliseconds (about 1 second). In the lingo of this ASSR method, each of these time spans is called an epoch, and 16 of these epochs strung together is called a sweep. Each 16-second sweep is added to a total running average of sweeps. The default on the system is to collect 32 sweeps at each intensity level of 60 dB, 50 dB, 40 dB, and 30 dBHL. One can choose to have the intensity automatically drop in 10 dB steps until the test is finished at 30 dBHL.
The spectrum (FFT) on the top right of Figure 1 shows the ASSR of interest. Eight yellow bars rise above the eight individual modulation frequencies of the eight carrier frequencies (four for each ear). On the bottom of the figure are eight green circles. Each circle represents whether measured neural activity is actually following the modulations in the stimulus envelope (ie, a true ASSR); if so, it is assumed that the carrier tones must also be audible. The color green indicates an F value of less than 0.05. In Figure 1, all eight circles are green; this means there is a statistical probability of more than 95% that a significant difference exists between the amplitude of the modulating frequency (below the circle) and those of adjacent frequencies in the FFT. If the color of a circle is yellow, this is a caution: there only may be an ASSR present. That is, the F value is between 0.1 and 0.5, meaning there is a statistical probability between 90-95% that an ASSR is truly present. If the color is red, the F value is greater than 0.1, meaning there is less than a 90% chance that an ASSR is truly present.
Obtaining 32 sweeps takes about 9 minutes; doing so at each intensity level would take about 35 minutes. One can also use common-sense time-saving methods suggested by Hall.2 For example, if the ASSR appears satisfactory to the clinician before 32 sweeps (ie, at 20 sweeps), he/she can elect to drop the intensity. It is common to successfully obtain ASSRs before 32 sweeps at higher intensity levels; at low intensities, such as 30 dB and 40 dBHL, it may be necessary to complete more than 32 sweeps before recognizing a completed ASSR. For the subject shown in Figure 1, the ASSR at 60 dBHL is completed after only 11 sweeps.
A few tips. Bio-logic8 recommends that, once the F-values become less than 0.05 (ie, the circles are green), then test for another two or three consecutive sweeps before accepting the results, and then decrease to the next intensity. Furthermore, if the circles are green for 3 out of the 4 modulating frequencies, and one frequency is just not becoming significant, drop down to the next lower intensity and collect there; it is a better use of precious clinical time. As long as at least one green circle appears at any one modulating frequency, keep progressing to a lower intensity. On the other hand, if no response occurs for some frequency at a given intensity leveleven with 32 sweepsthen do not expect to see a response at a lower level.
In general, lower intensities require longer averaging time. Once you finish testing at 30 dBHL, look to see if there are any absent responses at 60 dBHL; if so, increase the intensity to 80 dBHL (the highest level where you can test all four pure-tone carrier frequencies at the same time). If the client is quiet, expect to see responses within eight-10 sweeps. A good thing to notice is noise levels and F-values going down from sweep to sweep. According to Bio-logic,8 acceptable noise levels are less than 20 nV; unacceptable F-values (>0.40), along with continued low noise, means no response. Remember that the ASSR occurs between 80-100 Hz (and its filters are 1-300 Hz); low-frequency noise is therefore criticaleven more so than for the ABR (its filters are often 100-3,000 Hz).
With the artificial intelligence used by the system, it sometimes happens that a response becomes absent at, for example, 40 dBHL and yet is deemed as present at 30 dBHL. Accordingly, it is important to have some internal rules for deciding where to decide the lowest estimated threshold occurs. The company recommends that, if you see green at any one intensity and not at 10 dB higher, check to see if more sweeps were taken at the lower level.8 Also, check if the noise levels are lower at the lower intensity; that is, did the client become quieter with further testing and start to sleep? On the other hand, if you see green at one intensity and not at 10-20 dB higher, then become suspicious of the green at the lowest intensity. John et al6 also posit the rule that a green at the lowest level should be taken as the estimate of behavioral threshold, provided that an absent response is seen at no more than one intensity higher.
The results from the eight subjects are shown in Figures 5-12. As in the previous ASSR article,1 the subjects ASSRs were most robust when they were sleeping. In each figure, the subjects behavioral audiogram is displayed, along with two panels which summarize the ASSR results when using mixed modulation (MM) and exponential amplitude modulation (AM). The carrier frequencies are displayed across the top of each panel, and the dB levels are displayed along the left vertical side.
The box colors in the charts are green, yellow, or white. Green indicates that an ASSR was successfully obtained at some specific pure-tone carrier frequency and at some specific intensity level. The number located inside any green box is less than 0.05, meaning that there is less than 5% chance that the amount of neural activity at the modulating frequency (compared to that of adjacent frequencies) occurred by chance. If the box is yellow, the number will be between 0.05-0.1, meaning there is between 5-10% chance that the size of neural energy at the modulating frequency occurred by chance. A white box corresponds to a red colored circle on the test screen; there is greater than a 10% chance that the neural energy at the modulating frequency occurred by chance. The levels of noise (in nV) and number of sweeps are each shown in separate columns along the right side of the boxes in each panel.
FIGURE 5 Exponential AM (top) versus mixed modulation (bottom) ASSR measures for a 49-year-old normal-hearing subject.
Figure 5 shows the ASSR results for the one adult male subject (the author), aged 49 years, who has normal hearing. Testing with exponential AM stimuli was done first (top right panel), followed immediately by testing with MM stimuli (bottom right panel). For the exponential AM stimuli, green boxes appear at two and three (out the four pure-tone carrier frequencies) at 30dB HL for the right and left ears respectively. The left ear shows an absent response for 1,000 Hz at 40 dBHL, while the response is present for the same frequency at 30 dBHL. According to the rules stated by Bio-logic8 and John et al,6 the 30 dB level should be chosen as the estimate because the absent response occurs at no more than one higher intensity tested. For the same subject, the MM stimuli did not render any responses at all at 30 dBHL. It would appear that exponential AM stimulation yielded better results in this case.
FIGURE 6. Exponential AM (top) versus mixed modulation (bottom) ASSR measures for a normal-hearing 45-year-old female.
The results for a normal-hearing 45-year-old adult female are shown in Figure 6. Note how many more green boxes show up for this subject compared to the first subject in Figure 5; in comparison to the first subject, this subject also renders remarkably clear, readable, tone-burst ABRs. The ASSRs recorded with exponential AM stimuli (done first, top right panel) yielded responses at all pure-tone carrier frequencies at 30 dBHL for both ears, whereas the MM stimulation (done directly afterward, bottom right panel) showed responses at only two frequencies. With exponential AM, a yellow box appears for the right ear for 4,000 Hz at 40 dBHL; again, for the decision rules mentioned for the first subject, the 30 dBHL response should be taken as the closest estimate of her behavioral threshold at 4,000 Hz. In addition, relatively fewer sweeps were required for the exponential AM stimuli to yield present responses. It would appear that exponential AM stimulation yielded slightly better results in this case.
FIGURE 7. Exponential AM (top) versus mixed modulation (bottom) ASSR measures for another normal-hearing adult female.
Figure 7 shows the results from another normal-hearing adult female subject. Here, MM stimulation (top right panel) was done first, while exponential AM stimulation (bottom right panel) was done afterward. Testing with MM did not go beyond 50 dBHL because, already at this intensity, an absent response was noted for the left ear for 500 Hz and a cautionary (yellow) response was indicated for the same ear at 4,000 Hz. According to the decision rules mentioned for the previous two subjects, however, testing perhaps should have continued but that is hindsight at this time. Exponential AM stimulation did not render any absent responses at 50 dBHL, and testing continued down to 30 dBHL. Even from these incomplete results, exponential AM appears to render the closest estimates of behavioral hearing thresholds for this subject.
FIGURE 8. Exponential AM (top) versus mixed modulation (bottom) ASSR measures for a 7-year-old girl with mild conductive hearing loss in right ear at low frequencies and a mild, flat conductive hearing loss for the left ear (due to otitis media).
Figure 8 shows the results from a 7-year-old female subject who had a mild conductive hearing loss for the right ear at the low frequencies, and a mild flat conductive hearing loss for the left ear (due to otitis media). Exponential AM (top right panel) was done first, followed by MM stimulation (bottom right panel). The subject had fallen asleep during MM stimulation, as can be seen by the lower noise levels for MM, especially at 30 dBHL. For this subject, the ASSRs appear slightly better with exponential AM than with MM stimulation, because it shows closer estimates of behavioral hearing thresholds for the right ear. (According to the decision rules mentioned earlier, the 30 dBHL responses for the right ear with exponential AM stimulation should be considered valid.) Note that, for MM stimulation, testing began at 50 dBHL; responses were not tested at 60 dBHL. It would appear that exponential AM stimulation yielded slightly better results in this case.
FIGURE 9. Exponential AM (top) versus mixed modulation (bottom) ASSR measures for a 32-year-old female who, five years earlier, developed sudden idiopathic hearing loss in the left ear for the mid to high frequencies.
The results from a female 32-year-old subject who, 5 years earlier, developed sudden idiopathic sensorineural hearing loss for the left ear for the mid to high frequencies are shown in Figure 9. For this subject, MM stimulation was done first (top right panel), followed by exponential AM stimulation (bottom right panel). Note that testing was not attempted for MM stimulation at 30 dBHL. It was hoped that a response would be detected for the right ear at 500 Hz; this indeed was the case for MM stimulation but not for exponential AM stimulation. For exponential AM stimulation, however, a response was indicated for 4,000 Hz; in view of her behavioral thresholds for 4,000 Hz, this was not expected. In this case, it would appear that MM stimulation gave the closest and the most accurate estimations of behavioral thresholds.
FIGURE 10. Exponential AM (top) versus mixed modulation (bottom) ASSR measures for a 10-year-old normal-hearing female.
Figure 10 shows the results from a 20-year-old normal-hearing female subject. For this subject, MM simulation (top right panel) was done first, followed by MM stimulation (bottom right panel). For this subject, similar ASSR results were obtained for both types of stimuli, because a similar number of boxes appear at the same low intensity levels of 40 dB and 30dB HL. Note that for exponential AM stimulation, testing was not done at 60dB HL. For this case, it would appear that exponential AM and MM stimulation yielded very similar results.
FIGURE 11. Exponential AM (top) versus mixed modulation (bottom) ASSR measures for a 46-year-old female with a long history of bilateral otosclerosis.
The results from a 46-year-old female subject with a long history of bilateral otosclerosis are shown in Figure 11. In this case, exponential AM stimulation was done first (top right panel), followed by MM stimulation (bottom right panel). Results with exponential AM stimulation at all frequencies are inconclusive; according to the decision criteria mentioned earlier, green boxes at lower intensities are accompanied by absent responses at more than one higher intensity. According to the same criteria, responses with MM stimulation do appear to be present for the left ear for 1,000 Hz at 60 dB HL. Note that only 18 sweeps were completed at 80 dBHL; this is a system default provided to avoid cochlear hair cell damage due to prolonged exposure to intense sounds. In this case, it would appear that MM stimulation yielded the closest and the most accurate estimations of behavioral thresholds.
FIGURE 12. Exponential AM (top) versus mixed modulation (bottom) ASSR measures for a 46-year-old female with a history of noise-induced hearing loss for both ears.
Figure 12 shows results from a 46-year-old female subject with a history of noise-induced hearing loss for both ears. For this subject, MM stimulation was done first (top right panel) followed by AM stimulation (bottom right panel). It was expected that responses would be present at 500 Hz and 1,000 Hz for both ears. The MM stimulation appears to be slightly more accurate because responses, indeed, do appear at these frequencies at 50 dBHL for both ears, whereas they do not with exponential AM stimulation. Furthermore, with exponential AM stimulation, the right ear response at 4,000 Hz cannot be accepted as shown here, because there is no response at 60 dBHL. In this case, it would appear that MM stimulation yielded the closest and the most accurate estimations of behavioral thresholds.
FIGURE 13. Same subject as Figure 1, except this time, exponential AM stimuli was combined with FM stimuli to create a new type of MM stimuli (exponential AM + FM, instead of AM + FM).
Figure 13 shows results from the first subject once again; this time, exponential AM stimuli was combined with FM stimuli to create a new type of MM stimulus (exponential AM + FM, instead of AM + FM). This is presently an option on the MASTER software. The top two panels show the same results as those from Figure 1 (exponential AM and MM stimulation). The bottom panel shows the results obtained with a combination of exponential AM and MM stimulation. The results were obtained in the order that the panels are arranged in this figure, with exponential AM first, MM second, followed finally by the combination. Results obtained with the combination are very similar to those obtained with exponential AM alone. The MM results (center panel) show the poorest estimates of behavioral pure-tone thresholds (see Figure 1).
For most (3 out of 4) of the normal-hearing subjects tested here (shown in Figures 5-7), the exponential AM stimulation with the ASSR tends to render the closest estimates to behavioral pure-tone thresholds seen on the audiogram. One normal-hearing subject (shown in Figure 10) had similar results with both stimuli.
Results followed a different pattern for the 4 hearing-impaired subjects. For most (3 out of 4) of these subjects (shown in Figures 9, 11, and 12), MM stimulation tended to yield the closest and most accurate estimates of behavioral pure-tone hearing thresholds. For one hearing-impaired subject (shown in Figure 8), exponential AM stimulation yielded the closest estimates to behavioral thresholds.
A possible way to maximize the benefits of both methods of stimulation would be to combine exponential AM with FM stimulation (Figure 13). The large responses with MM stimulation could be due to the addition of neural responses to the separate stimulus properties of AM and FM. Furthermore, MM tends to increase ASSR amplitudes at 1,000 Hz and 2,000 Hz. On the other hand, exponential AM, with its faster and steeper rise/fall times, might increase ASSR amplitudes due to increased synchrony of the neural response. Additionally, exponential AM tends to increase ASSR amplitudes at 500 Hz and 4,000 Hz.6
This article was prepared without remuneration from outside entities. The author did consult and was assisted in the preparation of this article by several parties including his wife, Laura Venema, and Tang Chow of Electro Medical, Mississauga, Ontario, who graciously lent the lab the Bio-logic equipment.
|This article was submitted to HR by Ted Venema, PhD, an assistant professor at the University of Western Ontario, London, Ontario, and the author of the book, Compression for Clinicians. Correspondence can be addressed to HR or Ted Venema at [email protected].|
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